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The equipment within the picture above consists of, from left to proper: meccano hand-cranked gramophone with acoustic recorder (out of body) to cut 78 rpm disc records on wax or celluloid; 'Crosley' 1926-vintage two valve regenerative receiver (above) with 1925-vintage crystal radio below it. Stromberg-Carlson portable battery-valve broadcast band receiver c.1948 used as a BFO for my foremost transistor receiver. Small multimeter in entrance of S-C radio. National Panasonic four-band 11-transistor (all germanium!) radio set receiving 160 metres (VK3AML on that occasion) with my Emmco headphones linked to it. The curly wire above the transistor radio was our communal private phone line to my buddies David and Bruce Bowden, who lived across the nook in Pleasant Road. HMV portable wind-up gramophone kind C101, circa 1923, with carbon microphone to transmit 78 rpm discs down the phone line. Various broadcast-band and shortwave DX QSL cards and a single-sided disc on the noticeboard above (sung by a Madame Alma Gluck, from memory!). In my left hand, I held the oldest disc file then in my collection, London, circa 1903, of a brass band enjoying the 'Tancredi' overture. On the right, spare elements for our personal neighbourhood line, and an old electrodynamic speaker. This was the only photograph of my 'radio shack' taken within the 1960s. It was the 'sleep-out' behind 6 Torring Road, East Hawthorn, 5 miles East of Melbourne's GPO (Australia). The house was demolished in 1997. Three house items now stand there.

Author's note, 28 April 2005: As this web page deals with my own modulated mild experiments I hope readers will tolerate these reminiscences earlier than I reproduce my 1979 article on the topic from 'Amateur Radio' magazine:

WHY MODULATE Light?

Today's younger Australians can barely imagine the frustration of teenagers within the so-referred to as 'radical' 1960s where digital communication was involved. By modern standards, radio was locked in legislative totalitarianism, dominated by the governmental communication monopoly of the Australian PMG's Department, and run on rigid submit-colonial British public service strains. Something as innocuous as an intercom line run along your back fence to adjoining friends was unlawful - 'a financial risk to the monopoly of the public telecommunications network' in the eyes of the PMG. I found this when i set up a neighbourhood phone network to other local kids' places early in 1967! Music transmission by way of novice radio had been banned in Australia since 1939, so that the material legally conveyed on any 'wireless' system was limited, significantly for music-mad teenagers. CB radios, mostly of the one channel hand-held selection, have been imported in restricted portions and bought freely - however paradoxically they could not be legally licensed or used. You needed to be fifteen years outdated to sit for the amateur radio license, then involving a trifecta of exams in full principle, regulations, and Morse at 12 words per minute. For the speculation, discursive essay solutions were required, theprimefans.com and there was no level of principle examination below the single maximum standard. From a younger particular person's perspective, you had to drag yourself over a area of damaged glass to attain a authorized entry level. Even then, you had to wait to your sixteenth birthday to function ham radio legally in Australia.

If, like me, you had been thirteen years previous in 1967 and had digital experimenter pals close by, your aspirations to communicate faced years of seemingly endless frustration. There have been good causes to analyze 'various' communication technologies - including modulated mild.

Along with that we had the Vietnam War; the imminent threat of the compulsory military draft; a conservative authorities in power since our delivery; and British-styled college uniforms unsuited to our scorching summers. Australian teenagers of the 1960s had many legitimate causes for discontent. The protest motion had its roots in lots of aspects of Australian society, not simply the political issues for which any street march was a convenient excuse...

During the following decade Australians saw the advent of full citizenship for Aborigines (1967); the Vietnam Draft Resister's Union, and its pirate radio station '3DR' (1971); a radical Labor authorities coming to power (1972); the first Sunbury rock festival, Australia's 'Woodstock' (1972); novice newbie radio licensing (circa 1973); group broadcasting (1974); ethnic, multicultural broadcasting (1975); and the native legalisation of CB radio (1977). Finally, in the 1980s, non-public carriers in the telecommunication marketplace had been allowed to compete with the PMG and its descendant organisation, Telstra. Communication monopolies have been broken and the authorized restraints on private electronic communication eased markedly. With the advent of the Internet, how may it's otherwise immediately?

I'd argue that Australia was fairly in contrast to America within the 1960s, in that conservatism reigned through most of the 1960s here, significantly in our communication laws. Australia's 'liberation' - if you could call it that - largely got here after 1970...

The advent OF Educational Solid-STATE KITSETS

So, how did your average kid with greater than average curiosity get into electronics, and more significantly into optical communication, forty years ago?

In the mid-1960s, when the price of germanium transistors had fallen to a reasonable degree, electronic development kits have been made obtainable to stimulate the minds of future radio hams and experimenters. These kits have been in contrast to the single-venture meeting outfits then available from Heathkit and Lafayette. Their accent was on schooling, providing a collection of electronic components to be arranged in different configurations on an insulating 'breadboard' with spring clips and connecting wires. With assistance from a guidebook explaining the operation of each association, more than twenty different circuits could be constructed, together with Morse code oscillators, simple radio receivers, excessive-achieve audio amplifiers and low-power radio transmitters. At this time, we additionally had the bonus of a neighborhood Melbourne journal for younger experimenters, 'Transistor Kits' revealed by Colin Witchell - extra not too long ago of 'Talking Electronics' journal fame - from a tiny shop in Church Street, Brighton. For many who had been fascinated, the mysteries of electronics held the key to a new world of technical potentialities - and many of Colin's projects concerned gentle detectors in some creative method...

An previous good friend from my main faculty days, Rowland Legg, acquired a Philips twenty-in-one kit for the Christmas of 1965, and I was given a similar Japanese 'Eleco' equipment on the identical day. My mother and father had apparently noted my early aptitude for constructing crystal radio receivers, so they'd determined to 'push the envelope'. The natural outcome was that over the following 5 years Rowland and that i spent many afternoons collectively, unravelling the mysteries of digital amplification. Whatever time wasn't spent indoors with the kits or on other electronic initiatives, we spent up on our rooves erecting lengthy wire radio antennas, or making an attempt to get them up on progressively larger trees or supporting masts.

During these antenna-raising periods, Rowland Legg and I discovered that we may see one another's homes from vantage points on our rooves. In an try and set up a signalling system, we spent many evenings up ladders with kerosene lanterns, using a black card to chop off the sunshine and send messages to one another in Morse code. The usual Morse alphabet appeared fairly complex, so Row devised considered one of his own, following a logical mathematical progression:

This Also proved to be too complicated to memorise and, with light flashes, too troublesome to relate to any typed checklist! We searched for a extra facile technique of communication, with adequate security not to draw consideration from the licensing authorities.

