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Re: Making contact with AO27



On Wed, 24 Jun 1998 JDWalkerJr@aol.com wrote:

> << I have been trying in vain to talk to AO27. >>
> 
> I have had a similar experience.  It is difficult to work AO27 with a hand
> held and whip, but it can be done.  I'm not sure why it works when it does but
> I'm thinking it has something to do with the orbit and Doppler.  Sounds
> strange, but I'd like to know if anyone agrees.  

It likely has to do with congestion on the uplink first and sufficient
signal to the satellite second.  During the *early* days of AO-27's
operations, there was a station way up in Canada (off Hudson's Bay, I
think) that consistently accessed the bird with an HT.  Lack of
competition for the uplink was a MAJOR factor in his success.  (And no, I
sadly don't recall the gent's call.)

> competition.  Weekdays are the only possibility.  Forget weekends.

Field Day is also going to be a mess.  *If* one big-gun station could
"hold" the frequency and work everyone else as in a DX pileup, more QSOs
would happen.  But with multiple stations trying to be "in charge", no one
gets through.

> As for Doppler, I'm thinking that the uplink works better at certain points in
> the pass.  I know the satellite's band pass is fairly broad but I'm guessing
> the Doppler effect puts the signal into a better part of the pass band along
> the way.  I use 145.85 all the way through the pass.  I understand there is no
> reason to adjust for Doppler on the uplink.  

The uplink has AFC.  There is no need or reason to tune one's uplink.  But
the uplink antenna is the main Microsat-style whip; so fading from
interactions between the satellite's attitude, antenna patterns, and your
location relative to all the above will affect the uplink as well as the
downlink.

> I have a sense that the satellite locks on to my signal at a certain point for
> a period of time even though someone else may appear stronger.  I would be
> interested in knowing if that is possible.

My understanding of this is not what it could be, but I think that
the FM-capture simply requires that one signal be sufficiently stronger
than any others to "win".

> Another problem with the hand held is that you almost lose the satellite
> between tuning increments.  I get a good signal on 436.805 and 436.800 and
> 436.795, but it almost disappears between those points.  That costs valuable
> time during a relatively short pass.

I expect that is a function of the particular receiver; some
5kHz-increment FM rigs seem to be more forgiving than others.  My Standard
dual-bander, for example.  But "continuous tuning" (supported by most
multimode rigs) will always be better.

Multiple stations, HT's, whips, etc. are all compromises from an ideal
situation, which in turn compromise performance.  YMMV.

It's been a long while since I seriously attempted AO-27 QSOs, so others
may have more useful recent and practical experience to share.

73, Steve  KA1LM@amsat.org




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