18 May 2017

HB-100 300 metres "DX" and opaque windows

When the first "long distance" 10 GHz reception test failed (about 300 m as the crow flies), I knew something was utterly wrong. Based on the indoor NLOS SNR I recalculated the link budget and determined I had enough signal to cover a much longer distance.

The setup was as follows. TX in a plastic box outside at the far away site. RX indoor at home, with RTLSDR and laptop. Just bare HB100 radar modules. Full optical visibility.

No reception at all. I suspected broken wires and cold joints, so I re-checked everything and successfully re-tested at home.

For the second attempt the RX station was on the balcony and I replaced the RTLSDR with the spectrum analyzer: ta-da! My transmitter' signal was there, at 74.8 MHz I.F. as expected. It was fun to notice how millimeter waves bounce everywhere if you're surrounded by concrete buildings and combined with the broad radiation pattern of HB100 antennas I could not get a null.

Then a doubt arose: could the RTLSDR dongle be too insensitive at that frequency? So I powered up the laptop on the kitchen table, the dongle and the receiving module. This time the door to the balcony was open, and I was standing between it and the nearby closed window. Since I was holding the RX in my hand, I moved it around looking for a peak on the spectrogram and noticed it would appear and disappear, loosing 20 dB down into the noise. What was going on? Just five minutes before I couldn't get a null and now the signal was gone still pointing the antenna at the right direction?!

Slow down Paolo. Repeating my movements I could see that the signal was lost when the receiver was behind the window, and present when looking out of the open door. What?! Wow!

The window is made of double-glazing and I know it contains some form of Sun shield: my home windows are also a very effective 10 GHz filter!!

Now I can look for a longer LOS path!