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Re: Orbit 250 apogee

The S-band downlink is very much stonger than that on AO-13. I was using a
4-foot diameter dish (26 dBic gain) and a 0.7 dB NF preamp for AO-13 mode S.
I am now using 3 16-turn helices (20 dBic gain) and the same preamp for
AO-40. When the squint angle is low, signal strengths on the AO-40 downlink
are about the same as I remember from AO-13 even though the antenna is much
smaller and AO-40 is much further away than AO-13.

This is a big improvement over AO-13 mode B. A 20 dB gain antenna at 2400
MHz has a very small capture area compared to the long yagis used with the
VHF downlink on AO-13. A 19-foot long CP yagi with 15 dBic of gain was the
minimum that I could get away with on AO-13 mode B. It had the same capture
area as a 39 dBic gain antenna (18 foot dish) at 2400 MHz.



----- Original Message -----
From: "Estes Wayne-W10191" <W10191@motorola.com>
To: "Lee" <lee-fl@cfl.rr.com>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, 16 May 2001 23:44 UTC
Subject: RE: [amsat-bb] Orbit 250 apogee

> Lee wrote:
> ...100w EIRP should provide a 23db SNR at the AO40 U-rx.
> Wayne replies:
> 100W ERP may give 23dB SNR at the AO40 receiver, but the corresponding
> downlink doesn't give most of us any where near 23 dB SNR on the S2
> downlink.  For me, the average SNR with SSB signals is in the 0-10 dB
> Most of us need more than 100W uplink ERP to get a readable SSB downlink.
> Ideally we would all get 0.5 dB NF downconverters and 24+ dBi antennas,
> precision rotors to handle such a narrow antenna beamwidth.  But most of
> don't have all that, and should we really need that with a 50W transponder
> and high gain antennas?  Maybe the current LEILA-controlled "power per
> is appropriate for some time in the distant future when there are 100+
> signals on the transponder.  But it doesn't seem reasonable now when there
> are few users and most of us are struggling to hear the satellite.
> Lee wrote:
> There may be some imbalance between the ul receiver outputs but a
> review
> of the tlm archive doesn't show the 23cm rcvr AGCs being hit hard if
> at all.
> Wayne replies:
> Telemetry watchers say that 23cm uplink signals hardly budge the AGC even
> when the downlink signal is strong.  In comparison, 70cm uplinks affect
> AGC a LOT even when the downlink signal is weak.  To me, the telemetry
> suggests that something is wrong with at least one of the AGC's.
> Lee wrote:
> Driving AO40's ul AGC
> primarily acts to keep the offending downlink signal from getting
> any
> stronger.  It does this by reducing the effective sensitivity of the
> ul
> rcvr.  As a result, the aligators aren't really that much stronger,
> but they
> prevent the lower powered stations from accessing the satelllite.
> Wayne replies:
> I know the theory, but that is NOT what I observe.  I observe a linear
> relationship between U uplink ERP and S2 downlink signal strength when
> increasing uplink ERP from 100W to 500W.  In my case, the downlink signal
> strength goes from about S-zero to S2, and readability of SSB signals goes
> from 0% to 100%.  For CW, 100W ERP does give me a perfectly readable
> as long as I use a narrow IF filter.  With 500W ERP my downlink signal is
> about one S-unit weaker than the Middle Beacon (don't know how many dB per
> S-unit on my FT-847).
> I'm curious to hear from former AO13 mode-S users.  How does the signal
> strength from AO40's LEILA-controlled 50-watt U/S2 transponder compare to
> signals from the tiny 1W U/S transponder on AO13?
> I don't want to advocate an ERP arms race, but I do want to advocate
> the U/S2 transponder "usable" with commonly available equipment.  In my
> opinion that requires a slightly higher LEILA setpoint.
> The S1 antenna with minimal squint would provide a much stronger downlink,
> but I fear that off-pointing requirements for solar illumination will
> us to use the wider-beamwidth S2 antenna most or all of the time.
> Wayne Estes W9AE
> Mundelein, IL, USA
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