----- Original Message ----- From: G0MRF@aol.com To: kd6ozh@comcast.net; amsat-bb@amsat.org 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, kd6ozh@comcast.net 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].