OUR FIRST OPTICAL COMMS - JUNE 1968

In 1967 I became aware of the potential for transmitting speech over modulated gentle beams when i purchased a replica of an historic, leather-bound book 'Science For All' (1884), containing William Ackroyd's account of Bell and Tainter's then-new 'photophone'. Another early influence was a e book published in 1921, 'The Boy Electrician', which gave constructional details of selenium light sensitive cells, Tesla coils, audio transmission by multi-flip induction loops (which I constructed) and even the details of a small X-ray machine (which, thank God, I did not assemble).

Like many teenagers of the late 1960s, I experimented with modulated gentle communication using amplifier-driven torch globes or neon lamps for transmitting and CdS photoconductive cells or OCP71 germanium phototransistors for receiving. A web page from my bench notebook dated 25 May 1968 - a number of weeks after my 14th birthday - exhibits my earliest plans for an optical system:

On the following weekend, 1st June 1968, a highschool friend named Howard McCallum and i set up the deliberate modulated gentle system with an incandescent torch bulb for transmitting and a CdS LDR in sequence with a 9 Volt battery and headphones for receiving, collimating a gentle beam between the two with magnifying lenses. The results over a distance of 4 metres had been loud, however very distorted with frequency doubling effects - a results of our preliminary attempts to transmit without DC bias on the filament lamp!

Soon afterwards, a Mr A G Murrell of Penola, South Australia, printed the details of his simple 'photophone' in the 'A Reader Built It' page of March 1969's 'Electronics Australia' (pps. 91-93). The challenge's simplicity made it an attractive proposition for younger folks, and plenty of Australian experimenters of my era will remember it:

The geographical setting for our personal reconstruction of Murrell's device was less than salubrious. In actual fact, it was probably the most eccentric radio 'shacks' that I've ever encountered. At the underside of my buddy Rowland Legg's back yard, a big wood packing crate originally used for shipping a Volkswagen automobile to Australia was set up by Row's father, Ern Legg, as a tiny electronics room - 'the tin shed' as we referred to as it. To maintain out the drafts, its walls have been papered with out-dated advertising posters for Melbourne's weekly scandal-rag, 'The truth', obtained from our kindly native newsagent around the nook in Tooronga Road. It was the kind of newspaper that no person would admit to buying, although the newsagent assured us of its wonderful native circulation. From every angle in Rowland's radio shed, headlines in an unlimited typeface assaulted the attention: "SHOCK BIKIE Film" - "BLACK PANTIES Murder" - "CATHOLIC FATHER Wants Sex Surgery"! The piece de resistance amongst these posters was tactfully hidden behind a cupboard door, its wording being one thing like - "UNWED Mother TELLS Court: 'HE SHAGGED ME; THEN WE HAD INTERCOURSE' !!" - and I'm nonetheless wondering about the implications of that!

In these inglorious surroundings, and within the backroom of my dwelling proven within the picture at the highest of this web page, we spent many weekends constructing electronics tasks of every conceivable sort. Together, we quickly had a version of Murrell's photophone transmitter built into the optics of a small ex-WW2 Aldis signalling lamp with a concave parabolic mirror of about 7.5 cm diameter. The Aldis housing was finally mounted (with yards of PVC tape!) on a scorching water overflow pipe protruding from the tiled roof at 1 Cole Street, Rowland's dwelling.

However, my own initial optical comms exams with the Murrell equipment have been finished around March 1970 from the the backyard radio shack then occupied by Hughie Paton, VK3ZEP, at forty nine Havelock Road, across that home's again backyard, down its back driveway and across Torring Road to the glassed-in entrance veranda of our household home (to the left of the entrance door within the photo below) - a distance of about 50 metres. The system provided a link in one course only. These assessments have been logged on open reel audio tape, in order that I may test my audio high quality with out the necessity for an assistant. My mother sometimes came onto our veranda to take heed to the end result and communicate her reaction by waving through the windows. The tape still indicates the pretty good audio quality that one may get with a restricted depth of modulation. It also demonstrates the very excessive hiss degree of the OCP71, the actual machine used on that day being seen within the photo on the right.

I was out in the open air at the back of the Havelock Road property, across Torring Road from my home, shown above, with the Aldis lamp's glass parabolic reflector optics on a telescope tripod. This had a three volt 300 mA torch globe at its focus. I later found that the high frequency response of the system was inversely proportional to the thermal inertia of the filament. The very best excessive frequency response was obtained with lamps of lower rated maximum present, a skinny filament with thick lead-out wires to dissipate the heat quickly.

The transmitter gear, consisting of a microphone pre-amp, 3 watt transistorised audio power amp with transformer output, and two dimension D torch cell battery supplies (one for the amp and one as DC bias for the lamp) all sat in a carton between the legs of the tripod. On the receiving end, a 7.5 cm diameter magnifying lens focussed the image of the Aldis lamp onto an OCP71. I am unable to remember whether or not the OCP71 was instantly linked to the microphone enter of the Sanyo 5" open reel tape recorder (photovoltaic configuration), or whether or not it was working into the road enter via the same old two-transistor pre-amp (photoconductive arrangement). I seem to keep in mind that each were tried at different times, with a minimal distinction of results. An occasional brief buzz could be heard when frequent Australian blowflies happened to fly via the beam, modulating the sunshine with their wing beats. In the latter a part of the extract, a gentle shower of rain occurred, the droplets falling by the beam producing a type of delicate 'plip-plap' sound. The effect of waving one's hand by way of the beam was additionally demonstrated. Whenever the beam was interrupted the hiss degree rose considerably - an impact for which, even now, I don't have any rationalization, except that it obviously had something to do with the impedance of the phototransistor supply rising as the sunshine enter fell.

The audio tape of the take a look at was made on a warm, fairly overcast Saturday afternoon. Within the background, Ghera Harris (1896 - 1991) and her architect daughter Berenice Harris (1925 - 2002), who owned the Havelock Road property, could be heard washing dishes after lunch and planting hop bushes in the garden. The primary World War veteran aero-engine fitter Reg Harris (1894 - 1979), Ghera's husband, was apparently planning a new ingredient for some home brew! The son of the family, Brian Harris (1936 - 1992), was briefly VK3ZFH in the late 1950s. Brian confirmed the writer methods to tune up a transmitter and browse an oscilloscope on the age of 4, in 1958. This set Chris on the lifelong 'downward' path into technical interests... One in every of Brian's last jobs concerned the design of some of the communications gear for the Hubble house telescope. Suburban life might be attention-grabbing with neighbours like the Harris household at forty nine Havelock Road!

