[amsat-bb] AO-7 Mode A vs. Mode B Performance

John Papay john at papays.com
Wed Jul 27 06:54:43 PDT 2011

There is a huge difference in signal quality between
Mode A and Mode B on AO-7.  I've been working Europe on
Mode A days and have found that the audio quality is
excellent.  I've experimented with higher power levels
to see how the transponder performs and have found that
it can handle the highest power levels on the uplink
without fm'ing, distorting or creating images across
the passband.  Of course it is harder to hear with severe
fading on a horizontal beam antenna, but it is very usable.

This morning there were three French stations on in Mode A.
F6FLE was pounding away on CW and loud.  But there was no
apparent degradation of the other voice stations using the
transponder at the same time.  Strong CW on Mode B just kills
voice readability and strength on other stations.  No images
were heard, the passband was quiet except for other ham radio

I have several antennas for 10m.  The one I use for Europe
is a 5 element beam at 120'.  It can see the horizon in all
directions and can hear AO-7 when it is below the horizon.
For higher elevations I use a single element at about 15 feet.

The Mode B transponder never sounds very good.  And it seems
to be in a power limited situation even when there are no
ham radio transmissions in the passband.  What you do hear is
all kinds of noise and I suspect this noise is the culprit in
robbing all of the available power to run the transmitter.  The
noise moves and is probably being received from ground sources,
hence the apparent doppler.  User stations sound choppy even when
running minimal power.  When you run more power, signals FM and
images are heard within the passband.  The question is what is
the source of all the crud that AO-7 is hearing in Mode B?  After
all, 432 is supposed to be used for weak signal operation and should be
very quiet.  Are there issues within the satellite that contribute
to this?

If you were a user of AO-7 back in the 70's, what was Mode B like
back then?  Of course it had a better power source when the batteries
were still functional, but was the signal/audio quality much better?
Did it FM and was the audio choppy?

We are very fortunate to have AO-7 regardless of the performance issues.
It is challenging to work at low elevations but the payoff is worth it.
What we would really appreciate here in North America is more activity
from Europe on the passes that reach the across the Atlantic.  Last week
I had a qso with IK8YSS in Mode B, 7554KM.  I have seen his callsign in
the AO-7 logbook for a long time and finally we had a mutual window with
good conditions.  LA4FPA/p was on yesterday as promised from JP22.  AO-7
might not be the HEO that many wish we had, but it is the highest bird
we have right now.  Let's use it while we still have it.

John K8YSE

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