So, what's available for radar components on the open market? Most radar guns (aka cheap hardware) is either in the 10GHz band or the 24GHz band. These high frequencies are also used by ham radio operators, so they sometimes try to use these as high bandwidth line of sight radio links.
So, where to get them?
IF you want new, expect to pay $500 to $1000 per node. Advanced Reciever is one that lets you know without requesting a quote.
How about used?
Well, depending on availability, you can assemble much of the parts you need from SHF Micro.
So, my friend Creuzer was wondering about interesting feedback effects versus police radar, so let's go over what you'd have to do. Police radar works purely on doppler effect. Gunnplexers use a pair of tuned diodes to produce a beat frequency from a continuous wave source, so speed is a simple conversion of output frequency to speed. Now, how do you play games with one?
If the police radar is "dumb" (common), the radar electronics will lock onto the most powerful signal. Because your return is FAR more powerful than his reflection, you should win this fight. Now, the tricky part. You'd have to take in his signal, determine the initial doppler shift from knowing your own speed, and then fire off the right frequency to add or subtract whatever you want to to that radar signal.
A "smart" radar system (does it even exist?) that looks for a specific strength return requires more time and investment. You'd have to apply something (like, say SHF's Eccosorb AN-73) that will reduce your radar profile. Of course, you also have to protect the usually expensive components, which would require a good dielectric (radar transparent) coating of plastic over the top. You might also need to apply a fine metal mesh to your windows to keep the RF from bouncing off your head and back to the gun (and then you can control the radiant direction more). Anyway,
you'd have to take a power measurement and try to radiate back a certain percent of that. This would also help with the "dumb" radar but it's far more expense than it's worth.
Sounds simple? Well, it might be, but there's an issue. Radar gunnplexers tend to "float" their tuning all over the place within a 3GHz band, and you usually only have 100MHz or so of available bandwidth. So, know your police radar bands and hope they have it tuned! Mind you, the old hardware worked at either X band (10.5 to 10.55GHz) or K Band (24.05 to 24.24GHz). Relatively more modern hardware runs somewhere between 34.2 and 35.2GHz. I've only found one reference to these, and it was from Japan and cost about $3000 per unit if my translation is right. 35GHz is becoming more common, though, I've seen a lot of miniature radar units (for UAVs) being built around hardware at this frequency.
And that is why I don't mind spilling this little tidbit. Unless your local police is using hardware from the 80's (they might be though!), you'll probably never be able to mess with their readings.