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@jrmiller.demon.co.uk To: amsat-bb@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
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