Satellite radios: Don’t lock those VFO’s on LEO’s (satellites)

DISCLAIMER: This entire blog post is focused on manually tuning your satellite radio to make contact via linear transponders (SSB/CW.)  If you are using computer control of your rig for Doppler Shift correction, some of the comments  below will not apply.

Most popular satellite radios such as the Kenwood TS-2000, Icom IC-910h, and the Yaesu FT-847 offer a mode to “track” satellite frequency movements.  There are normally two types of tracking, Normal and Inverse.  Your rig or software may refer to Inverse as Reverse.

Inverse tracking moves your uplink and downlink frequencies in opposing directions.  If you move one VFO up 10 kHz, the other VFO will go down in frequeny 10 kHz.  The majority of amateur satellite transponders in orbit utilize inverse tracking.  Inverse tracking is the most common practice.

Normal tracking moves your uplink and downlink frequencies together in the same direction.  Only one satellite operational today does this – AO-7 in mode A.  (note: Mode B is inverse)

It is important you have the correct tracking mode selected for each satellite before you go any further.

Most of the manuals for true satellite radios explain how to find your own uplink or downlink signal.  I won’t go into that on this blog post.  I want to cover an important topic: DO NOT DEPEND ON YOUR SATELLITE RADIO’S LOCKED VFO’S TO KEEP UP WITH DOPPLER SHIFT.  I intentionally bolded and capitalized that entire sentence.  It is very important.  You will always need to make manual adjustments as long as you aren’t controlling your rig’s VFO’s by computer.

The radio’s internal VFO locking does not really have anything to do with Doppler shift.  It simply locks the VFO’s so that they move equally with each movement of the main VFO knob.  This will not properly tune your station to the same frequency as another station on the satellite passband.

If you try to have a QSO with someone while your VFO’s are locked, you will look like a lid because every time you move your VFO knob, the uplink/downlink frequencies are technically moving away from each other in an unnatural manner, not how Doppler shift is affecting the uplink/downlink relationship.

It’s fine to lock the VFO’s and move to another part of the passband but it is not sufficient for staying in one place and having a QSO.  Once you “land” somewhere, unlock and tune the sub or main band VFO depending on the satellite mode (V/U aka J or U/V aka B.)  This is where the  Updated One True Rule comes into play.

Remember, we tune the higher of the two frequencies.  Examples:
AO-7, AO-73, XW-2 linear transponders: tune uplink
FO-29: tune downlink
If you lock the VFO’s, you will be tuning BOTH!
(that’s okay if you are using computer control and it’s done automatically and properly)

Final thoughts:

I highly recommend reading “The One True Rule for Doppler Tuning” by Paul, KB5MU and the updated “Bringing the One True Rule of Doppler Tuning into the 21st Century” by Alan, WA4SCA.

Buy a copy of AMSAT’s “Getting Started With Amateur Satellites.”  It covers this topic well and includes the above two abstracts.

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