15 Feb 2015 dangermaus   » (Journeyer)

There is a hidden and invisible world that passes through us at crazy speed in any moment, it is the world of radio waves. At midnight, the radio silence is broken only by whistles and by electrical discharges of a distant storm...

An ear to listen to the weather birds
I read in Internet about the Quadrifilar Helix antenna (QFH) built by Chris van Lint with coaxial cable RG6. G4ILO did a version with RG58 coax cable and having plenty of this cable around (which was earlier used to power the first Ethernet networks at 10Mbit/s), I decided to build my own.
I started to cut old PVC tubes. My first mistake was to misinterpret the symmetry of the antenna. It is important to understand that the big loop (B) uses supports of 38cm and that the small loop (S) uses supports of 34cm and therefore the mast tube of 57cm supports 3 tubes of 38cm and three tubes of 34cm.
It was easy to soldier braid and signal part of the cable together as described by Chris, but it was very difficult for me to soldier only on the braid. Each time I checked the integrity of the cable with a voltmeter, I still had an unwanted resistance of 3 to 20 MOhm, instead of reading infinite MOhm on the voltmeter.
To solve this problem, I used a BNC connector with the needle carrying the signal removed and only the shield connected in combination with a T-BNC connector to avoid soldering on the braid.
I then checked with my antenna analyzer (the Mini VNA BT Pro) that the antenna was resonant as expected on 137 MHz, measuring a SWR of about 1.4, which meant that the circularly polarized QFH was ready to be mounted on the roof (after some simple impermeabilization for the top part of it).
Satellites transmit with circular polarization because waves travel better through rain, snow and clouds (due to the interference effect of rain and water drops) and also because the antenna orientation also changes in respect to ground, if the satellite is not stabilized.
Once on the roof, the antenna was connected to a Funcube Pro Plus Dongle. My current setup uses HDSDR and Wxtoimg with VB Audio Cable to relay the output signal of HDSDR to WxToImg. Also APT Decoder works well, but it needs to load Keplerian elements in text format (it assumes a .tlx extensions which needs to be overriden to .txt).
In the beginning, it is important to tune HDSDR with a shift between the tuned frequency of 27kHz and the Upper Spectrum Center Frequency (e.g. tuned on 137.100 with USCF on 137.137) and setting a large receiver bandwidth to 68KHz, because of whistles and artifacts created by the cheap Funcube receiver.
It is much easier to receive good signals when the satellite passes close to the Zenit (in the meanwhile I learnt to read the ephemerids). Weather conditions are not so important thanks to the circularized polarization of the antenna. Only if it is a windy day, I have some white noise lines in between of the pictures. Even with my cheap setup, I receive weather images without problems with a signal to noise ratio (SNR) of 25 dB and less.
In the beginning, we looked several times at black images of APT decoder, until once hacker Vir asked me if the monitor was dirty. She then realized that the image was not only black, as the dirt moved with the APT Decoder window. After proper denoising we had our first satellite image which you can see on the bottom of our QRZ page. By the way, the satellite NOAA 19 has an interesting story on Wikipedia, as it had an accident during manufacturing.

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