22 June 2019

A second dish for 3 cm

I got a surplus 40 cm parabolic reflector for my 10 GHz experiments. It doubles the caravan set where I will mount the LNB and use in the receive chain. Since the caravan set can be mounted with a suction cup, it will probably stay on the roof of the car, while the TX dish has to be adapted to sit on a photographic tripod.

The picture depicts the way I found to attach it to the tripod: a removable vice. The tripod is lightweight, but on a field without vegetation I don't need to extend it fully, so the center of mass will be as low as possible.

Then I needed to see how it performs, depending on the position of the TX signal vs focal point. Here things got interesting. We have installed locally up in the mountains a WebSDR equipped with just an LNB pointing towards the Pianura Padana. It covers 10368.0-10369.0 MHz and receives signals beyond 300 km. It is at about 40 km from my home, slightly behind a building, into the Fresnel zone I dare to say.

The "red cover" TX was put into service. Its TX patch antennas are quite off the focal point, closer to the dish and on a side. The resulting beam, as seen on the remote RX waterfall is quite wide, with strong sidelobes. I could not get a null.

Second TX head.
Then a second TX head "white cover" was tried. It is shorter and closer to the focal plane, slightly lower than the focal point. The beam is sharp now! Because of the RTL-SDR AGC I cannot estimate the gain, but any movement impacts the signal strength, move few degrees and it is gone from the waterfall!

I am worried that dealing with two dishes will make beaming twice as time consuming, but at least the HB100 TX has a form of passive thermal stabilisation that keeps it relatively still in frequency once it reaches equilibrium.

Unfortunately the dish weight makes it difficult to make smooth movements on the cheap tripod, but that is what I have for now.

Last but not least, going from about 25°C (home) to full sunshine (35°C, not measured) resulted in 2 MHz frequency change downwards of the whole system. Good to know when arranging for real QSOs!

20 June 2019

Thermal isolation for HB100 transmitter

The actual frequency of an "HB100 10 GHz radar doppler HAM RTX module" is very sensitive to external factors: supply voltage, proximity, temperature.

Voltage is easy to stabilize. Proximity too, and if the IF port it becomes less sensitive.

On the other hand, temperature has a devastating effect, both on short and long term. On the long term, the frequency will drift until the HB100 reaches a constant temperature. But even a quick breeze will produce a snake on the waterfall. So, in order to minimize the effect, I found my way to add short-term thermal inertia to my 10 GHz transmitter:

Transmitting end on 10 GHz, modulated on the IF port.

Variations on long term ("minutes") will still occur, but at a slower pace so that it's easy to do a manual AFC on the receiving side. And the trick is to let it reach a "working" temperature before use and avoid variations over the whole QSO.