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

Zach Leffke zleffke at vt.edu
Mon Aug 8 21:02:53 UTC 2016

I seem to recall a while back that Arecibo was having funding 
difficulties and was looking for new programs.  There is another 
'something that shoots straight up' with a little bit of ability to tilt 
the beam.  They have to move the feed around to change pointing angles, 
no idea if they can track fast enough for a cubesat or what their 
lateral limits are.  I think they can at least track as fast as the moon 
moves because they've done EME stuff in the past.

using a quick online dish gain calculator, and i know arecibo is 
spherical not parabolic, 
(http://www.satsig.net/pointing/antenna-beamwidth-calculator.htm) at 437 
MHz, with 50% efficiency and a 300m diameter, your looking at almost 
60dB gain on boresight.  So 1W at the feed gets you a Megawatt EIRP.  
The price you pay for that though is a 0.2 degree 3dB beam with probably 
minimal tracking capability.

could be a neat experiment though to try to time things right with the 
orbit and give it a go.  I'm pretty sure Arecibo is 'ham-friendly' and 
would probably be open to the idea.  I think the folks at Greenbank are 
similarly 'ham-friendly' because I've heard stories about the use of old 
systems there (not the 100m telescope that is actively used) for EME as 
well (maybe a better tracking capability?).

Also, I cant remember clearly, but I think there was a program back in 
the 60s (maybe 70s?) to deploy little X shaped dipoles into orbit.  I 
think these were designed to resonate at microwave (possibly X-Band) 
frequencies and the idea was to create a 'cloud' of them and bounce 
comms off them between ground stations (maybe it was an initial 
investigation into X-band for satcom work?  I can't remember, I think i 
read it in the history of the DSN or something like that).  I bet a 
cubesat could be a handy little 'deployment mechanism' for something 
like this (1u's worth of rolled up thin tape measurers or piano wire cut 
for 70cm, pop the door just like a PPOD and now you've got a cloud of 
70cm dipoles).  There was a lot of concern even back then about causing 
significant orbital debris problems, but as predicted the little tiny 
hair-like X's decayed relatively quickly after a few weeks. If you 
deployed at a low altitude (maybe 350-400km or less?) I bet you could 
get past the orbital debris mitigation requirements. Slap a transponder 
on the thing too, so it can be used while we wait for the orbit to decay 
low enough for the filament deployment.

Fun mind game.

-Zach, KJ4QLP

Research Associate
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

On 8/8/2016 4:05 PM, Robert Bruninga wrote:
> 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.
> http://www.arrl.org/news/haarp-facility-to-reopen-in-2017-under-new-ownership
> 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...
> Bob, WB4APR
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