[amsat-bb] Re: The Moon is our Future / antennas

i8cvs domenico.i8cvs at tin.it
Sat Jul 4 11:27:17 PDT 2009

----- Original Message -----
From: <G0MRF at aol.com>
To: <kd6ozh at comcast.net>; <amsat-bb at 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 at 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


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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