Hi Peter:

If you want to pick up microwave radiation based on temperature, like from a 
warm body against a cold background, Google on Dickie Radiometer.  The idea is 
to switch the gain polarity at the same time as the input is switched from the 
antenna to a load at a known temperature.  Adding a PR coded modulation will 
greatly increase the sensitivity.

A easy way to detect an RF signal is to use a simple crystal radio where the 
input tuned circuit selects what frequencies it receives.  If the signal has 
on/off keying either in the modulation or just in that it starts and stops, 
then you can A.C. couple the crystal output and use high amplification.

So the circuit looks like an antenna, a tank circuit followed by a diode 
detector and an AC coupled audio (video?) amplifier, a rectifier driving a switch.

I recently saw somewhere on the web a key chain device that did this for cell 
phone, blue tooth, and some other signals.

I have a WiFi detector that uses SMT microwave filters and amplifiers ahead of 
the crystal radio.  The output consists of a number of LEDs that light to show 
signal strength.

Here's an example of a microwave crystal video receiver where I designed the 
limiter-detectors:  http://www.prc68.com/I/ALR54.shtml

-- 
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com

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