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Re: FO29/RS13 compared to UO14 - Why?



Jim,

As other users have pointed out, FM birds put their full downlink power on
one frequency instead of across a full passband.  Additionally, SSB readings
on an S-meter are generally lower because it is an amplitude modulated
signal.  The highest meter reading and therefore, the largest signal occurs
on a voice peak.  Your S-meter on your radio is probably not fast enough to
catch the voice peak so you get a more "average" reading than you do with
the constant carrier of an FM signal.

Secondly, there is also the fact that with a linear transponder bird, the
whole goal is to conserve the birds power.  The downlink signal on a linear
bird is directly proportional to the uplink signal.  In other words, if I
run 150 watts into my 22 elements on the uplink and get the strongest signal
at the bird, I will get the largest percentage of power delivered to the
downlink.  Say the bird has a total output power of 10 Watts.  If there are
10 users on the bird and the bird hears each of us at the same level, we
each will get about 1 watt on our respective downlinks.  Say however, I have
10 times the power level of everyone else at the bird.  I will then take 10
times more power on my downlink than everyone else.  Therefore, I will get
well over a watt on the downlink and everyone else will get significantly
less than a watt.

So good satellite users keep their transmitter power as LOW AS POSSIBLE so
as to maximize the total transponder power available to other users.  P3D
will limit you and even give you voice feedback when you start pushing the
AGC of the transponder.  FO-29 doesn't have this.

There is no need to be heard 59 through the satellite.  Most people run
enough power so they can comfortably hear themselves and the other person.
Satellites are a WEAK signal mode.  This is why it is SO essential to have
really good ears when working the birds.  It's more important to HEAR well.

Now, I question how well your preamps are working at the mast.  Just last
night, I was listening to FO-29 (I can't get into the bird because my 2m
beam is hosed right now).  With my new preamps on (>20 dB gain on 432), I
was getting a 59 + 10dB signal from FO-29.  Without the preamp (just using
the internal preamps on the 847), I was getting about 53.  I receive FO-29
as well as I do UO-14.

I think your issue with receiving FO-29 better could be that something is
amiss with your preamps or that the people you hear are just keeping their
transmitter power low so that they don't overload the bird.

73,

Jon
NA9D

----- Original Message -----
From: "jim" <jmadden@seanet.com>
To: <amsat-bb-digest@AMSAT.Org>
Sent: Wednesday, September 20, 2000 9:03 AM
Subject: [amsat-bb] FO29/RS13 compared to UO14 - Why?


> I've been playing with satellites for a few months now with my Yaesu 847
and
> autotracked M2 antennas.
>
> My question is why do I get such fantastic reception of UO14, but FO29
> signals are down in the static most of the time.  Does UO14 have
> considerably more power? both satellites are  leo. Both satellites use 70
cm
> for the downlink. They do use different modulation methods but would that
be
> a problem?
>
> I'm using preamps mounted on the antennas with low loss coax. What do I
need
> to do to improve reception of FO29?
>
> thanks. Jim
> kd7gnd
>
> ----
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