Richard Crisp <crisp@max.bitnet> (02/21/90)
I'm interested in receiving WEFAX photos directly from satellites. If someone out there has experience with this, I'd like to hear your comments on receiving and decoding equipment, antennas etc. It seems that something like an ICOM R7000 should be capable of receiving the 1.6GHZ signals etc, but what about antennas and fax decoding? Will the PK232 be able to decode these pictures like it can the HF WEFAX photos? Will I need a dish to receive the signals or will another less bulky antenna do? -- Making the world safe for technocracy.
Eric Roskos <roskos@IDA.ORG> (02/22/90)
crisp@mips.COM (Richard Crisp) writes: >I'm interested in receiving WEFAX photos directly from satellites. If >someone out there has experience with this, I'd like to hear your >comments on receiving and decoding equipment, antennas etc. It seems >that something like an ICOM R7000 should be capable of receiving the >1.6GHZ signals etc ... What is the bandwidth of the R7000? You need a ~30 MHz bandwidth. If it is narrower, the lightest and darkest areas of clouds will have "snow" in them (or will appear almost black depending on the radio). If it is wider, you will get excessive noise throughout the picture. Usually the best approach is to buy one of the radios made specifically for the purpose, which are around $300. I tried using a Kenwood R5000 before buying one of the latter, and the Kenwood just won't do it, even when the FM filter is completely bypassed, due to a 20 KHz filter in the first IF stage. Also, you would probably find it much easier to start with the 137 MHz transmissions from polar satellites for a variety of reasons. >but what about antennas and fax decoding? Will the >PK232 be able to decode these pictures like it can the HF WEFAX photos? No. The satellites transmit AM WEFAX (i.e., the 2400 Hz subcarrier is amplitude modulated), the HF stations transmit FM WEFAX. I've tried a lot of different demodulators and output devices, including an FAA surplus WEFAX recorder, and the best I've found is the A&A Engineering demodulator, which comes as a kit for around $50 and produces a 4-bit parallel output that you can connect to the game button connectors on a PC's game port. There is also software available from several sources to display the data from this demodulator. You can also buy devices from Universal that are supposed to work with a dot matrix printer, but the quality of the picture is *very* bad in comparison to the A&A device, because they use a pixel-averaging algorithm for grey scale that gives large pixels that blur fine details of the picture. Also, a lot of the picture quality is a function of grey-scale resolution, which just won't work when displayed via a halftone shading method as in the Universal devices. >Will I need a dish to receive the signals or will another less bulky >antenna do? For 137 MHz you can build a relatively small X-shaped antenna, about the size and height of an office desk chair. I built one out of a piece of PVC pipe, four pieces of copper house wiring, and a Le Menu dinner plate (for the base). You can receive transmissions from three US and 2-3 Soviet satellites via this method (the Soviet satellites keep getting turned on and off periodically so it's hard to know how many are operating at a given time). These satellites give higher-resolution pictures than GOES, because they have much lower orbits. But they go by at specific times of the day -- about 8AM and 8PM, 5AM and 5PM, and 2AM and 2PM. The exact times vary from day-to-day, and can be computed using the information TS Kelso publishes in rec.ham-radio periodically, using a public-domain "orbit" program that's on SIMTEL20. For the S-band transmissions from GOES, ideally you should have a dish antenna. Several companies sell "loop-yagi" antennas which are very long rods with a large number of small c-shaped loops all along it; these are supposed to be "almost as good," but so far I haven't found anyone who uses one, so I don't know how well they actually work. That approach is considerably more expensive than the 137 MHz satellites because you need a downconverter, a preamplifier, and the antenna, each of which is fairly expensive. If you want to see some examples of pictures received from the polar satellites on 137 MHz, look on VMD.CSO.UIUC.EDU in the directory PHIL.515 - I put new pictures in there every couple of days for use by an amateur meteorology mailing list. Those are received using the A&A demodulator, a Vanguard Labs receiver ($300), and the above-mentioned x-shaped antenna. The ARRL Satellite Experimenter's Handbook tells how to build the antenna. It's easy; the only moderately difficult part is you have to make a fairly precisely-measured segment of coax to connect the two axes of the "X" so they are 90 degrees out of phase with one another, and the only trick to that is you need to take into account the "velocity factor" of the coax (which is given in the ARRL manuals) rather than just making it 1/4 wavelength long. You can receive the signals fairly well, though, with just a simple dipole, except there tends to be periodic fading. Vanguard Labs sells a somewhat more elaborate X-shaped antenna ready-made. -- Eric Roskos (roskos@IDA.ORG or Roskos@DOCKMASTER.NCSC.MIL) "Folks shouldn't be messing around with other folks interrupts." -- E. Larsen