Reception Puzzle

[M0XDL]

I have been using my Elecraft K3 (with 2m transverter) to receive Funcube 1 telemetry for the past few weeks with success, however there is one aspect of my experience which puzzles me:-
My antenna is a 5 element steerable 2m Yagi and reception involves steering as per the satPC32 azimuth data and tuning (for doppler) manually while watching the very clear Funcube signal on my Pan Adapter.
The puzzle is that while the bird is coming towards me can see it before it gets a positive elevation angle, REGARDLESS of whether it is coming NtoS or StoN. BUT when it is overhead or receding the signal quickly fades to nothing at all ranges. This is not a doppler thing, but it could be a polarisation or antenna aspect thing. Help! 73deRay M0XDL

[g3vzv]

Hi Ray,

Yes - that does sound a bit of a puzzle...especially, as at the moment, the spacecraft is "spinning" quite quickly - about 1 rpm. This means that the transmitted signal should be rotating from vertical thru horizontal polarization very 30 secs or so.

This doesn't explain why when it coming towards you it seems to work best. I am assuming your yagi is linear and not circular polarized?

It sounds a bit rude but are you 100% certain you have the right keps, time and location in Satpc32?

Please let us know when you find the answer

73

Graham
G3VZV

[pa3weg]

Something else: are your rotators keeping up? Especially during high elevation passes, your antenna system may be slewing quite a bit?

[M0XDL]

Regarding the Keps: I could immediately see how big(ish) changes there might help explain things, and I haven't updated since my initial set up about a month ago. A possible Eureka moment I thought. I have been using NASA.all and I just updated it. There were changes in AOS/LOS but only by a few minutes. Is there a recommended update interval? I'll keep a closer eye on THAT in future!I suppose the PC clock would also be a suspect but I am using DIM4 for synchronisation and I can tell that I am always within a fraction of a second of UTC.
Regarding rotators I am doing that manually and it certainly does explain things when the satellite is near to overhead, especially since I don't have any elevation control. I wasn't surprised by the resulting issues when near to overhead, the real puzzle is when the bird is moving away at a fairly steady azimuth angle.
By the way I didn't mention my 2m yagi is horizontally polarised, and interestingly (now that I know about the rate of rotational change in polarisation) I haven't really noticed any significant dips in signal strength during the "good" period when it is heading towards me.
By the way, the graphical display on my Elecraft P3 Panadapter is a great help in tracking doppler and azimuth, although the wide beamwidth of my 5 element Yagi makes azimuth tracking less of a problem.
A couple of supplementary questions if I may:-
When it is approaching AOS I see strong signals when SAT PC says it is below the horizon by up to 7degrees or so, especially looking South. Is this normal? Perhaps my QTH on the south coast gives me some advantage which is relevant.I'll keep an eye on it with regular KEPS updates and let you know if that changes!
I notice the Telemetry power changes from as little as 40mW to 350+mW, I haven't noticed any power issues on the bird so is this something pre-programmed or maybe an artefact of the measurement system? Thanks, 73deRayM0XDL

[M0XDL]

Embarrassment all round; it WAS the KEPS. I just monitored a South to North overhead pass. Funcube 1 rose just about on AOS and I tracked it departing without a problem. It didn't go all the way to LOS (it disappeared when about 7degrees above the horizon) but I know that is because of a ridge of hills behind me here in South Dorset.
Needless to say my azimuth tracking worked better too (kept a strong signal all the way); that even gave me time to notice that there are short term fluctuations in signal strength after all, presumably due to the rotation effect.
I have already ordered a 70cm crossed yagi, a masthead diplexer, and a kit for an ft817/SatPC32 CAT sequencer from the ARRL, so my next adventure will be to try to get up on the transponder, although that will probably take a few weeks to animate.
You don't have to be a teenager to have fun with Funcube1! 73deRayM0XDL

[g0mrf]

Hello Ray.
We look forward to hearing you on the transponder.
Next Friday is FUNcube's 'birthday' and the transponder will be on from Thursday evening (20th) through to Sunday evening.
I've only just started using it myself and it can be a little tricky at first. If you have the ability to send a few CW 'dits' it's easier to hear those than trying to listen for your own voice.

Good luck and Happy birthday FC-1

David