On this ten-minute mp3 extract from the original 45-minute tape, the author at the age of 51 in 2005 introduces his squeaky-voiced 16-yr-old self in 1970. Computer nerds didn't exist then - WE have been RADIO nerds! Anyone involved can obtain the audio log file of this March 1970 check as an mp3 file right here:

(Currently broken - coming soon) March1970.mp3

One curious facet of the germanium phototransistor OCP71 was its excessive infra-red sensitivity. It was attainable to transmit audio, as one can hear on the tape above, with the torch bulb working at a voltage so low that it had no perceptible visible output. The germanium detector may resolve the modulated HEAT from the lamp filament. Its work perform as a photodetector was very low, which meant that it was inherently topic to massive amounts of thermal noise, much greater than silicon and very much greater than photomultipliers:

On 1 September 1970 we extended the vary of the system to transmit audio from Rowland's electronics room ('the tin shed') at 1 Cole Street to my dwelling at 6 Torring Road, East Hawthorn, where I stood atop a ladder subsequent to our backyard bungalow with an an optical unit product of tin cans, like Murrell's within the photo above. The audio link covered about 600 metres, and that i could clearly see the light focussed on the OCP71 within the receiver tube. In truth, I might purpose the receiver by having the focussed spot disappear behind the sq. delicate materials inside the phototransistor. The hiss degree from the OCP71 germanium phototransistor was gross, the 7.5 cm diameter of our optics was inadequate for the range and the trebles were restricted by the thermal inertia of the torch bulb, however this was our first real 'light beam DX'. At the age of 16, the pleasure of listening to these outcomes induced my hands to shake, so that my optical receiver's goal was erratic, however I managed to log the whole contact on an audio tape in two 'bursts', with a pause to phone Rowland to tell him that each one was being obtained, midway. Initially, Rowland and his friend Neil Florence have been merely relaying the 7:30 pm news broadcast from the Melbourne broadcast station 3AK with its items on the Vietnam War, and floods in New Zealand. As I slowly managed to align the receiver, the transmission steadily rose out of the noise - though not by a lot! Later, typical teenage music of the day might be heard - The Crystals singing 'After which He Kissed Me' (with Rowland attempting to interpolate a 1970-vintage form of 'karaoke') and the document of 'Lay Down' sung by Melanie Safka. When Rowland switched to his carbon microphone to announce 'Hello, Chris... that is being transmitted on the first day of Spring, the 1st of September 1970 - and if you can't hear this after all this hassle I'll should kill you!', I may easily have fallen off the ladder in amazement. We'd lastly devised a technique of circumventing the illegal utilization of a radio transmitter - though to be real looking, with this appalling sign-to-noise ratio the success was only marginal:

http://www.bluehaze.com.au/modlight/1Sept1970.mp3

I've not often skilled more thrill from experimental work than I did on that night time 35 years in the past, and that i need hardly add that instantly afterwards, adrenalin-charged, I ran non-stop to Rowland's to report our success! The three of us posed for a photograph around that point, with me holding a pair of 1920s-vintage headphones which we continuously used. The image could possibly be titled 'hear evil, SEE evil, DO EVIL' - but largely, our backgrounds have been simply too 'Eastern Suburbs' and discreetly shy for any of that!

OPTICAL COMMS IN SOUTH AUSTRALIA - 1968 TO 1972

Elsewhere, different Australian experimenters have been attempting the potential of atmospheric optical communication extra critically. In Adelaide throughout 1968, my future collaborator Mike Groth (at the moment VK7MJ, then VK5ZMG) and a lab assistant friend, Stewart Powell, built a pair of optical communication items in the suburb of Hammersmith using torch globes and OAP12 germanium photodiodes. That they had a maximum range of about 1 km, and, as Mike places it, "awful fidelity". Mike's spare time for optical tests was restricted at the time by the calls for of doing the third yr of a BSc - with a new spouse.

However, in 1969 Mike Groth did his Honours in Adelaide, and one of many course tasks in that year concerned modulated light. He explained the next events to me in a letter dated 28 February 1988:

"The project involved the evaluation of modulated light as a means of transmitting geomagnetic knowledge over brief distances to keep away from wire links to the remote sensors, which tended to introduce hum loops if not rigorously balanced. This was a golden opportunity to be taught the speculation behind optical hyperlinks and search again by means of the literature at the University Library. It grew to become apparent that it could be no drawback to transmit knowledge over just a few hundred metres on a transparent night time using the brand new infra-purple diodes as sources, but the reliability of the link was unknown, especially as it was to be used at a subject station in the Adelaide hills, where fogs and mists have been common. I constructed an infra-crimson hyperlink at 930 nm which measured the path loss over the winter and spring of 1969, and my estimates of the path losses [in the 1987 'Amateur Radio' article 'Photophones Revisited'] were based mostly on this data.

Optical communications fell into the background for the next sixteen years, as I spent 1970 in New Guinea instructing and moved to New Zealand in 1971 to do postgraduate work at the University of Otago in Dunedin. I returned to Australia at the end of 1979, but was fairly inactive in experimentation till 1985, once i determined to write down my experiences as a review of the prospects of optical hyperlinks. The hassle took practically 18 months..."

Mike's resultant article published in 1987 with revisions from 2005 may be discovered at:

Probably the most highly effective novice atmospheric optical communication tests in Australia around 1970 were carried out by an Adelaide workforce and reported in an extraordinary Tasmanian magazine. The Hobart-based 'Electronics Exchange Bulletin' was printed round this time by the Tasmanian team of Leo Gunther VK7RG and Rodney Reynolds VK7ZAR (now VK3AAR). Their extraordinary magazine inspired articles by local experimenters on every conceivable subject related to electronics and communications. Through the pages of 'EEB' between August 1968 and October 1972, two university students, Kingsley Burlinson VK6ZEA and Robert Averay VK5ZGE described their experiments in modulating fluorescent and mercury vapour fuel discharge lamps, attaining atmospheric ranges in excess of 3.5 miles (about 5 km), just exterior Adelaide in valleys shielded from the city lights.

Though their tools was massively bulky by fashionable requirements, Burlinson and Averay pursued a novel line by driving their gas discharge lamps with audio-modulated 10 KHz pulse width modulation from 'class D' switching output transistors, thereby avoiding linearity and modulation efficiency problems. In this fashion, the performance was just like that of the celebrated (on the time) class-D British Sinclair 'X-20' transistorised audio amplifier of the mid-60s. I used to be not conscious of the reports of those assessments until the start of 1976, otherwise I could have skipped the next few steps in my very own tests...

OUR Switch TO Gas DISCHARGE LAMPS

A search for higher modulated light sources than incandescents drew my consideration to neon lamps. Gas ionisation is a much quicker process than the incandescent heating of a filament. In these days, neons had been cheaply accessible in any size from a pea lamp to a full size "beehive" bulb from one marvellous supply. Melbourne experimenters energetic in the 1960s will remember Waltham's Trading Company in Elizabeth Street, Melbourne. At the top of a slim stairway main down from the pavement was a quite grubby Aladdin's cave of tables laden with cable, conflict surplus junk (each war except Vietnam), bins of valves and khaki-painted gadgets of indeterminate origin. Cash-strapped adolescents milled about with down-turned eyes, slowly sifting row after row of cartons and crates full of technical cast-offs of each description. I acquired several packing containers of neons there, together with varied different gas discharge units to strive. Many years later, I discovered that some of these contained traces of radioactive material to help ionisation. Let the buyer beware!

