|
|
|
ATV Contesting
(the way to do 'em)
by G8MNY
ATV contests do not
have the same pace as phone or CW contests, and can therefore be more
enjoyable. Here are some tips from 12 years of contesting.
Talkback
During
contest 144.750MHz becomes a calling channel, not a talkback channel,
so please QSY ASAP. On QSYing both station should check out the new frequency
by asking "IS THIS FREQUENCY IN USE PLEASE?" before hand, to
avoid loosing the contact and causing QRM to others. FM is the most commonly
used mode for talkback but it is often difficult to find a clear channel
when on hill tops. Remember the all mode section of the band has many
allocated channels:- Beacons, Raynet, Packet, RTTY, etc. Deep QSB causes
difficulty on FM, so why not use SSB, which won't block the ATV half as
much, and give yourself a chance in maintaining contact with the Dx! Also
don't forget to listen around 144.17 SSB as this frequency is used abroad.
Where
to beam
With narrow beam aerials
aligned on the same mast, the use of headings based on QRA locators is
highly desirable, assuming you can calibrate the rotator that well, and
you have a RF quiet electronic abacus! Otherwise beam up on the 2M signal,
if you are the same polarization. On 70cm, best headings are obtained
from using a narrow band (SSB) Rx tuned into the carrier, once you can
extract the exact frequency from Tx station. For 24cm exchange, send the
70cm pictures first is an obvious rule, but be prepared to deviate the
heading slightly, as the paths can bend and reflect differently.
What
to send
The contest rules
require a 4 number code per band to be sent in vision, these numbers must
not be sequential or repeating. So sending pictures of that nice tidy
shack, is NOT REQUIRED (that's a relief), nor is that colour test card,
only the 4 number code. However one should identify the code as being
yours, so also include the callsign. The numbers should be designed to
fill most of the screen (allowing room for callsign). They need to be
thick, this is especially true for all the vertical portions of the digits,
as this is the first detail to be lost on poor paths.
Black
or White Caption?
At first sight you
may think that white numbers on a black background would be best, as noisy
blacks are less noticeable. However to see anything, first the TV set
has to lock the picture, this means SYNCs must be sent. With white numbers
on a black background, the TV has a reduced signal to noise ratio of 0.3:noise,
assuming no sync clipping in practice this could be reduced by 6dB with
slight clipping (0.15:noise)! Now for black numbers on a white background
the sync circuit has it easy, a full swing at sync time 1.0:noise. This
is true for both FM & AM TV, with AM TV there is an added bonus, as
less mean power is required, sync pulse linearity is less important so
more drive can be used, giving greater peak power, with the resultant
improved S/N ratio at the Rx. The use of white background upsets some
operators who like to see lots of mean power leaving, so to them I say
"get a PEP meter".
Power
The UK licence limits
the power to 400 watts peak at the aerial. The aim under contest is to
work as many stations as possible, thus the higher the power the better,
unless you have neighbours that is. In that case try going /P or get new
neighbours (move).
Modulation
Information is only
carried in the modulation, so full modulation is the key. To get as much
modulation of the carrier as possible is not always easy, unfortunately
there is no video equivalent of `speech processing', only the camera and
video gain controls are available. An evenly lit code caption card, or
computer video with no video droop, are the best that can be done. On
FM TV the amount of deviation used depends on the Rx station's IF bandwidth,
so the picture content becomes the only adjustable item.
Tx
Bandwidth & Frequency
For just code captions,
curtailed video bandwidth should be used. This can be as low as 0.5MHz
@-3dB and 2MHz @-20dB. A simple 75j repeated c section filter can provide
this. On 70cm, this enables other stations to work alongside you in this
restricted band, as well as keeping other 70cm users happy. With such
reduced bandwidths every thing looks blurred, but the caption is still
just as readable as with the wide band, because fine detail is drowned
by the noise on weak pictures anyway. With the narrow bandwidth, the Tx
frequency can go much higher in the band, up to say 438Mhz. This can also
enable each station to be readily identified with its unique spot operating
frequency, and not all on 435.6MHz. On 24cm, the aerials, preamp, Rx,
and PA are often frequency selective, not to mention the RADAR interference,
so a compatible frequency can vary around the country. 1255MHz is favoured,
but as for 70cm a spread in frequency can help when the band is congested.
Receivers
For 70cm a TV tuned
down to the band is best, as down down-converters can generate unwanted
broadcast images. Reduced IF bandwidth from 5MHz to 0.5MHz with a switched
`Q' multiplier stage after the tuner, will give up to 10dB improvement
on weak signals. This is much greater improvement than a low noise preamp
could produce, but you will still need one of those as well. Splitting
the preamp output into both the TV and a 70cm phone Rx, enables monitoring
of signals well below P1, this will give you better judgement of the contact
chances under QSB conditions. With a good quality weak signal, the TV
should be able to lock up before the numbers are identifiable. On 24cm
a narrow IF down to 6MHz, will improve low grade pictures, if the Tx deviation
is kept low. Adjusting the PLL on weak signals before a contest is a good
idea, as it is quite a different setting than that for P5 colour and sound.
Dx contacts can be as much to do with "Aircraft en route" than
equipment sensitivity.
Rx
QRM
At /P sites other
strong signals may cause problems, on:-
70cm 24cm
Strong PMR on 440MHz band Extremely strong Radar V not fV
Broadcast TV image at +2xIF Broadcast TV on IF
Broadcast TV due to VHF mix Broadcast TV's 2nd harmonic (Rx)
Syledis wideband location system Cellphone/VHF PMR mix
Aircraft altimeters (discontinued)
70cm repeaters users
70cm satellite users
Some of these can
be cured with inter-digital filters, others can not.
Viewing
Contest pictures are
best viewed on a small screen TV (6") from a long way away (5') in
subdued lighting. This is because one's brain can do a lot of `state of
the art' picture processing, and act as a frame store, under these conditions.
Reports
Only the check sum
(the code numbers added) is permitted to be communicated on the talkback
link, the code number itself must not be communicated on pain of disqualification!
Don't forget to log the code number seen.
A report of P0 means
no usable picture was received.
P1 is a picture just interpretable.
P2-P4 increase in viewability, until,
P5 no apparent noise.
Under contest conditions
it is often possible to extract part code numbers from fleeting glimpses,
where aircraft QSB just lift the signal long enough to get some of the
code, several QSB peaks may be required for the whole code. Usually a
P1 report is given, though P+ would really be correct.
In addition to the
QRA, check sum, report, the exchange needs a contest serial number per
band, as with other contests this is a 3 digit number starting from 001.
Scoring
A BATC logsheet should
be used, this will give all the right boxes to fill out as you go. Unlike
most other radio contests scoring is based on Km of video worked. Normally
1 point per Km per direction. Bands higher than 70cm, 2 or 4 point per
Km may be used, so check the contest details in CQTV.
Be
Brief
Please remember also
that others want to exchange pictures too, so remember to cut transmissions
once the code number has been seen or when the other station in not in
a position to Rx.
73 de John G8MNY
|
|
|
