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RE: 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 range.
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, plus
precision rotors to handle such a narrow antenna beamwidth.  But most of us
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 user"
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 the
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 signal
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 making
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 force
us to use the wider-beamwidth S2 antenna most or all of the time.

Wayne Estes W9AE
Mundelein, IL, USA
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