I initially wired a neon lamp into the anode circuit of a Philips battery triode from the 1920s, sort B406. The orangey-pink glow across the cathode on these neons is fairly intense, though it doesn't even strategy the intensity of modern LEDs or lasers. After organising the neon modulator I used to be rewarded with near perfect audio from my OCP71 for the primary time. Fortunately, a parental veto on constructing mains-fed power provides ended on my fifteenth birthday (8 March 1969), so the required 300 volt rail was no downside.

SIDETRACK INTO 'MECHANICAL' Television - 1971

I used the neon modulator and OCP71 as the idea for a easy television system in 1970, by including a couple of Nipkow scanning discs to the outfit. This diverted me from the modulated mild communication assessments for a number of years as I delved into all of the pre-struggle Baird television texts to bring the pictures to an acceptable standard. This introduced me into collaboration with the late Dan Van Elkan (b.1952 - d.1986, name sign VK3UI) and Tony Sanderson (b.1945, VK3AML), now the moderator of the 'bluehaze' internet site. They had been the 'ringleaders' of a larrikin group of amateurs working residence built AM transmitters on the 160 metre band, then occupying 1800 KHz to 1860 KHz. These guys' novice radio interests were unique and individualistic. Nearly all of conservative hf (shortwave) operators had a narrow and obsessive emphasis on 'communication high quality' modulation, normally 300 Hz - three KHz, clipped, non-linear and cruddy. Dan and Tony were each hi-fi enthusiasts. Their transmitters were comparatively broadbanded and immeasurably low in distortion, exceeding the audio specifications of many broadcasters. AKG or Western Electric microphones and broadcast-high quality audio peak limiters with twin time constants had been involved. The design and construction of their modulation transformers and amplitude modulators was nothing wanting an artwork. Their 160 metre receivers, additionally, employed biased, low distortion envelope detectors working by way of carefully designed audio amps into enormous speaker methods with vented enclosures. The content material and audio quality of their in-depth conversations on communications expertise made listening to their transmissions an absolute pleasure. Even their customary of audio compression served to convey the listener into the acoustic surroundings of their properties - an underestimated side of making a sensible auditory illusion. Their exercise ruffled fairly a few amateur operators' feathers on the time... which solely increased my youthful admiration for each of them.

I really met Dan (3UI) - dare I admit it - on the air on 31st December 1969, as the results of a quick dalliance I had with pirate radio transmissions on 1.Eight MHz. He lived in Hawthorn only a mile from my dwelling, near the corner of Glenferrie and Riversdale Roads. Naturally he was amongst the first to listen to my feeble and unstable transmissions, and he inspired me to experiment additional to gain the data to get the novice 'ticket'. We were each given a 'cease it or else' ultimatum by an over-zealous radio inspector identified to the locals as 'Uncle Ugh' (many will nonetheless know who I mean), but Dan and that i turned nice friends as a result. Dan was about two years older than I, and was extremely influential on the instant future course of my life. Because the yr 1970 progressed, I found myself spending extra time with newbie radio mates and fewer with Rowland and the old school mob. Rowland ultimately carved out a really successful profession in the Victorian Police Force (ironical, isn't it?), and i consider he still does, but his early interest in electronics declined...

In subsequent many years, the amateurs have legislated themselves into 'band plans' with 'accepted modes' and 'accepted bandwidths' for various frequency segments. Consequently - and I'll express an opinion right here - they have systematically eradicated the authorized basis for the type of justifiable experimentation that 3AML and 3UI used to undertake. Many 'hams' are now operators of economic 'black field' transceivers which may elegantly provide single channel phone high quality (or worse) on every obtainable band for $4000+, but which regularly can't be correctly adapted to every other mode or type of experiment. From my perspective, the result's that beginner radio has had progressively less attraction as a hobby, and I'm sure I'm not alone in expressing this opinion. For my cash, should you legislate against experiment you kill the one attraction that newbie radio ever had. So long as the emissions don't unfold past the novice band edges, what is the issue? Anyone who pushes the hoary previous argument that "bandspace is at a premium, so transmissions should be of the minimum attainable bandwidth" have to be deaf and blind to the steadily declining level of newbie band usage over the previous fifteen years. Who's to say what beginner radio ought to entail, so lengthy because it supplies coaching and encourages experiment? Many people surprise why I've never bothered to pursue an newbie radio license. I hope that I've justified my position. Thank God for alternatives with extra freedom of content material and bandwidth, like mild beam communication!

Dan (3UI) and that i eventually arranged test transmissions of slim band television using mechanical disc scanners over his 160 metre transmitter early in 1972. The subsequent couple of years gave all of us an excellent grounding in the rules of light detection and modulation, video amplification and optics.

Eventually, with D B Pitt and others within the United Kingdom, we formed the Narrow Band Tv Association, still in energetic operation and now represented on a website:

http://www.nbtv.org

One significantly type donation to this mechanical Tv scanner project was supplied by the late Kevin Duff, VK3CV (b.1927 - d.1996). Kev worked in telecine at Melbourne's authorities Tv station, ABV channel 2 in Elsternwick, operating an archaic monochrome Marconi 35mm movie scanner. This was initially mentioned to have been designed for the 405 line British service and used at BBC Tv's unique studio on the Alexandra Palace in London. By 1972 it was solely being used for half-hour a day, at about 4:00 pm, to broadcast 35 mm film episodes of the kids's serial "The Cisco Kid" - the only regular program materials they'd which still used that gauge of film. The EMI 6097 photomultiplers on this Marconi telecine had been written off as soon as they developed spots on their photocathodes - Kev referred to as them 'dynode spots' - however they were still fairly serviceable for light detection. In 1972 Kevin saved some of these from the dustbin for us.

Dan (3UI) and i adapted the EMI 6097 photomultiplier for use in our experimental digicam and for the modulated mild receivers. Its sensitivity was such an enormous quantum leap from the OCP71 that it opened a complete new world of technical potentialities to us. Further photomultipliers have been acquired from the late A H 'Mac' McKibbin, VK3YEO, who used 931A's for gradual scan television scanners in these days.

