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Re: AO40 downlink signal strength



The AIDC - 3733 measures better than your numbers.  WC0Y measured a noise
figure of 1.2 dB and a 36.7 dB gain out of the box (no mods).  This unit has
an integrated antenna (No feed line).  I'm sure the manufacturer claims
something better.  The significance of the 150 MHz feedline is greatly
reduced by all of that gain in the downconverter (in fact most of us are
adding attenuation between the radio and the down converter).

I have not had any real trouble hearing anything but myself.  I don't think
I will have any trouble hearing myself when there is less than 10 dB of
attenuation cranked in by the AGC.  Just as the command team is learning as
they go, so will the operators.  It is quite possible to drive the AGC to
the rail on the FO or RS birds as well, but it rarely happens.  The
operators on those birds have learned if you want more people to talk to
back off the throttle and they will come up out of the noise.

73,
Joe
ka0yos@amsat.org


----- Original Message -----
From: "Scott Townley" <nx7u@mindspring.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, May 07, 2001 1:40 PM
Subject: Re: [amsat-bb] AO40 downlink signal strength


> I have no practical experience with this (yet!) but I can't help but
> wonder...
>
> The modified commercial S-band downconverters seem to generally be
> around 2dB NF and 20dB gain.
>
> Assuming the following (probably close to typical) station parameters:
> no cable loss between downconverter and antenna (probably the area of
> maximum variability.  Definitely a simplifying assumption)
> 100' RG-8 foam (2.3dB loss at 150MHz)
> 15dB RX NF (which seems generous to me)
>
> The system NF of a modified commercial system will be
> NF=F1+(F2-1)/G1+(F3-1)/G1G2
>
> F1 2dB  G1 20dB
> F2 2.3dB G2 -2.3dB
> F3 15dB
>
> NF=3.25dB
> Noise Floor=-139.5dBm (approx.)
>
> Now use a customized unit like the UEK3000 (no commercial endorsement
> intended)
>
> F1 0.7dB G1 30dB
>
> NF=0.9dB
> Noise Floor=-147dBm (approx.)
>
> So a 7.5dB improvement in receive ability.  This is a LOT!  And if 15dB
> RX NF is indeed generous, the difference in Noise Floor between the
> modified commercial system and the customized system will be even larger
> (with these specific numbers, the relative contribution to the Noise
> Floor of the RX NF is 1.25dB for the modified commericial system, and
> 0.2dB for the customized system).
>
> Obviously I haven't considered external/galactic/sky noise.
>
> Is this an accurate synopsis of the situation?  Is it possible that
> "easy to get on mode-S downlink" is that suboptimum?
>
> I'm not trying to ruffle feathers here, but these thoughts are what so
> far have paralyzed my efforts to get mode S going.
>
> Estes Wayne-W10191 wrote:
> >
> > When transmitting with 20W to a 15+15 CP yagi on 435 MHz, I got an
easy-to-copy downlink signal.  On the Sunday pass my downlink was stronger
than most of the other stations I heard.  I'm amazed how WEAK most other
downlink signals were.  Most of the stations had much weaker downlink
signals than mine, yet they were making contacts.  They must have much
better receiving setups than mine, because I had a very difficult time
copying most stations.
> >
> > My receiving setup is a 60cm (2 foot) dish with an AIDC 3733
downconverter.  I didn't have to "see through trees" for the Saturday and
Sunday passes.  The downlink is not near as strong as the old AO-13 V-band
downlink, even though my S-band antenna has much more gain than my V-band
antenna.
> >
> > So I'm really curious how much more stronger downlink signals will be
with the "final" transponder configuration.  Right now the downlink signal
is marginal for the commonly available receiving equipment.  It's very hard
NOT to be an alligator with the current configuration.
> >
> > Other comments:
> >
> > 1. The "transponder frequency" was 36 kHz lower than the expected
2401.225+435.800 MHz.  I had to turn my sub tune knob for a LONG time before
I finally found my downlink.  Then I noted how far I tuned and put a new
value into ITUNE.CFG.  Now my downlink is right on frequency every time I
start up InstantTune.
> >
> > 2. This weekend was my first time to copy the beacon with my AIDC 3733.
With a 60C PTC thermistor on the crystal, I found the L.O. frequency to be
about 2277.98 MHz (20 kHz low).  After putting this rxconverter frequency
into ITUNE.CFG, the beacon is audible every time I start up InstantTune.
Hopefully I can tweak the L.O. at work to be exactly 2278 MHz.  That would
make it much easier to determine the "actual" frequency.
> >
> > 3. Signals seemed a lot stronger on the Saturday pass than on the Sunday
pass.  This is probably because the satellite set on the Sunday pass before
getting to the strongest part of the orbit.  On Saturday I made SSB
contacts, but on Sunday I could only make CW contacts.  Sunday night I
worked two LZ stations on CW. (don't bother QSLing to LZ1JH, he won't
respond).
> >
> > 4. I sure would like to know the ALON/ALAT prediction for the next
several days.  Are the high-gain antennas used now?
> >
> > 5. I tried spotting my first AO40 contact to the local PacketCluster.
I entered the actual RX frequency of 2401.385 MHz and PacketCluster returned
an amusing error message: "invalid frequency".
> >
> > Wayne Estes W9AE
> > Mundelein, IL, USA
> > ----
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