[amsat-bb] Re: eggbeater rx performance (Mobile Operation)

Bruce Robertson ve9qrp at gmail.com
Mon Jun 1 10:18:42 PDT 2009

On Sat, May 30, 2009 at 12:01 PM, Bob Bruninga <bruninga at usna.edu> wrote:
>> I found a trunk mounted 2 meter half wave
>>(dual bander) to work well when the satellite
>> was behind me, but not at all overhead...
>> I currently have a 2 meter quarter wave
>> (dual bander) mounted... into the center
>> of the roof. It works in all directions...
> Any "gain" antenna on a car is not good for satellites.  Because by definition, "gain" in a mobile means "gain" on the horizon where mobiles need it, and this means LOSS everywhere else.  The better the antenna is for "mobile" the worse it is for satelliites.  (basic law of physics)..
> Eevn the "dual bander" 2 meter 1/4 wave has a decopuling loop for UHF, to keep the angle of UHF radition low on the horizon, because without it, the 1/4 2m whip is a 3/4 UHF whip and results in gain ABOVE 25 degrees which is BAD for terrestrial work (but GOOD by 2 dB or mnore for satellites).
> So, when we say a 19.5" 1/4 wave 2m whip for mobile satellite, then that is all it is.  No matching, no coils for UHF, no nothing.  Just a 19.5" vertical whip.  BUT for satellites, this 19.5" whip is also a 3/4 wave UHF whip which has almost 7 dBi of gain ABOVE 25 degrees which is where it really yields solid performance for LEO satellites when above 25 degrees.
> A 5/8 wave 2m whip has a NULL at 15 degrees and so it is useless for satellites.  They are too far away below 10 degrees to hear, and then there is the null up to say 45 degrees.  And above that the satellite is only in view for 2 minutes or less!  AND this antenna cannot be used on UHF at all.
> So that is why the 19.5" vertical in the middle of the roof is the best for mobile satllite (and not all that hot for terrestrial... but useable, since repeaters are a lot closer).
> See details about 75% down this page:
> www.aprs.org/astars.html
> Good luck.
> Bob, Wb4APR

I'll take this opportunity to expand on Bob's advice. I think it is
spot-on for data reception. I'm using poorly-constructed 1/4 wave
antennas just for fun, and I note that I cannot get good 1200 baud
downlink from them off of cubesats: it seems that there's just not
anywhere where the gain vs. path loss curves match nicely. It's my
guess that by pushing the gain up above 20 degrees, where the pass
loss is diminished, I would have telemetry decoded nicely.

On the other hand, if you want to listen for DX on, say, VO-52 or
FO-29, or download CW telemetry, you might consider a fixed antenna
with a lower azimuth peak gain. With a 1/4 wave antenna and clear view
to most horizons, I can hear some good signals down around 10-20
degrees, and since many passes occur in that range, it is a fun place
to have some gain. (And LEO satellite operations are great for
experiments confirming the 3-d plots of standard antenna designs!)

Survey question: who has been able to download 9600 baud telemetry
with a fixed antenna, and what design was used?

73, Bruce

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