JOTA 1997 DUPLEX ATV LINK

By Barry Cleworth VK5BQ & David Minchin VK5KK

Scene from VK5BQ's Shack showing vision switching and two camera's

Asked to do the Impossible,

making it work!

There is no doubt that among the many modes of transmission available to us, television has the highest potential to capture and maintain the interest of JOTA (Jamboree on the Air) participants. A full Video Duplex system, enabling scouts and guides at both ends of the system to both see and speak to each other simutaneously, improves the interest factor many times over. The facial expressions seen in the accompanying photos perhaps emphasizes the interest factor!!!.

However, to ensure that a Duplex ATV link will operate satisfactorily, providing good quality pictures and sound in both directions without interaction between transmitters and receivers, requires considerable forward planning and expertise. The first decision made was to employ FM TV as the prime carrier, to ensure the best possible picture quality over the link.

The execution of such a link between Scout/Guide Halls may well result in poor or no results due to a variety of factors, invariably the Halls' location and it's lack of a suitable UHF or Microwaves path to the other participating Group. Distance between stations is obviously the main limiting factor although if one or both end's of the circuit has some height above sea level and have a minimal number of obstructions over the path then hopefully the calculated radio horizon will match the total distance.

For the past three years the authors have tried various ATV link-ups for JOTA involving near line of sight paths and various frequencies using both AM and FM between their stations with varying success. It has become clear from these experiences that the following requirements are essential. Firstly it is wise to calculate and design each link, on paper, to ensure noise free pictures at all times regardless of propagation enhancement or degradation. Where possible, it is advisable to have atleast a 10 db safety margin, over average conditions, to guard against fading. Ultimately this will translate to proper site selection, link frequency compatibility, sufficient power output and functional high gain antennaes. But what happens when both Groups are 90Km's apart and less than 30 metres above Sea level??????

In this particular case, VK5STW at Reeves Plains (Near Two Wells), 50 Km's North of Adelaide (30 Metres above sea level) is situated some 90 Km's from VK5BQ (20 Metres ASL), at Stansbury, Yorke Peninsula. The calculated combined Radio Horizon of both stations leaves a 40 km short fall, let alone obstructions between VK5BQ and VK5STW! Faced with the logistical problem of shifting 150 Scouts and Guides at VK5STW to a more suitable position, it was decided to introduce a third location as a translator point. As luck may have it, VK5KK's QTH (185 metres ASL) was line of site to VK5STW at 26 Km's and the VK5BQ to VK5KK path of 86.5 Km's is only 5km's past Radio Line of site.

Map showing the Path between VK5STW & VK5BQ via VK5KK

VK5KK's Link tower showing all 4 antennae used.

How it was done

After doing all the calculations we settled on 1250 & 2439 MHz as the main duplex frequencies for the longer path (86.5Km). We had a lot of data on the Path on 1250MHz (typically +15db signals over threshold) so 1250 was the easiest part. On 2439 calculations showed that with VK5BQ using a 52 element Loop Yagi, VK5KK's 5 Watts would need a 1.2 Metre dish to get some margin (+5db). This proved to be accurate within a couple of db. After a little experimentation, sufficient antenna isolation was obtained to get P5 Duplex pictures. But that is only half the story! We still had to set up a shorter Duplex link to VK5STW from VK5KK.

In principle, the shorter link should be easy. But we were running out of channels, that didn't have harmonic relationships to those already in use. Also, as we have three local ATV repeaters, other channels like 426/444/1286 AM and 2415 FM are all in use! The final selection went as follows.

VK5STW was to transmit on 1283 MHz FM (1 Watt). This meant that both link receivers at VK5KK were on 1200 MHz, ie no interference problems from the 2439 MHz transmitter. The Link back from VK5KK to VK5STW proved to be the final logistic problem. We had run out of FM receivers so we could not use 3 or 5 GHz, as planned. An old 444.25MHz VSB AM Tx was placed into service. The commercial specs of the transmitter gave excellent results over the 26km path, only 1 Watt was required over the path. Next year we will use 5815 MHz FM, as we have since developed some 2 Watt transmitters from ex commercial units.

We had no dress rehearsal for setup. Actual setup took about 3 hours, mostly spend debugging problems at VK5KK, nothing that a few filters didn't solve! Next time we will have a shakedown the day before (didn't we say that last year!) The end result was P5 signals, both ways with better than 35 db signal to noise pictures!

As can be seen a lot of equipment was used. In fact more than double that used for a straight duplex link, let alone a simple one way connection! What is not shown is the audio mixing and video buffering at the VK5KK translator site, to maintain correct levels. Three monitors were used at VK5KK for each link plus video playback. As VK5KK acted as central control, audio could be directed in either direction to control the scen should things get out of hand!

