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Jim Lux wrote:
Dave Typinski wrote: On Sun, 21 Oct 2007 01:01:19 -0400, "Jimmie D" wrote: "Dave Typinski" wrote in message ... Anyone have any ideas about what one might do with a television receive-only (TVRO) antenna? The thing is 10' diameter and it's mine for the price of dismantling it and hauling it out of the present owner's back yard. I'm thinking radio astronomy. Might be nice to make my own radio map of the galaxy. I'm guessing that this would work okay somewhere between 1 and 10 GHz... which means making a feed horn... which is easy enough to do. What else could I do with this antenna? Other than covering it in polyethylene sheet to make a really big bird bath... -- Dave Typinski AJ4CO I know of a couple of guys that took two of them, one for rx and one for tx, tx was a microwave oven mgnetron. He and his frined talked to each other via troposcatter. I do know if a maggie operates within a ham band or not but it didnt matter too much for these guys because they didnt have a ticket anyway. Jimmie I thought about magnetrons... but not for communication. My idea is to set it up as a radar station to measure the distance to the Moon. A microwave oven magnetron operates at 2450 MHz, which is right at the upper edge of the 13cm ham band. Unfortunately, magnetrons produce a really dirty, wide output. Worse, being right at the edge of the band, half the RF energy would be out of band. Just how precise a measurement do you want to make? You could do quite well using a 100W 2m SSB rig and a decent yagi antenna and some signal processing software. See, e.g. http://home.earthlink.net/~n5bf/eme2.html I don't know if one can re-tune a magnetron by the addition of external components; I'd like to get the thing down to 2.42 GHz to put the emission in the center of the 13cm band. There are such things as tunable magnetrons; but, they aren't found in microwave ovens, which means they won't likely be had for free. That said, I'd sure like to look at the output from an oven magnetron with a spectrum analyzer. Maybe the output isn't as dirty and wide as I think it is. it is quite dirty, even more so in an oven because: 1) it's fed by a half wave voltage doubler, so it's pulsed at line frequency 2) the voltage waveform is hardly nice and regulated 3) the temperature of the tube changes during operation Why not build/buy some small no-tune transverters for 3.4 or 5.7 GHz and use those. At least then, you can do coherent processing over some reasonable time span, and what you lose in raw pulse power, you pick up in processing gain. The beamwidth of your 10ft dish at 2.4 GHz is going to be pretty wide, so you're illuminating a lot more than the moon (which is wasted power). In absolute terms, on receive, the fraction of the signal power received from that power which hits the moon will be the same (same physical aperture.. gain in dBi is larger, but that's just because an isotrope has smaller aperture at higher frequencies). So, your goal should be to illuminate JUST the moon (and a bit more to cover pointing errors). Beamwidth is approximately = 70*wavelength/d. So GHz m deg dBi 2.4 0.125 2.9 35 3.4 0.088 2.0 38 5.7 0.053 1.2 43 Given that you want to light up the moon with the same power, at 2.4 GHz it will take 8 dB more than 5.7 GHz. Since the moon is about 1/2 degree wide, even 5.7 GHz is a bit low for a 10 foot reflector. There's a little more than this to the whole problem, but, in general, if what you want to do is measure the distance to the moon, a radar built with an oven magnetron probably isn't the best way to go about doing it. It's probably cheaper than buying a tape measure long enough to do the trick... KA1LHZ |
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