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There are three parts to my amateur weather satellite station, an antenna to pick up the signals, a receiver, and a PC with a sound card.

Quadrifilar Helix antenna - loft mounted The antenna I went for is called a QFH - quadrifilar helix - and appears to be only commercially available in expensive marine or professional versions.   However, there have been descriptions in the magazine of the Group for Earth Observation (GEO) of how to build a QFH using easily available copper pipe.  I chose to use 8mm pipe, and the Design by Bill Sykes G2HCG and Bob Cobey G0HPO).

I wasn't very happy with the self-tapping screws into copper pipe at the top, and we had great fun using a blow torch to solder the elbow joints (the local fire brigade was on hand...) but the finished result seems to work quite well!

I mounted it in the loft, as I felt the plastic pipe construction might not be well suited to an outdoor environment and it's the middle of winter here, and you can see the result to the left.  The antenna is held from the roof simply by string, as it is quite light, but it also rests lightly on the roof insulation.

I also have a Turnstile antenna mounted outside - and comparing results between the loft-mounted QFH and the outdoor Turnstile, I find that the outdoor antenna wins.   I also have an outdoor QFH, but find its better response near the horizon also produces greater interference from pagers necessitating an external filter....

Turnstile antenna links - please let me know of more!

The turnstile has a poorer response near the horizon, which doesn't help if you are trying to get the maximum range i.e. near horizon passes, but which may help if you are getting interference from pager transmissions, for example.

  • Mark Griffiths' (Amateur Radio callsign G6KIZ) page includes a turnstile antenna for 137 MHz and some design notes with EZNec plots.

QFH antenna links - some still work...

  • Walt Maxwell's Web page http://www.iag.net/~w2du - with two largish PDF files.  He performed the original development work on the quadrifilar for use on the NOAA polar orbiting satellites. Quadrifilars are used on these satellites on frequencies in the 136-137 MHz band, at 121.5 MHz and 243 MHz for Search and Rescue, and on the 1.6 GHZ band for video.  He wrote an R&D report after measuring more than a 1000 different configurations of the quadrifilar - the report which led to the later development. You will also find a copy of a revised chapter that appears in the 2nd edition of his book 'Reflections --- Transmission Lines and Antennas', which will be out soon from WorldRadio Books.  The 1st edition was published in 1990, but had sold out 10,000 copies by 1995!
  • Ruud Jansen, chairman of the Dutch Workgroup Kunstmanen, who have their homepage at http://www.kunstmanen.nl, with Technote 1999_01 and RQHAdims.pdf (click on Antennes)
  • Paul Hayes - http://homepage.ntlworld.com/phqfh1/ for his complete QFH diy Guide and Balun Guide. 
  • Paul Marsh "Buidling a WXSAT QFH antenna".
  • Steve Blackmore's Tall, Narrow QHA.
  • Julian Moss, G4ILO, describes constructing a QFH in some detail.
  • "Taming the QFH" by Bill Sykes and Bob Cobey at http://www.askrlc.co.uk/
  • For members of GEO, the GEO Shop (UK) offers an updated PIC for the RX2 and turnstile and QFH antennas.
  • You can purchase a turnstile or QFH antenna from the GEO Shop.

Other antenna links

To convert the 137MHz FM signals into audio, you can use a scanner, such as the Icom IC-R2, but as the FM deviation of the signal is 17KHz and scanners expect 5KHz, white parts of the signal will be lost or distorted.  It's a good way to start, though, if you already have a scanner.  Having tasted what was on offer, I bought a more specialised receiver.  This receiver scans the satellite channels and beeps when a signal is available.  Specially modified scanners are available from the Hoggwild site in the USA, and I can recommend the R2FX and R2ZX receivers available from the GEO Shop.

RX2 receiver, switched on, in situ.
My RX2, boxed, showing connectors, meter, and channel LED

Between the antenna and the RX2, you can add a pre-amp, which will boost weak signals when the satellite is near the horizon however if, like me, you live in a good VHF location (on a hill or near other transmitters) you may find that a pre-amp simply exacerbates interference from other signals.  In the UK we have pagers sharing the same frequencies as satellites (no, I don't understand why, either) so interference from strong transmissions on adjacent frequencies is a real problem.  I found that the audio level from the RX2 was higher than that from the scanner, so I attenuated it before feeding to the sound card of the PC, you may not need to do this. 

On the PC, free private use software is now available called WXSat from Christian H. Bock that will do a software demodulation of the AM subcarrier that comes from the receiver.  (The original video signal from the satellite is first used to amplitude modulate a 2400Hz carrier, which then frequency modulates the 137MHz downlink signal).  Les Hamilton has some useful Beginner's Notes on WXSat.  This software can compensate for sampling rate errors in your sound card, and it is well worth taking some time to set up the software correctly.  This involves editing a data file with results from dialog boxes but is quite straight forward.  A great feature of this software is the automatic recording mode - if you have a reasonable signal and not just a piece of wire as the aerial - you can leave your PC on and any satellite signals that appear from your RX2 can be saved to disk as wave files, pictures or both. 

I subsequently ended up writing my own software called SatSignal for off-line decoding of signals with accurate temperature information, and I used Max Hadley's modifications for serial remote control of the RX2, again with my own control software, thus automating weather satellite reception.

You might also want to know when satellites are about to appear over your horizon - the program I started with was C. D. Gregory's WinOrbit.  I found that as supplied, the program ignores drag terms, which make a substantial difference to the results, so be sure to enable drag terms and use the most accurate orbital model your processor can handle. The Space-Track Web site is an excellent source of up-to-date orbital parameters.  Again I have ended up writing my own software for orbit prediction, WXtrack, which allows overlay of a predicted and actual satellite image, thus helping you to judge where you are seeing when the clouds predominate!

You can see a small set of results on this Web site.

 
Copyright © David Taylor, Edinburgh   Last modified: 2016 Nov 13 at 10:40