----- Original Message ----- From: <[email protected]> To: <[email protected]>; <[email protected]> Sent: Saturday, July 04, 2009 1:21 AM Subject: [amsat-bb] The Moon is our Future / antennas > > In a message dated 03/07/2009 20:46:44 GMT Standard Time, > [email protected] writes: > > Building a prototype that works on Earth for project like this is only a > few percent of the effort required. Treating it as a radio club project > won't be effective as people need to sign up for a 5-year project. > > Hi all. > > John is absolutely right in saying the complexity cannot be easily > compared to a terrestrial radio project. One other thing that stands an > almost zero chance of succeeding is a dish antenna that needs to point > towards the earth. If NASA and the ISS have trouble with moving parts > on the solar array you can imagine how much more difficult it would > be on the moon. > > However, how about this. > The problem with the higher bands is power generation / path loss / > antenna gain. Any higher band like 1.2, 2.4 or 5.8G would need a high > gain antenna to offset the increased path loss. > > But, instead of a conventional steerable dish....with its unreliable > moving joints...How about an electrically steerable array of patches / > dipoles / or any other type of antenna element. > > But how to 'point' it? > > Well. actually I think Tom Clark provided the answer for that with his > proposal of a few years ago. The principle is this: If you have 2 > arrays. One say on 5.6G uplink and one on 5.8G downlink, then the > receiving array can electrically look in different directions for a signal > from the Earth. > Once the receiver has identified a signal and optimised the RX Antenna, > the information on the direction of the Earth i.e. the direction of the > strongest incoming signal can be used to configure the transmit array > which will then beam a signal back to earth with high ERP. > > Directional, high gain, and no moving parts. > > Thanks > > David G0MRF
Hi David, G0MRF The following article from G3RUH is a good additional answere to your message. http://www.amsat.org/amsat/articles/g3ruh/110.html I have extracted from it the most important following part: 73" de i8CVS Domenico Extracted from G3RUH article "THE EARTH MOVES" > Moon Downlink > The maximum total excursion of 9.5° is the same as the beamwidth of a 5 > wavelength diameter dish antenna. This has a gain of some 20 dbi, and > represents an upper limit for an unsteered Moon-based antenna. However the > higher the frequency used, the smaller mechanically is the antenna, which > makes 2.4 or 5.6 GHz a good choice. Five wavelengths is 60 cm and 26 cm > diameter respectively; quite small. > For a given TX e.i.r.p., signal strength received at Earth depends only on > the mechanical size of the RX antenna; frequency is irrelevant [1]. Noise > level however is not, and S-band (2.4 GHz) is a sensible downlink choice > because very low noise performance is robustly obtainable "off the shelf". > An example, 1 watt transmitted from a 20 dbi gain dish on the Moon, > received on a 1.2m dish at Earth with a system noise temperature of 100K > results in a signal to noise ratio in 2.4 kHz bandwidth of 10.5 db. (Note > that frequency matters not). This would support one rather noisy SSB voice > signal. > Alternatively it would carry an error-free 2400 bps binary PSK data > transmission without coding, 9600 bps with modest coding [2]. _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
