Finally, I got some test equipment for 2.4GHz up and running. It is not yet complete, but I could start testing the up converter and power amplifiers in the lab.
A few tests and measurements were done, with the following results.
For temperature stability reasons I decided to place the up-converter per se indoors. The unit can deliver between 1.5 and 2W. The outdoor unit contains a Chinese WiFi booster, a so-called 8W unit, the Edup AB-003. Tests show the saturated output power just under 4W, and with 3W it would probably have sufficient linearity for SSB on the satellite.
The test was done in the lab, with a cable length approximately the length that will be used from the indoor to the outdoor unit, 15 - 20m. I decided to go low cost and use low cost, i.e higher loss, cable, since a really low loss cable would be overly expensive and un-flexible.Loss is about 15dB.
With this setup it was impossible to get more than 2W out of the Edup (actually a bit less). Ach! too little drive for the Edup. What to do?
Next test, simulated indoor unit with an extra Edup amplifier, only a little less than 3W out. Hmmm! Now what?
One more test: (Simulated) outdoor unit with 2 Edup amplifiers in series. That helps. Just under 4W is now easily possible, the system gain is good.
Next "problem": With lower gain/output of the indoor up-converter the local oscillator and image rejection is reduced. I want as clean a signal as I can reasonably get. OK, a 2.4GHz PCB "hairpin" filter mounted between the two Edup amplifiers in the outdoor unit should do it. The filter should improve the LO rejection about 30dB, easily compensating for the reduced rejection. On top of that the output of the up-converter has to be increased due to 5-6dB loss in the filter. Still sufficient system gain, as expected. The image rejection for a 432MHz IF is improved about 45dB. All looks good.
Testing this setup results in just under 4W saturated out of the Edup amplifiers, and we are ready for an initial live test. The amplifiers and the filter are mounted in an (electrician's) distribution box A 2.4GHz patch antenna is connected to an extended cable. At this point, the first test will provide 2W saturated at the patch antenna that will be mounted on a 60cm dish. This should deliver a good CW signal over the transponder. SSB, however will sound ugly because of the threshold for the RF sense in the amplifiers.
A set of attenuators in series have been connected, and should provide sufficient attenuation of the 35W from the transceiver, not over-driving or destroying the up-converter mixer.
The system has now been set up, and is tested. It is far from ideal, many improvements and optimizations are possible.
The test with CW is a qualified success. The signals are not very strong, and the first evening there were no replies to my CQ calls, even though I could receive my own signal (too late for much activity). The afternoon/evening after (today, Saturday) 12 QSOs have been made, and most of the stations could receive my faint signals.
I was warned that the patch antenna is not ideal, so at some stage I will make a helical feed - and yes, I will have to make sure to get it wound the correct way.
A shorter cable from the outdoor PA, combined with the helical, should provide at least 5dB better signal-to-noise ratio, and a bigger dish (the present one is 60cm) will provide even more gain. It looks like I can find a 110cm dish from a local amateur.
A modification of the Edup amplifiers for constant TX mode will have to be done, so SSB transmission will be possible. About 3W to the antenna should be possible. A higher power amplifier may be convenient to have.
All in all much to do to improve the system, a part of it (indoor activities) can be done anytime, but a bit of the outdoor stuff should be done soon, before winter.
Other improvements will be increased frequency stability and precision. At the moment I have completely separate transmission and receiving systems, and the frequency offsets are different, so tracking transmit and receive frequencies is cumbersome. I will likely end up with a GPS locked system, but slowly, slowly, not too hasty.
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