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is 145.800 an IARU "illegal" frequency?

sw9ofo – June 24, 2009 – 09:50

When voice downlink is set for 145.800, theoretically falls within the IARU border for amateur space communications. But since the IARU bandplan has right underneath the amateur space band on 145,800-146,000 the Region 1 Repeater 12.5 KHz slots (R0 - R7B) with the latter falling on 145,787.5, when ISS sets and doppler draws the transmission frequency maximum 12 KHz down, (due to enormous relative to observers speeds ISS excibits, a result of a lower than average spacecrafts orbit)

mutual interference with the repeaters is assured.

Let me ask for a downlink frequency change, then. Maybe the packet frequency at 145.825 is best suited for voice downlinks, too.

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illegal downlink and repeater interference

FelixP – June 25, 2009 – 06:13

Dear sw9ofo,
thank you for your kind thoughts on the possible interference of the 2m ISS downlink with the R0-R7B repeaters.
I do not think that such an interference is likely to occur since the doppler effect on the 2m downlink frequency used by ISS is ±3.5kHz and not ±12kHz as you have stated in your kind posting.

This also implies that the lowest frequency to be heard would be about 145,7965 while the maximum would be 145,8035.

On the 70cm Band Frequency used for the ISS communication a doppler shift of about ±10kHz can be observed.

Best Greetings,
Felix

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Not illegal downlink (no QRM) but victimized

sw9ofo – June 25, 2009 – 15:05

relative doppler shift is 3:1 on Uhf-VHF bands respectably, I know.

But those are not unmodulated carriers. They do carry modulation and the two modulated carriers tendency to overlap is greater than one can simply "estimate". You 'll have to witness it, maybe measure it, or even (why not) emulate it on lab bench setup, to understand what am I talking about.

It isn't that ISS interferes with the repeaters, signal strenght difference is enormous. But it is the opposite. The repeater has a narrow FM setup but it still renders ISS unworkable with conventional setups, like a Dual-Band handheld or even an all mode-all band transceiver without extremely narrow IF filters. Usual receiver passband for most commercial transceivers is 6 KHz, narrower IF filters are extremely rare. Happens all the time in most transceivers, who maintain ability for normal FM reception, swichable to narrow or wide only on their TX part, like two of my yaesu dualbanders.

This is why narrow channel spacing in FM mode is set up to 12,5 KHz. Not less than that. Even AM channel spacing for Air band has a spacing of 8,33KHz.

Thus, if two signals come closer than 12,5 KHz, FM discrimination process including limiting WILL render the weaker signal unreceivable.

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