[amsat-bb] Re: Lunar Architecture Moon Base

Bruce Bostwick lihan161051 at sbcglobal.net
Wed Dec 6 07:45:28 PST 2006

There are definitely a lot more Part 15 radiators on 2.4 GHz than  
there were 10 years ago.  How much of an issue they pose in terms of  
raising the noise floor of your receiver depends on where you are and  
how well your antenna rejects off-axis signals.

Not sure how useful this would be, but a lot of TV stations using C- 
band and Ku-band dishes in high-noise areas build RF-opaque walls  
around their dishes that block signals to the feed up to just above  
the horizon, or in really severe cases, dig a pit deep enough to get  
the feed of the dish below ground level so all it hears is what's  
reflected off the dish.  The reason it may not be that useful is, of  
course, that they're aiming at GEO sats that stay in the same spot in  
the sky, and you wouldn't be able to track the moon that well when it  
got low on the horizon that way.  But it's worth a thought.  I can't  
see any other way terrestrial 2.4 GHz signals would make a difference  
one way or the other except in terms of raising the noise floor of  
your receiver's LNA.

As far as engineering the transponder itself .. the two biggest  
problems of putting a transponder on the moon (1, getting it there  
and deploying it properly, and 2, keeping it powered up through the  
lunar night to survive the cold soak) get a lot simpler if there's a  
spacecraft already going there that you can hitch a ride on (at  
however many $M a pound!), astronauts (i.e. trained eyes and hands)  
there to set it up, and an external source of power to keep it  
running without sunlight.  It starts to get maybe sort of practical  
if you squint real hard, once those two problems are solved.  Not  
*really* practical in the sense of being affordable on an AMSAT-type  
budget, but getting an order of magnitude or so closer at  
least .. :) .. enough so that if NASA can be sold on it as an  
emergency communications backup (with the caveat that this means it  
could be taken over in the event of a station emergency), it might  
just become feasible.

The libration problem does get tricky, because the transponder's  
antenna either has to have a wide enough main lobe that Earth stays  
in it most or all of the time (the exact percentage being a very  
critical engineering tradeoff), or there's some tracking mechanism to  
steer the antenna for a tighter beam, which gets into the additional  
failure modes of moving parts as well as the control system to steer  
the antenna (and ways in which that can get out of sync with the  
Earth's motion causing intermittent LOS and requiring Earthside  
control ops commands to get it lined back up!), also a critical  
engineering tradeoff.  Which of those two solutions is better is for  
better minds than mine to decide, but those are the choices antenna- 
wise ..

On Dec 5, 2006, at 11:08 PM, Greg D. wrote:

> Great article, James.  Finally I understand what is going on.
> But, I wonder if the last section should be updated, and if so, to  
> what?
> The statement is that 2.4 ghz is a nice quiet band, and would make  
> for an
> excellent downlink, was very appropriate 10-ish years ago, but now...?
> Greg  KO6TH
> ----Original Message Follows----
> From: James R Miller <g3ruh at jrmiller.demon.co.uk>
> To: amsat-bb at amsat.org
> Subject: [amsat-bb]   Re: Lunar Architecture Moon Base
> Date: Tue, 05 Dec 2006 08:10:07 +0000
> On Dec 04, Pat, KA9SCF wrote:
>> With the amount of libration on the moon, what's the maximum  
>> antenna gain
>> per band that you can get before it becomes too much of an issue.
> http://www.amsat.org/amsat/articles/g3ruh/110.html
> Perhaps this should be in the Amsat FAQ   =%-)
> 73 de James G3RUH

"Go ahead and do it, you can apologize later." -- RADM Grace Hopper,  
"The sunset is an illusion, but the beauty is real." -- Richard Bach

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