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

Robert Bruninga bruninga at usna.edu
Mon Aug 8 20:05:47 UTC 2016

At the SMALLSAT conference there are always presentations on modulating
optical retroreflectors so very low power passive pspacecraft can still
communicate telemetry by simply modulating the mirror of a laser reflector.

Can we do this at RF?  Unfortunately the 218 MHz megawatt radar fence in
Texas is now shut down... But, now with the amateur radio announcement of
the return of the HAARP ionosphere transmitters in Alaska, there too is
another high power CW system.  But it is at HF.

So what other HUGE power continuous RF emitters are there that transmits
straight up that could be used by a passivle ON/OFF modulation of a
resonant dipole in space to convey a few bits of data from a piece of wire
in space?

I assume this is a 1/R^4 range equation.  If I do the numbers right, a
passive dipole only 200 miles up with a 10 megawatts ERP transmitter at 300
MHz could be detected by a good low noise CW receiver using a 15 dBi
receive antenna at -128 dBm in a CW bandwidth?  How many dB can this be
improved with DSP processing?

Its just a mind game.  A small matchbox size satellite with dipole antenna
could at least report a few bits of data per pass over the radar beam?

I think the Airforce is re-building the radar fence but at S band.  At that
frequency some gain can be added with multi passive dipoles on a cubesat
size satellite gravity gradient stabilized to keep it pointed down.  If it
could be made to work, what could we do with it?

Just thinking...


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