1997 DUPLEX ATV LINK
Barry Cleworth VK5BQ & David Minchin VK5KK
from VK5BQ's Shack showing vision switching and two camera's
to do the Impossible,
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
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.
showing the Path between VK5STW & VK5BQ via VK5KK
Link tower showing all 4 antennae used.
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
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!
find someone who isn't watching the TV!!
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
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.
Barn "Studio" complete with Dirt Floor, Mice, .....
End of the link
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!
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.
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!
system consisted of a 15 element DL6WU for 444.25 MHz and a 22
element DL6WU for 1283 MHz.
Antennae up 7 metres, beaming back to VK5KK
1998, VK5BQ & VK5KK
RETURN TO VK5KK's HOMEPAGE