Omitting the detailed discussion of the Doppler effect, the maximum doppler shift on UHF for our satellites is approximately +/- 10 kHz for a total of 20 kHz. The maximum doppler shift on VHF for our satellites is +/- 3.3 kHz for a total
of 6.6 kHz.
When using manual tuning, the recommended procedure is to try to keep the lower frequency (usually VHF) fixed, and tune the higher frequency (usually UHF). For an non-inverting transponder, the maximum shift is the sum of the UHF and VHF
shifts, or 20+6.6=26.6 kHz. For inverting transponders, the maximum shift is the *difference* in the UHF and VHF shifts, or 20 minus 6.6 for a total of 13.4 kHz.
So the inverting transponders are “easier” because you don’t have to tune as far across the entire pass, 26.6 kHz vs. 13.4 kHz. “Easier” isn’t “easy”. “Easy” is computer controlled. (Grin)
73, Steve N9IP
From: Calvin McDonald <7ndel3fr@gmail.com>
Reply-To: "ck1@ckmcdonald.com" <ck1@ckmcdonald.com>
Date: Monday, August 29, 2022 at 1:54 PM
To: AMSAT-BB <amsat-bb@amsat.org>
Cc: "ck1@ckmcdonald.com" <ck1@ckmcdonald.com>
Subject: [AMSAT-BB] Re: Linear Tuning and Doppler
Tuning:? Yes, I was referring to the RIT-type tuning required for the transponders.? My question is:? Why are the transponders not designed to hold the frequencies more exact??
I'm guessing it's possible so there must be a design trade-off?? I've recently got into FT4 on the birds and the shift of the frequencies is made very observable by WSJT-X.? The spread is different bird to bird, pass to pass for the same birds and even changes
noticeably during a pass with some birds.
Inverting:? I have read several times that transponders typically invert because it makes dealing with Doppler shift easier.? I've laid in bed more than a few hours trying to understand why that would be true - can't figure it out.? That might be because my
entire experience with satellites I have used computer controller doppler compensation.
On 8/29/2022 11:25 AM, Burns Fisher wrote:
Thank you Jean Marc.? I have had no experience with manual turning, but I guess that is the answer to Calvin's question.? Maybe you can say more about why it is easier; I'm not quite seeing it.
Burns,
Just practical/experience sharing.
Actually it does matter using a computer aided tuning if both in a QSO using same e.g. using SatPC32 works very well and may be only slight adjustment of RIT or XIT to keep you in sync through the whole pass of the bird.?
Manual tuning also possible and very often used but one need much more skills and mental calculation to keep on a common QRG.
Inverted make it easier for manual operations.
Jean Marc (3B8DU)
Ok, let me try this:
I'm not 100% sure what you mean by "tune to the linear birds", but I can guess.? Ask again if this answer does not help.? A linear satellite has a 20 to 40kHz range in the uplink band where anything it receives is sent to the same (but
usually inverted) location?in the downlink band.? If you want to hear yourself or anyone responding to you, you must tune your receive frequency?to match your transmit frequency.? ?It would be nice if such satellites had an exact local oscillator so that you
could get the tx and rx frequencies all set ahead of time, but these oscillators do drift.? That means you generally have to "tweak" the tuning a bit to get the receive tuned so that you hear yourself clearly.? ? Is that what you are asking?
As to inverting/doppler, yes it does say that in the amateur exam (Extra, I think).? I have to say, I thought I understood this, but everytime I started to write about it, I realized "no, that is not right".? So for now, I think I'll just say that it does not
really matter if you have computer-aided tuning.
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