- Original Message -
From: g0...@aol.com
To: kd6...@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,
kd6...@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 dishwith 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].
___
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