Tuesday, August 25, 2020

QO-100 ground station update

My previous post show tons of opportunities for improvement. The more I was thinking about those, the closer I came to my initial gut feel, i.e. using a helix as a feed for the up-link dish.
In fact, the helix was sitting on my desk for quite some time by now. I had doubts how to mount the helix in the first place, also matching appeared to be a big deal, without the help of any decent GHz-capable measurement equipment.

However, seen that no matter what I did, I could not really improve the performance of the up-link. Essentially, I spent two entire night to find the best position of the dish. (Night-time, so nobody is on the satellite, avoiding to disturb more senior users)

When listening to traffic on Es'hail 2, I often heard that operators switched from patch feeds to helical feeds for their up-links and reported better results with those. Was my gut feeling correct?!

So, today, I returned to trusting my gut feeling. So I removed the patch from the weird reflector / holder shown in the last post. I used a small piece of PCB to form a base, essentially a mount, for the helix. Yes, I could pretend that this was a carefully calculated matching strip for matching my 75Ohm SatTV cable to the helix, but it wasn't. I dimensioned and adjusted the PCB just such that the helix would be - more or less - in the center of the reflector.
note the remainders of glue where the patch was

Feel free to count the turns, which would result in a little of 6. My idea was, that I could always cut some off. It is much harder to cut wire on ;-)

The photo shows, it is night-time again. So, why not firing up the station and do some test transmissions?
And how right was my gut?!

lower beacon at 69dBuV

What a difference! My signal is now just 9dB weaker than the lower beacon!
Of course I had to do some side-band test. And yes, now I think I am QSO worthy.

The present setup still employs the "attenuator" made of about 10m of 70Ohm SatTV cable. So, as long as I still have not yet figured out how to build an outdoor unit (PSU,TX-converter and PA in a weather-proof box), I still will be able to do one or the other QSO.