MOD Light ON 160 METRES - FIRST CROSSBAND Tests 1974

In the course of the Autumn of 1975, I built a modulated gentle communication hyperlink that was used briefly between two members of the 160 metre AM group, Paul Higgins (then VK3BEK, now VK3EN) and Dave Stewart (VK3ASE). Both had radio shacks at first floor level, dealing with each other throughout suburban Glenhuntly with an uninterrupted line-of-sight, and separated by about 700 metres. The modulator from my Baird mechanical tv receiver by then used a 6L6 beam pentode in sequence with the neon, which was re-mounted at the main target of a tough 30 cm moulded glass parabolic reflector supplied by Tony (3AML). The reflector had initially been used in a site visitors signal.

This optical transmitter was placed on Paul's balcony in Glenhuntly Road, on the nook of Clarke Avenue. The receiver was positioned within the attic window of Dave's QTH in Burrindi Road, Caulfield South. It used a 13 cm diameter magnifying lens focussing onto a 0.5mm focal plane aperture, with a 931A photomultiplier catching the transmitted light behind it.

Using Dave's 160 metre novice transmitter as the return hyperlink - the ultimate in cut up frequency operation - Paul managed to hold on a crossband contact through the neon lamp. Sig/noise was poor owing to the very poor spectral match between the orange neon and the blue sensitive photomultiplier. The neon was pushed past its present rankings, in order that its bulb was rapidly blackened by cathode sputtering, however at least it could possibly be totally modulated. I was later in a position to measure the bandwidth and was astounded to seek out that the neon might be modulated to around 500 KHz, which was a terrific improvement on the torch bulbs. Distortion was severe. We had been pushing the modulation fairly arduous, owing to the poor sig/noise ratio. A sample of the audio log of the contact is currently out there on Dave's web site. The results had been sufficiently good for us to realise that we have been on the right track for further enchancment. The contact was logged on tape and an extract may be heard on VK3ASE's internet site:

[ Actually, Dave appears to have removed this one for now. (Tony, VK3AML) ]

The primary drawback to be overcome was the low characteristic depth of the neon discharge. We additionally needed a supply with significant blue output, to match the spectral response of the photomultipliers, which may solely detect violet, blue or green mild. Development was accelerated by the involvement of one other member of the 160 metre cross-band contact clique, John Eggington (then VK3ZGJ, now VK3EGG) whom I met at the top of 1975.

BREAKING THE ONE-MILE BARRIER - DEC. 1975.

At my residence, I used to be lucky in having an elevated place near the top of a hill in East Hawthorn, South of Camberwell Junction and quite close to the excessive copper dome of 'Our Lady Of Victories' catholic church. Standing on our roof, the view of downtown Melbourne to the West and of the suburbs around to the North was unobstructed, encompassing all of Hawthorn, Kew, Toorak, Kooyong as well as parts of Malvern, Richmond, Abbotsford and Northcote in an unbroken a hundred and twenty degree arc. Just on my aspect of the Yarra, on the highest of a rise in clear view, was VK3ZGJ, with his shack facing me on the third ground rear of an outdated Victorian mansion at 29 Shakespeare Grove, West Hawthorn. The topography was ultimate for optical communication exams, exactly two miles (about 3.5 km) on an East-West path.

In the early 1970s, before I met him, John (VK3ZGJ) constructed a sequence modulator for fluorescent lamps, consisting of a number of 807 output valves in parallel, with the fluoro within the anode return. He also built a portable light dependent resistor (LDR) receiver with a FET preamplifier. The LDR operated with bias into a load of 10 megohms or more, and with its low noise preamplifier it had much better sensitivity and spectral match to a fluorescent lamp's output than my previous OCP71. You needed to arrange for the image of the fluoro to focus exactly onto the gap between the conductive combs on the LDR's delicate floor. This concerned peering on the LDR by a 'spy-hole' in the optical mounting while deftly manipulating the receiver's alignment.

LDRs have a very slow response, rolling off a minimum of 6dB per octave above 50 Hz. Treble boost might only partly appropriate the problem, as John found. My photomultipliers supplied the answer to John's receiver issues, simply as his fluoro transmitter modulator permitted advances over my feeble neon. We pooled assets over the subsequent 5 months.

Late in December 1975, John and that i received the communication system working between our houses, using a vertically mounted fluorescent lamp on the roof which "broadcast" light in all directions. The modulator consisted of several (four, I feel) kind 807 beam pentodes feeding the 40 watt fluoro of their anode circuit. A rail voltage of about 600 volts DC was applied to the lamp. We did not use a lamp starter or a ballast choke to operate our fluoros. Instead, we had a novel starting association consisting of a band of aluminium foil wrapped across the glass close to the cathode end of the tube, which was connected to the secondary of an automotive spark coil. To start the discharge, you'd apply the 600 volt rail, then energise the ring across the cathode by applying a battery briefly to the spark coil primary. The excessive-voltage spikes utilized to the glass near the cathode started a barely perceptible glow discharge inside that finish of the tube, which would instantly spread the whole length of the tube below the affect of the 600 volts DC rail. Standing present was varied by altering the worth of the cathode resistor on the sequence 807's. The filaments at either end of the tube have been by no means heated up with this association, which appeared to extend the working life of the tube.

Our first 40 watt 'fluoro' test transmission came simply after the Christmas of 1975. It was in a single course solely, with John transmitting and me receiving. 3ZGJ managed to arrange some fairly elaborate music programs interrupted by bulletins, one in every of which I recorded on tape. In the early 1970s a British group, calling themselves "Radio Love" (hey man, real groovy) had proposed a system of local mild beam broadcasting. This seems to have been an attempt to bypass the heavy hand of British officialdom, which at that time had forced a number of impartial broadcasters (eg 'Radio Caroline') to transmit from ships anchored in International waters off the English coast. Nothing was heard from the group after the publication of its initial plans, including the diagram beneath which neatly summarised their ideas:

John 3ZGJ, in imitation of this 'Radio Love', jokingly introduced his programs as being transmitted from "Radio Hush". The title was a bit less poofy, yet still retained the important environment of the authorized fringe-dweller! Later we duplicated the system to provide full duplex communication (simultaneous transmit and receive) in each instructions. A typical extract from these fluorescent light transmissions within the final week of December 1975 could be heard on this tape, recorded from the photomultiplier output at my finish of the link, two miles from the transmitter:

RadioHushDec75.mp3

We used no reflector or collimator with these fluorescent lamps. The photomultiplier receivers solely managed to attain 15 dB sig/noise over the 3.5 km range with this arrangement. Fluorescent lamps had more than their fair share of problems as a modulated supply. Their phosphor coating had time lag, the persistence limiting the upper modulated frequency to about 5 KHz. The time lag was not constant with the wavelength of the emitted mild. The pink phosphor components had very lengthy persistence, whereas the blue phosphor was a lot faster. The system's upper audio frequency limit due to this fact various with the spectral response of the detector, but with a blue-sensitive photomultiplier it was greater than adequate for audio.

