[amsat-bb] AO-7 V/A Transponder (Mode A)
Rsoifer at aol.com
Rsoifer at aol.com
Thu Dec 8 11:14:03 PST 2011
Operational linear transponder satellites have dwindled to only two: VO-52
and AO-7. The latter, now 37 years old, spends approximately half its
operating time on the V/A transponder, or what we used to call Mode A:
145.85-145.95 MHz uplink, 29.4-29.5 MHz downlink.
Unfortunately, activiity on the V/A transponder is quite low compared with
what it was in the 1970s. Today, as then, much of this problem can be
attributed to the V/A transponder's relatively weak downlink signal. In the
1970s, however, many users were able to solve this problem and work DX through
this transponder out to nearly 8,000 km.
How did they do it? As one of those 1970s users, I'll tell you: low-noise
preamplifiers and good 10-meter antennas. In 2011, most modern radios have
enough sensitivity that an external 29 MHz preamplifier should not be
required. That leaves the antenna. Most satellite users wouldn't think of
operating Mode U/V without good antennas. The 29 MHz band is no different.
As any successful 1970s Mode A user can attest, a good 10-meter beam is by
far the best choice. For best results, it should be pointed at the horizon,
not elevated. Why? For most users at temperate zone latitudes, AO-7's
elevation angle will be 10 degrees or less about half the time it is in range,
and 30 degrees or more only about 10% of the time. During that 10%, the
inverse-square law of distance will keep signals strong despite AO-7 being
outside the beam's major lobe..
As in HF operation, the beam should be mounted as high above ground as
possible. My friend Ben, W2BXA (now SK), with his beam at 20 meters, was able[
to work stations up to 300 km farther away than I could with mine at only 10
meters. That is a large part of the reason why Ben held Satellite DXCC No.
1 while mine is No. 13.
If you cannot put up a beam, a rotary dipole is perhaps the next best
choice, again mounted as high as possible. If you cannot do that either, try a
ground plane antenna, mounted so as to be clear of all obstructions. No
matter what your specific circumstances, put up the best 10-meter antenna you
Let's use and enjoy BOTH transponders of AO-7!
73 Ray W2RS
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