Re: [Re: [amsat-bb] Re: Moon-based amateur radio transponder]

[Bruce Bostwick <lihan161051@xxxxxxxxxxxxx>]

> I never said that the moon was not a worthy goal, although a somewhat
> difficult one.
>
> I am asking if Amsat members would like to spend their money on a lunar
> repeater INSTEAD OF the Echo and Eagle projects.

At least.  Possibly much more.  Add some for each 2 week lunar night 
you want it to survive.

> I would like someone with EME experience to comment on the assumption 
> that a
> transponder on the moon would improve the EME link margin. It seems 
> that even
> though the old moon is a passive reflector, it is also a VERY LARGE 
> passive
> reflector, having a cross section area of about 9.5 E+12 meters 
> squared. A
> transponder antenna on the moon would have a capture area of a few 
> tens of
> square meters at best. Let's see the calculations.....

Well, but the point is it's a passive reflector.  And a very 
inefficient one.  The lunar surface itself doesn't reflect laser light 
very well, and if you shine a laser beam at the moon and look for the 
beam spot in a boresight-mounted telescope, chances are you won't see 
much.  But if some of your beam happens to hit the array of 
retroreflectors set up on Apollo 14 (I think), you'll see a bright 
flash even though that array is only a foot or two square.

I would think that an active transponder with a large enough dish would 
do two things: 1) provide a more or less coherent return signal with 
substantial gain, and 2) provide a frequency split between uplink and 
downlink.  Both of those are a substantial help, the latter even more 
than the former.  Working split means you're not having to dodge S9+20 
outgoing signals via groundwave or tropo or half a dozen other more 
efficient return paths and listening for an extremely faint return 
signal from EME.  If the uplinks are on 440 or 2.4 GHz, and the 
downlinks are on 2 meters (or whatever scheme turns out to be most 
practical to avoid QRM) all you're listening for is downlinks, ideally, 
and your radio's AGC isn't getting pumped down by the stronger nearby 
signals.

But the size of the dish and the power available to the transponder 
amps are big and expensive things, mostly weight at you don't want to 
know how much a pound ..

           --... ...-- -.. . -. ..... ...- -...
                   Bruce Bostwick N5VB

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