The fluorescent lamp discharge would wander and 'snake' inside the tube, especially at switch-on, interfering with the modulation. For some reason which we could never explain, the output at the cathode end of the fluoro, and solely on the cathode finish, was modulated by an erratic whine at about 400 Hz, probably caused by the discharge hopping about from one a part of the coiled cathode filament to another. This always set a particular restrict to the sig/noise achievable, even where there was loads of mild sign to demodulate.

The audio frequency response of the fluorescent tube was unexpectedly uneven, an impact undoubtedly caused by acoustic resonances of the modulated mercury plasma column within its tubular enclosure. One might certainly hear a faint acoustic radiation from the tube while it was in operation, as these acoustic waves really penetrated the glass partitions. Certain modulation frequencies, obviously related to 'organ pipe' resonances throughout the fluoro tube, would cause the mercury discharge to extinguish, or to interrupt up into a series of spaced glow discharges along the tube. I later found that this problem had been noted by N C Beese, who wrote a chapter on "Light Sources for Optical Communication" in the guide 'Infrared Physics' (Pergamon Press Ltd., London, 1961, Vol. 1, pps 5 - 16). To quote from Beese (pps 13 - 14):

"Enclosed arc lamps operated on a.c. energy within the audio-frequency range, or on d.c. and modulated by a.c. currents could cause sound vibrations to be produced within the arc chamber. They are brought on by thermally induced variations in fuel stress that outcome from modifications in present density in the arc. At sure essential frequencies, resonance of appreciable intensity is constructed up by reflection from the bulb walls. The scale and shape of the bulb, kind of gasoline or vapour filling, temperature and operating conditions decide the frequency of the plasma oscillations which are similar to standing sound waves within the discharge. Ordinarily this phenomenon is not noticed as a result of lamps are operated on d.c. or low frequency a.c. with enough ballast to make sure stable operation. In long tubes the discharges assume a constricted, snakelike appearance on the essential frequencies and are brought on by sound energy reflected from the ends of the bulb. In a spherical bulb the sound waves unfold to the bulb walls and are then focussed back upon the arc to supply instability at the electrodes [...]"

Beese goes on to analyse a Xenon discharge lamp 1.5 cm in diameter and 13.Three cm lengthy with three Amp d.c. current applied with a 2 amp a.c. modulation applied. He noted:

"[...] the arc showed violent distortions at 2250 Hz but was quiescent at 2000 Hz and 2500 Hz. With 5 A d.c. and 3 A a.c. modulation at 2500 Hz, the discharge again showed pronounced disturbances, but was stable at 2300 and 2700 Hz. The instability might begin at both electrode, whereupon the discharge constricts into a skinny luminous ribbon with sinusoidal shape, and the voltage increases because of elevated arc size. [Instability at] harmonics of the elemental frequency may also be observed[...]

"[...] In a spherical bulb with electrodes on the centre [like a Xenon arc] acoustical resonance happens when the bulb diameter is equal to at least one-half [of the acoustic modulation] wavelength [...] An arc centred in a spherical bulb will actually 'blow itself out' by its personal sound waves if any of the robust resonance frequencies are utilized to the lamp for an appreciable time [...]

[...] In a low pressure discharge lamp [eg fluorescent], assuming an average gas temperature of 250 levels Centigrade, the velocity of sound in mercury vapour was 19,000 cm/sec, calculated by Laplace's formula. At 600 Hz, the wavelength equals 31.7 cm [...] Maximum disturbance or turbulence at the electrodes occurs at a half wavelength from the nodes which are at the ends and centre of the lamp."

Owing to the low intensity and prolonged supply space of the fluorescent lamp, the radiated flux might never be properly collimated. These limitations, and the frequency response irregularities clearly indicated that our light transmitter wanted a change of method.

At this level, Rodney Reynolds VK3AAR drew our attention to EEB's publication of the earlier work of Burlinson and Averay, whose modulator circuit and mercury arc source was a wholly novel approach. Their optics, however, had been fairly crude, not practically directive sufficient for our work in suburban Melbourne, where road lights and illuminated advertising indicators proliferate.

FROM FLUORESCENT LAMP TO MERCURY ARC

We would have liked the next intensity source, no fluorescent coating, and an output rich in blue gentle to match the photomultipliers. From the electrical provider Arthur J Veall in Bridge Road, Richmond, John and that i obtained some Philips excessive pressure mercury arc lamps in January 1976, of the type used for manufacturing unit lighting. Most of these had inconvenient fluorescent coatings, aside from the smaller lamps under one hundred watts output, which sadly had frosted glass envelopes. We purchased a couple of HP80 mercury lamps rated at eighty watts. Driving these was an actual drawback. The fluorescent lamps that we previously used have been low present, high voltage units, modulated simply with commonplace output valves. By comparison, the excessive stress mercury arcs ran at medium voltages (30 to a hundred volts) however with high present (0.5 to three amp standing current), and they exhibited a severe unfavorable resistance characteristic - their voltage drop decreased sharply with rising current circulation.

Other problems arose owing to the instability of the mercury arc. Its striking voltage varied broadly with ambient temperature, and its standing current diverse with time as heat precipitated the mercury to vaporise and the arc pressure increased. At its peak, the quartz arc tube needed to withstand inside pressures of around 30 atmospheres, so it was a device that you needed to treat with nice respect!

The slightest overmodulation peak would extinguish the discharge, and the entire system then had to cool before it could be re-struck at a moderately low voltage. The lamp polarity also needed to be reversed at frequent intervals with a DPDT swap to avoid premature cathode failure by ionic bombardment. The lamps had been designed for AC operation, where the effects of cathode heating have been shared 50 instances per second by each electrodes. Despite using DC bias on the tubes, for which they were not designed, we managed to get a few hundred hours out of them, which was Ok when the lamps only price $7 apiece. Today they value about ten instances that.

The modulator needed to steer the present passing to the arc. Quiescent class-A amplifier conditions demanded that a minimum of half of the supply rail ought to be dropped by the collection modulator, the remainder being dropped by the mercury arc. The damaging resistance of the arc was our stumbling block. As the present by means of the arc elevated its voltage drop decreased. This dragged the voltage applied to the collection control system up in direction of the rail voltage as current reached a most. With 200 volts-on the rail and a peak present approaching 5 Amps, no transistor generally accessible in 1975 might handle the job. They could not take the high peak currents concurrently with the high emitter-collector voltage that this load with its unfavorable resistance would current to the output transistor in class A. We had some expensive pyrotechnic displays of 2N3055 and BUX80 transistor failure owing to this secondary breakdown level being exceeded. Like many foolhardly experimenters, we rarely used quick-blow fuses in our gear. Youthful arrogance, I suppose!

