With your 2-radio setup, you should definitely leave your uplink frequency
fixed, and tune only the downlink. Unless you have full computer control
for doppler, this is the One True Rule anyway: tune only the higher of
the two frequencies, where the doppler shift is greatest (about 9 kHz at
435). (If you ever get on VO-52, note that it has a VHF downlink and a UHF
uplink, so you would tune the uplink there instead.)
Remember that the transponder is INVERTING: you must uplink on LSB, and
listen to the downlink on USB. Also, the higher your downlink frequency is
in the passband, the LOWER your uplink frequency will be. CW is generally
found in the lower half of the passband (435.715 - 435.740) and voice in the
upper half (435.740 - 435.765). Therefore, you should pick a downlink
frequency above .740, and set your uplink radio to the corresponding
frequency (the same amount DOWN from 145.950, the center of the uplink
passband, as your downlink frequency is ABOVE 435.740, ie, an uplink of
145.940 for a downlink of 435.750) Pick a clear frequency, but don't go too
high up, as most voice activity tends to stay pretty close to the center.
For convenience, you might want to make up a little nomograph or chart that
has the downlink frequencies on one line/column, and their corresponding
uplink frequencies on another.
As far as power goes, the general rule has always been that your downlink
should be no stronger than the beacon. That way, you're not tripping the
AGC knocking down someone else's signal. The FCC rule is, the minimum
power necessary. Others have reported that 5 watts to an Arrow is
sufficient.
Transmit either a slow voice count or a string of CW dits or V's while
tuning your receiver around the predicted downlink frequency until you hear
yourself. Early in a pass, while the bird is coming toward you, it'll be
higher, and drift downward as the pass progresses. Call CQ, and listen for
someone coming back. They probably won't be right on your downlink
frequency, as doppler shift will be different at their location than at
yours, so you'll have to tune your downlink to them. Hopefully, they will
tune theirs to match yours. You'll get the hang of it pretty quickly.
Good luck, hope to hear you on the birds!
George, KA3HSW
- Original Message -
From: Mike Benonis mailingli...@benonis.net
To: BB amsat-bb@amsat.org
Sent: Tuesday, December 29, 2009 1:41 PM
Subject: [amsat-bb] HO-68 Linear Transponder - Newbie Questions
I'm a relative satellite newbie, and hence I don't have a base station set
up. I do have a 2m SSB rig and an HT capable of SSB reception up at 435
MHz, as well as an Arrow antenna. With that in mind, does anyone have any
tips for making contacts via the linear transponder on HO-68? Other than
keeping my antenna pointed in the direction of the bird/rotated to avoid
fades, how much do I need to compensate for doppler on the transmit and
receive rigs?If I only tweak the RX rig, is that acceptable? Also,
how much power should I use? Thanks for any advice this list might have!
73's de Mike KI4RIX
Best regards,
Mike Benonis
m...@benonis.net
KI4RIX
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