Hi all,

A while back I bought a 10GHz gunnplexer (combination 10GHz low-power
transmitter and simple homodyne receiver) and attached a sensitive audio
amplifier in an attempt to make a simple doppler radar. It was quite
successful; I can point it at cars up to about 100 feet away and hear a
tone which varies with their speed. I also can get some very interesting
sounds by pointing it at my PC's power supply fan or the CD drive while it
plays a CD or is ejecting a CD. I recorded a WAV file of this and I put it
up on my web site:

http://www.rocket-roar.com/BT
(look under the section on "Small Doppler Radar Experiment" about 1/4 down
the page)

For the most part, I understand how this works. However, one thing puzzles
me: when I point it at a fluorescent light, I hear a VERY loud 60Hz hum
(might actually be 120Hz, it is hard to tell because most of what we
usually attribute to 60Hz is really full wave rectified 60Hz which is
actually 120Hz).

If it were simply a case of the audio amp picking up 60Hz noise, I wouldn't
think that the antenna direction would make a difference, only the
proximity to the lamp. But it makes a profound difference whether I point
the horn antenna at the lamp or not. So, my only guess is that the radar is
seeing an echo from all the mercury (?) ions being jostled around inside
the light at 60 or 120Hz. I'm not surprised that I would be able to detect
such an echo, only surprised that it is SO strong, it is much stronger, for
example, than the echo from my power supply fan.

Any ideas?

Sean

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