The elegant solution, instructed a few years later by Rod Reynolds (VK3AAR), was to run the management transistor in parallel with the arc, with a single series resistor up to the availability rail from each. In that configuration, the transistor handed most voltage at minimum current, and vice versa, in order that a much smaller transistor may very well be used than in the sequence modulator configuration, the tradeoff being solely a decreased general power efficiency. However, at the age of 21 in 1975 - and John was 20 - neither of us considered that...

Instead, we used a class-A vacuum tube amplifier of elephantine proportions in collection with the arc. The facility provide was able to 200 Volts DC at three Amps, and the reservoir capacitors totalled 1500 µF at a 350 Volt score! For the modulator, 4 hefty 6080 or 6AS7 excessive present, low gm triode regulators have been positioned in parallel with low-value balancing resistors of their cathode returns. The anodes had a nasty habit of glowing a dull crimson when the arc was initiated, as it took a couple of minutes for the mercury discharge to run up to its ordinary a hundred Volts drop. During that time, the major a part of the availability rail was utilized to the valves, which have been sent past their dissipation restrict. Under these conditions, one might see alarming high resistance 'spots' scintillating with a brilliant and sparkling yellow gentle on the huge oxidised cathode surfaces of the 6080s. I at all times anticipated a 'bang' however someway I was all the time fortunate...

Each 6080 filament consumed 2.5 Amps at 6.Three Volts. The valve heaters alone consumed a total of sixty three watts! It was all brute drive, ignorance, bulk power and heat! Fan cooling was obligatory. We used a home fan of 1928 vintage with a bum sleeve bearing, which rattled constantly throughout QSO's. Nevertheless the system labored remarkably well, and because of Melbourne's many disposals shops, it may very well be constructed for a couple of dollars. Not so now! Electronic disposals stores of that sort at the moment are largely a factor of the previous.

This mercury arc transmitter was quite unstable in operation, vulnerable to permitting the arc to drift right into a gradual thermal runaway. To correct this tendency, one had to trace the present of the arc by manipulating the grid bias of the output tubes - hence the explanation for the metering of arc current and voltage within the modulator circuit, above. The temperature of the quartz bulb was the most important variable, as this and the arc's standing current were carefully associated. The bulb temperature managed the mercury vapour pressure within the quartz phial, and due to this fact the electrical resistance of the arc. With inadequate common present movement, the heat of the arc could be too low to maintain the mercury vapour pressure, inflicting the voltage across the arc to slowly fall, in turn resulting in an extra fall of current. If the standing present was too excessive, the arc would grow to be so scorching that it not only turned harmful, however was not possible to modulate fully. One could not leave the machine unattended for greater than about 5 minutes. Arc present would fluctuate, notably in the time simply after 'change on', as the log record below clearly signifies. Warm-up would take around 15 to half-hour before the system lastly settled into static present situations, with the heat pumped into the arc electrically equalling the heat misplaced to the encompassing setting. The average standing present diverse with the audio program content material and with the symmetry of the audio waveform applied.

A typical instance is provided by the next log that I took of the arc transmitter's electrical status, measured via the night of Saturday thirty first January 1976. Operation on that night time was greatest described as 'intermittent'! Eventually, in response to 3ZGJ's insistence that I add a damaging peak clipper to my modulator, I fitted a diode to the grids of the output tubes to do the job!:

6:05 pm

80 V @ 0.Forty two Amp, 33.6 Watts

REMARKS: Test TRANSMISSION Only earlier than nightfall. Arc has been on for forty five mins of heat-up, approx 60% peak modulation, with music from 78 rpm discs (avoiding copyright issues!). Ambient temp. 350 C. VK3ZGJ not listening but. Power and heater transformers are each scorching to contact, however Ok.

PHOTOMULTIPLIER RECEIVERS - 1975/76

The receivers used in this system have been additionally designed around parts obtainable from disposals sources. Photomultipliers are superbly sensitive, and never almost so fragile or troublesome to set up as some so-known as specialists recommend. They require a clean 1000 volt supply at about 5 or 10 mA - simply arranged with an old valve radio transformer working right into a voltage doubler. The hand-held photomultiplier receiver was a relatively hazardous device whenever you were manipulating it at evening from the top of an earthed metal ladder! Particularly so when the 1000 volt provide had eight µF of oil-stuffed block reservoir capacitor behind it (ugh!) and was fed to the receiver by flat 240 volt twin lead flex. It grew to become much more dangerous when rain was falling! In my youth I had scant appreciation of my own mortality. Few teenagers ever do!

The previous disposals photomultipliers typified by the 931A had nearly no response to pink mild, in order that they couldn't be used with LEDs or HeNe lasers. Newer photomultipliers with gallium arsenide photocathodes have good crimson sensitivity, but at costs exceeding $1000 they're out of my league.

Overall outcomes with this cumbersome link system had been remarkably good. Considering the potential theoretical issues of thermal lag and ionisation time, the mercury lamp's restrict of 50% modulation at 10 KHz was fairly usable, solely acceptable for direct amplitude modulation at audio frequencies. Piles of signal had been obtainable. The output was terrific. At night, with one's eyes darkish-adapted, you wanted to guard your eyes with dark glasses or welding goggles. But information and narrow band video modulation revealed wild part shifts above 5KHz. I believe that the mercury plasma, when thermally modulated at an audio rate, was attempting to broaden and shrink against its quartz tube housing with each modu1ation cycle, interacting with the arc cavity in a fancy collection of acoustic resonances. Indeed, one might truly hear the modulation coming from the arc, significantly at high audio frequencies, by putting your ear down near the quartz bulb. One undesirable aspect effect was that sure modulation frequencies near the arc cavity's acoustic resonance extinguished the discharge. Sibilant sounds had been particularly risky. In reply to a query about the modulator I remember saying over the link "it's working completely!" At the opposite finish, all John heard was "it is working perf - Click". The arc extinguished itself, proper on cue!

Over the 4 months as much as April 1976 the system was in operation up to six nights a week between VK3ZGJ and my QTH at Camberwell Junction. It was unaffected by all but the heaviest fogs and rain showers. On extremely popular nights, low frequency noise and quick flutter fade had been evident, but hardly ever obtrusive. The impact was resulting from sizzling air cells rising from the warm floor into the cooler night sky. This brought on a heat shimmer, with a consequent scintillation of the received light. We thought-about using FM subcarrier modulation to clip off the scintillation on the acquired mild modulation, but the mercury arc lamp hadn't a sufficiently quick rise time to support this. Subsequently I discovered that the use of longer wavelengths - red or infra-pink light - tremendously reduce scintillation. FM approach may be fascinating for hyperlinks of over five miles vary, but for shorter distances amplitude modulation's higher obtainable base bandwidth in all probability makes it the popular mode.

