[amsat-bb] Re: HO-68 Linear Transponder - Newbie Questions

George Henry ka3hsw at att.net
Tue Dec 29 14:23:02 PST 2009

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 

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" <mailinglists at benonis.net>
To: "BB" <amsat-bb at 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
> mike at benonis.net

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