[amsat-bb] Modulating UHF Retro Reflector at RF?

Ross Whenmouth ross at topwire.co.nz
Tue Aug 9 08:50:37 UTC 2016

Years ago I built a "moving target simulator" for a K-band CW (homodyne) 
radar. I simply took the Gunnplexer module out of a K-band automatic 
door opener (a 24.125 GHz resonant cavity complete with feed horn, Gunn 
diode local oscillator and RX mixer diode) and fed my simulated doppler 
AF signal into it's RX mixer diode (I did not energise the Gunn diode - 
so the microwave oscillator was NOT operating).

When I illuminated it with 24 GHz RF from the radar, it produced good 
reflections at RF + AF and RF - AF. I did not have access to good K-band 
microwave test equipment so I was unable to quantify the strength of the 
reflections or mixing conversion loss, etc.

So I would say that yes, subject to sufficient link budget, if you had 
an antenna in orbit, coupled to a mixer diode, and you drove that mixer 
diode with AF modulation, then that antenna will re-radiate the mixing 
products of the RF illumination and AF modulation. Would this be an 
interesting experiment for an ulta-low-power bird that does not have the 
power budget to run an RF transmitter?

Ross Whenmouth

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