WX sat reception

The RX137 is a very simple, yet very effective receiver for WX satellites in the 137 MHz band. It is based on a TDA7000 IF circuit with a 10.7 MHz IF. It consists of a BF981 preamp, a NE612 mixer / oscillator , the TDA7000 if + demodulator and several opamps and a LM386 amplifier. Despite of its simplicity it works very well and is very sensitive.
The receiver has recently been rebuild and revamped. The tuning is now done with an MC145170 PLL synthesizer controlled by an Atmel ATMega8 processor. The processor also provides for an 80 segment S-meter, memory channels, a variable LCD backlighting and RS232 computer control, which allows automatic reception of pictures with WXtoImg. Have a look at the Recent Pictures page.

See also Circuit Diagram , PCB Layout and Synthesizer Diagram for further details

The "Quadrifilar Helix" antenna is mounted in the attic. This type of antenna is suitable for circular polarised signals like the ones transmitted by polar weather satellites.

Over the years I experimented with some other antenna types, but the "QH" is without doubt the best one for the job. The QH consists of two "cork screw like" closed loops connected in parallel. To get omnidirectional reception the signals from the loops should have a 90° phase shift.

This can be achieved in several ways. Most popular is the self phasing QH, with a larger and a smaller loop. The larger loop has an inductive impedance of R+jR and the smaller loop a capacitive impedance of R-jR (where R is the radiation resistance of about 30 Ohms) Fed from the same voltage source the current in the large loop will lag 45° and the current in the small loop will lead 45°, together creating a 90° phase shift.

The model on the left is made from coax, plastic tube, and pieces of wood. For further details look at the QHA page.

The results with this antenna are very good. The reception is noise free from about 10° of elevation. (due to obstructions in the surroundings) There is almost no fading noticable during a passage of the satellite.

The software I use is WXtoImg . The freeware version allows making false color pictures like the one above.The big advantage of this software is that it calculates the position of geographical borders and uses this information to treat landmasses different from sea surfaces. That is why the pictures look very convincing.The program has a lot of options and can produce pictures "in real time" or at any moment from a prerecorded wave file. It will automatically identify the satellite, and the time and will reconstruct the path and the appropriate border mask.

Taken on 11-12-2005.
The big black cloud from the fire at the Buncefield Fuel Depot stretches over a big area west of London.

It is very unusual to see the effects of something relatively local on low resolution images. This was clearly a major disaster.

Some examples (click on picture for full resolution)