Careful attention must be paid to the placement of antennas used for duplex operation to avoid interaction between receivers and transmitters. Shielding of the various components making up the complete receivers and transmitters is also very important. The use of the FM mode for ATV provides many advantages over AM and has been well documented in other articles, but perhaps the main advantage is that FM enables the use of class C Amplifier stages thus providing higher output powers from the various solid state modules in use. The trade off of course is the wider bandwidth of FM vs AM. Eg FM uses a 18Mhz bandwidth vs 6.5 MHz for VSB AM, making the use of our 1200 MHz and above frequencies mandatory.

The 1250 MHz, 1283 MHz and 2439 MHz receivers all use converters with an Intermediate frequency of the 480 MHz. A seperate 480 MHz FM IF and demodulator board produces video and audio output signals which were fed directly to the AV input terminals of a standard TV receiver or to the Mixing/Buffering stages in the case of the Translators. As is the case with FM reception, once a certain threshold is reached the signal advances to a noisefree picture within 4 - 6db of signal increase. Take our word for it, once you have experiened the advantages of FM ATV you will be hooked!

Try and find someone who isn't watching the TV!!

VK5BQ's end of the Link

Since the shack at VK5BQ is very small, it was decided to convert the lounge room to an ATV studio as for the previous two years. Interconnecting runs of coax for video and shielded lines for audio and liaison channels on 2 metres and 70 cm were already in place. The size of the lounge room after re-arrangement was quite adequate for the fifteen girls and their leaders visiting 5BQ's house. Two cameras and a VCR were employed via a vision switcher. Audio from two microphones and the VCR were fed through an audio mixer to the transmitter room (shack).

The vision switcher and fade to black circuits were derived from the BATC Compendium. Results were satisfactory despite a certain amount of video waveform distortion evident on the wave form monitor. Various homebrew VDAs (vision distribution amplifiers) were also used in the transmitter room to feed video to various destinations simultaneously. The two VDAs worked extremely well, one being a simple design derived from the BATC compendium using the Thomson TEA5114 video switch. Three outputs are provided with one input with the level at 1V P/P. The second unit was a bit more complex using a MAX404, featuring adjustable compensation, to cater for long video runs. These VDAs are now permanently installed in the shack. Multiple outputs make it easy to relay signals received to a transmitter on another frequency (e.g. 426Mhz AM).

Video output from the FM receiver is channeled through a VDA to the main viewing monitor (63cm) in the studio where the girls were assembled. Two other monitors were provided in the studio, one being for video foldback from the outgoing signal and the other to provide a large viewfinder for the cameraman to more easily frame his shots. These were both 34cm monitors. Audio from an electret microphone in the studio was fed to an audio mixer in conjunction with audio from the VCR and Microphones used in connection with liaison facilities via Two Metres and Seventy Cm's.

All equipment used for reception and transmission was home brew, constructed from kits of parts supplied by the South Australian VHF group and due credit must be paid to the designers and producers of these kits, namely Mark VK5EME, David VK5KK and Dallas VK5WA. Not withstanding the excellent design of these kits it is emphasized that careful construction and attention to proven microwave techniques is essential for successful results

The number of JOTA participants at the VK5BQ end of the link was small, however it has been found from experience that smaller groups are preferable to larger groups to maintain decorum and focus on the activities at hand. At the VK5BQ site, 15 Guides and Brownies proved an ideal number.

On the subject of HF contacts, it is pointed out previously that a mixture of various frequencies modes running simultaneously may cause excessive interference. Hash from TV monitors creates a huge interference problem in a sensitive HF transceiver as well as some digital modes. The TV link had to be shut down at VK5BQ to enable any HF contacts to be made! Not withstanding the considerable planning and effort involved in our JOTA exercise, the expressions of concentration and enjoyment on the faces of the guides and brownies as seen in the above photo's, provided all the satisfaction and incentive for us to repeat the exercise next year or even earlier as a special event.

VK5STW Barn "Studio" complete with Dirt Floor, Mice, .....

VK5STW's End of the link

Imagine trying to get sensitive ATV gear to work in a Barn with lots of Hay, Dust and 150 Kids. Welcome to VK5STW, located at Reeves Plains about 50 Km's North of Adelaide! Actually this end of things went well technically, the only downfall was the lack of promised lights (for night time operation) and the seeming lack of crowd control by Scout & Guide Leaders. Operating conditions can be seen to be the total opposite to the VK5BQ end!

Keith VK5AKM was the tireless operator, controlling the crowd, with skills honed after many years of dealing with High School level students. The amount of equipment can best described as basic. Video equipment consisted of a single Panasonic G100 Camera and a SONY 51 cm Professional Monitor. A single Audio mixer was used with a standard semi professional microphone.

What has since become apparent is that it would now be easy for other groups to join in the duplex system, just by having a 444.25 AM transmitter and a receiver on 1283 MHz aimed towards VK5KK. We also have access to the VK5RLZ 1250/2415 MHz repeater, giving further options for simplex add-ons. It is all a matter of equipment and operators!

The antenna system consisted of a 15 element DL6WU for 444.25 MHz and a 22 element DL6WU for 1283 MHz.

VK5STW Antennae up 7 metres, beaming back to VK5KK

Copyright 1998, VK5BQ & VK5KK

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