Weather satellite receiver
We have recently expanded the webSDR with a 137 MHz weather satellite receiver. Following positive reactions from users, after a short test period, we have decided to permanently add this band to the CAMRAS webSDR. These NOAA and Meteor weather satellites can be decoded with special APT software such as SatDump or WXtoimg etc. The satellite passing times can be predicted with the programs Orbitron or gpredict, for example.
It is recommended to adjust the FM audio bandwidth as well as possible to the width of the received signal. By default, this is set to 10kHz. Due to a technical limitation in the design of the webSDR, a maximum audio bandwidth of 30kHz is possible. In practice, it appears that this makes it possible to receive various NOAA satellites. The 100 kHz wide signals from some Meteor satellites can unfortunately not be received or can be received worse with the webSDR for the reason mentioned above.
After a few weeks of testing with a vertical Discone antenna, it turned out that the reception was somewhat disappointing. Despite the 15m height of the mast and a “free view” all around and the fact that the mast is set up in a “radio-quiet zone”. By then connecting the RTL-SDR dongle for a while to the horizontal 4-element Yagi antenna of the “GRAVES” meteor stream, it turned out that the signals came through considerably stronger.
After some research on the internet for an even more optimal WXsat antenna for us, I came across two articles on websites by an Italian called Jacopo (IU1QPT) and a Croat Adam (9A4QV). According to the authors mentioned, a dipole antenna folded at 120 degrees works excellently to receive weather satellites. They prove this by showing screenshots of beautiful weather satellite images they received, such as the ones below. The aforementioned people also claim that ‘this design, although theoretically 3dB less, in practice performs negligibly less well than a special quadrifilar helix antenna (QFH) or a turnstile antenna’. This is mainly because ‘such antennas are difficult to construct yourself and therefore often work somewhat less well in practice than theoretically calculated’, according to Adam. And ‘in the event that the wrong rotation direction LHCP instead of RHCP is accidentally chosen, the signals are even received 20dB less well than with the V-dipole’. This all sounds quite plausible. Because there is already a Discone antenna at the top of our mast, there is no possibility to place a QFH antenna here. So the perfectionist among the readers, who still wants to use a QFH or Turnstile antenna, should dig a little deeper into their pockets and buy such an antenna, which in my opinion is quite expensive.
Screenshots: “Jacopo’s Lair”website
Antenna construction
photos CAMRAS (Simon Bijlsma), 120 degree folded dipole antenna with an enlargement of the junction box on the right
The aforementioned 120 degree folded dipole antenna is easy to build and I did that for the 137 MHz ‘meteo band’ webSDR receiver. The antennas shown on the mentioned websites are in my opinion less suitable for a permanent installation because the used terminal blocks can easily corrode due to weather influences and the elements can easily bend due to strong wind or because birds sit on the antenna. That is why I used similar aluminium materials as with the other antennas, which I built for the 2m and 6m band, and of which I have written the construction manual elsewhere on this website. The above photos show the result of the WXsat dipole.
Note that the dipole elements, contrary to what Jacopo and Adam mention in their articles, are in this case 52cm long to be resonant at 137 MHz. The aforementioned authors mentioned lengths of 54cm and 53.4cm respectively. The 3mm and 3.25mm elements they used are much thinner than the 10mm thickness I used and that affects the velocity factor (vf) and therefore the physical lengths.
I estimate the cost of this antenna at 25 to 30 euros at most, I still had the materials lying around so I don’t know the exact current price. If you only want a temporary antenna setup, I advise you to use the design of Jacopo and Adam because their antennas are easier to make and also cheaper.
Antenna analyzer
Text contribution: Simon Bijlsma (PA7SB), radio amateur, amateur astronomer and volunteer at CAMRAS, the C.A. Muller Radio Astronomy Station.