The previous mercury arc/ photomultiplier system might have had appreciable lengthy vary potential but we by no means had the opportunity to check it. The heavy power calls for of the terminal gear demanded a mains provide at both ends of the link. We never successfully tried DX away from our respective homes and two miles (3.5 km) was the greatest distance tested. We might typically obtain forty five dB sig/noise over that distance.

Communication ceased during the day owing to the scattered blue mild of the sky. The photomultipliers were saturated by it. Interference filters might have been used to extract a dominant spectral emission line of the mercury discharge from the final background radiation, but I was unable to obtain one until the 1980s. They're expensive and appropriate just for parallel rays of mild. There are better methods of reaching good sig/noise through the day, utilizing a monochromatic source and a wavelength away from the peak output of the sun.

However, I would power up the photomultiplier with a comparatively low voltage - about 500 volts - simply earlier than sunset, to catch John's first distinct words because the ambient gentle pale. On scorching evenings, the signal was at all times lower than expected. I couldn't account for this, till one evening I climbed the ladder to regulate the receiver and noticed an odd odor. The receiver's optical cavity was full of smoke! I had forgotten that John was almost immediately West of me, and because the summer solstice handed, the place at which the solar set on the horizon was moving North on successive evenings. That evening, it was setting straight behind John's house - so my 13 cm receiver lens was focussing the solar's image on the back of the receiver housing, which was manufactured from black-painted particle board. Awk! We had been so fortunate to not have set hearth to all the things - however I saved the incident a secret from my of us. Things that my mother and father did not know couldn't hurt them! With some embarrassment, I fitted the optics with a removable lens cowl.

One potential use of the mercury arc transmitter that LED's, Luxeons and most low power lasers probably may by no means rival lies in non-line-of-sight (NLOS) linking by reflection off clouds. Some years in the past I acquired a pair of 1 metre diameter searchlight mirrors to try the idea. The mercury arc might be mounted in one, photomultiplier in the opposite, both tracking the identical cloud. Street lights right this moment use the same sort of mercury discharge because the modulated supply, in order that 100 Hz hum background plus harmonics could be unavoidable in an city setting. I've but to try the concept, as direct linking is undoubtedly extra dependable and environment friendly, and has therefore been my fundamental line of analysis.

OPTICAL COMMUNICATION'S 'Comic CUTS' - 1976

Almost as quickly as the optical hyperlink between John VK3ZGJ and myself was established, he relayed me to 144 MHz on the cross-band discussions radiated by VK3AML on 160 metres, several times every week. For all sensible purposes, I used to be on an intercom in 3ZGJ's shack, and there was no law towards that. Legally and technically, I used to be a supervised (ie, pull the plug out if he misbehaves) customer who just occurred to be current near 3ZGJ's microphone!

Through the early a part of 1976, there have been many cross-band contacts on 160 metres with me "on lightbeam" which seemed so as to add technical novelty to the proceedings. The discussions had been absolutely open to any subject, offered that the due decorum of the medium was maintained. On some occasions, it wasn't! One night time we one way or the other drifted onto the subject of funerals, morticians, Egyptian mummification and the 'expensive departed'. This contact, with many others from that 'mild beam hyperlink' period, was logged on audio tape by Tony VK3AML and by other listeners, and extracts may be downloaded from Tony's 'bluehaze' web site ("Multimedia Page 3") as "Death By Light Beam" (mp3).

Over time, the content material various from creditable technical depth to immature ratbag ramblings. I never stored track of all the log tapes, and that i never know when bits of it should re-emerge on the weekly replays which were maintained by VK3ASE - and others on Internet - haunting me ever since...

Most of our problems with the outdated gas discharge lamp system have been extra comical than theoretical, and most were completely unforseen. Moths, as an illustration, were irresistibly interested in the ultra-violet output of the mercury arc. Bogong moths, large Emperor Gum moths, Christmas beetles, flying ants - the total box and dice! They swarmed the transmitter arc on scorching nights, attenuating the beam flux and suiciding towards the recent lamp. Every couple of hours I'd should scrape their smouldering bodies out of the lamp reflector. Their fluttering wings modulated the transmitted beam, and the sunshine they mirrored formed a suggestions path into the adjacent, continuously running, duplex optical receiver. If I had my transmitter microphone operating close to the monitor loudspeaker, the presence of an Emperor Gum moth (which, I ought to explain for non-locals, are the scale of a small dinner plate) was signalled by ear-splitting feedback warbling at their wing-beat frequency of about 15 Hz. This was usually followed by our neighbours lobbing pebbles onto the tin roof of my radio shack to get me to 'flip my bloody noise down'! As we frequently operated between midnight and three a.m., I can hardly blame them. We known as the effect "moth-back" .

Another unexpected fault took ages to detect. After a seemingly harmless rainstorm, the mercury lamps would cease to operate. The lamp itself appeared completely Ok, even on close examination. We lastly discovered that water had drained down the bulb into to lamp's Edison screw socket. There, the DC bias set up an electroplating motion, dissolving the wires connecting the bottom with the bulb. The problem was fairly simply solved by mounting the lamps upside-down.

Our line-of-sight path, skimming the tops of suburban Hawthorn's buildings, backyard foliage and energy lines made steady contact between the terminal stations the topic of continuing concern - and some unusual QRM. The situation worsened in windy weather as there was a tall gum tree very close the the optical path, in a backyard a few half mile distant from 3ZGJ. In a Northerly wind this massive nuisance had branches that might sway back and forth into the beam path, slicing words and phrases out of our conversations. Eventually the damn factor became such a drag that we made half-critical plans about sporting balaclavas on an illicit midnight raid on its owner's backyard with ropes and a pruning saw! Another suggestion involved borrowing a surgical chopping laser from Melbourne University's medical faculty, and trimming the foliage from a distance - the space of 3ZGJ's shack veranda, the truth is. Just a few years afterward, I took a look at the same path with binoculars, and found that the nuisance tree, by then with several others, had grown to completely block the optical path. L.O.S. paths will be impermanent! The rising of tall bushes in suburban streets and gardens is now rather more fashionable than it was thirty years in the past. It may be unlucky for optical communication fanatics, nevertheless it supplies an aesthetically pleasing method of supporting h.f. wire antennas. (I'm humming a chorus of Monty Python's "Always look on the bright aspect of life" whereas I'm typing this)!

The color of the mercury lamp also offered problems. It seemed the same as every other avenue mild in Melbourne. One night, we drove up to one Tree Hill within the Dandenong Ranges exterior Melbourne in an unroadworthy Austin with the portable LDR receiver, optimistically hoping to set a distance report. At the top of the outdated lookout tower there (eliminated in 1981) we seemed down on the lights of Melbourne, which have been as numerous as the grains of sand on a beach. With John's optical receiver we searched the various lights seen in the final course of Hawthorn for about half an hour, vainly attempting to listen to audio modulation among the grunts and farts of mains-fed mercury avenue lights. We never did discov