I got the idea from a few meteor radar sites when I got a message from SpaceWatch about the Perseid shower in early August ( and ). Basically, these are pretty simple; an antenna pointed straight up with an SSB receiver tuned to an over the horizon TV station.Here's a couple of links:
I wanted a multi channel radar, though, so I could do some crude direction finding, or, at least, listen to the meteors in stereo. Without actually building multiple phase matched receivers (or modifying receivers to feed them from a common LO), I figured out one way to do it. The first idea was that the recording didn't have to be phase matched, as long as I could recover the phase later. I could inject a tone in the receivers' passbands from an external oscillator, track that tone and back the phase shift out. The oscillator didn't need to be all that hot stuff, just reasonably stable, and radiating a very low power, somewhere near the antennas. Here's a diagram illustrating it:
The next problem was that SSB receivers for 60 MHz aren't cheap. Some of the medium priced shortwave radios will work in SSB in the low VHF band, but not many. The Icom PCR1000 computerized receiver will do it, but will set you back $300+, and I wanted multiple channels, and wasn't willing to pay thousands of dollars to do it.
Sitting around contemplating a design for a CW FM radar in Xband, I hit on the idea that the difference between an AM and a SSB radio is that the SSB radio generally has narrower bandwidth (only one sideband) and that it reinjects the carrier (a BFO, in old CW receiver parlance). The narrow bandwidth wasn't something I particularly wanted, the 5kHz or so for the AM radio is just fine, and I could inject my own BFO, which would be much like the pilot tone in the previous scheme. All I'd be using the AM radio for is as a IF strip and detector (yes, there are some issues with images, and the like). And, of course, the previous problem with tuning the VHF band is still there.
So, the new scheme is this. Feed the antenna to a mixer, where I feed a common LO to mix it down to a convenient frequency for the next stage (I haven't decided yet... 1MHz is in the middle of the AM band, but a shortwave receiver might be a better way). Inject a BFO at almost the same frequency, and feed it to the receiver. Actually, since there aren't any filters in the front end, you can inject the pilot in the same way. I've also added a LNA of some sort (probably one of those TV booster amps would do) to overcome the loss through the mixer. Here's a diagram of this scheme:
Now, all I have to do is find a suitable IF strip.. that is, a cheap SW receiver with suitable bandwidth (wider the better, in this case). I wonder how stable the tuning is for the "slide rule" tuned units? I assume they use an LC circuit, but once you set it up, it's probably pretty stable, although, the intention is to set something like this up and leave it for extended periods of time (like overnight), so if it drifts too much, you'll lose the carrier from the TV station that you're using as your bistatic transmitter.. The direct entry PLL varieties are also widely available.. this weekend (24 August 2002) Radio Shack was blowing some of them them out for $30 a piece, but you're beholden to the XO in them, which is probably only good to 100 ppm, on a good day.
The LO can probably be one of those TTL clock oscillators, with a rudimentary filter on the output. OTOH, I'm going to be driving a saturated mixer, so a square wave probably isn't all that bad. They're readily available at 40 and 50 MHz. A capacitor will couple it to the mixer. The mixer can be any of a variety of cheap mixers; Mini-Circuits to the rescue. The reference oscillator's a bit more of a challenge. Maybe one of those channel 3/4 video modulators? Pick the LO frequency so that a harmonic is in band? Here's a case where spurs are your friend.
It also occurs to me that this same technique would work to build an interferometric radio telescope.
Low VHF TV
CH VIDEO carrier frequency in MHz
http://www.k5kj.net/meteor.htm - Sam's Meteor Radio Echo Page
http://www.imo.net/radio/transmitter.html - A page from the International Meteor Organization
http://page.to/meteor/ - Meteor and Colorgramme
http://www.saunalahti.fi/~oh5iy/msobs/msobs.htm - an excellent site with good trade studies on various system implementation
radio/meteorradar.htm - 26 Aug 2002 - Jim Lux
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