Amateur HAB launch

Yes, thankfully the drift isn’t anything more than that. But it’s FM, so even if there is a fast change in frequency FM should handle it pretty well, assuming it still fits within the bandwidth.

I have indeed tested! So far, when I point the nulls of the half wave dipole towards the HT (simulating low-SNR conditions) I can get it using the whip omnidirectional sometimes at about 1km. No doubt that with a 7 el it should go much further. Note that we have a lot of trees, snow, people, houses etc, not much LOS anywhere unfortunately.

Hmm, still not quite sure how you’re sending RTTY here? Is this RTTY ‘on top’ of FM (e.g. modulating audio tones into a transmitter, then receiving it with FM)

When we used RTTY, we were using the radio transmitter modules essentially as a voltage-controlled-oscillator, using an IO line from a microcontroller (with some level conversion) to shift the transmitter between two frequencies, producing frequency-shift-keying (which RTTY is). We would then receive this with a receiver in single-sideband mode. The frequency shift we were using was about 600 Hz.

Running RTTY (or any other data mode for that matter) on top of FM results in about 8 dB or so of link budget losses, from the extra noise introduced into the receiver system. It will probably still work, but will act to limit your maximum receivable distance by a bit over 50%.


Sorry for the late reply, but do you still have those transmitters on hand? What was the model name?

… this was a NTX2, which as you’ve found out elsewhere, is obsolete.

Yes. However, I’ve asked around, and I have a few old ones coming in from the UK.

Be aware that the original NTX2s are very drifty.

I would highly recommend making your telemetry compatible with the standard UKHAS format, as that way you can use the horusdemodlib decoder which is somewhat more tolerant of frequency drift.