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Re: Oscar 0

Bob did raise a good point which I had neglected.

 Although the moon is a very poor reflector, it does capture a large amount
of the signal as it spreads out from earth.  The moon's diameter is about
3475 km.  If you assume a 100% "reflection" from the transponder with a dish
antenna with a surface area of 2m instead of the moon's diameter, the
pathloss would  be about -365 dB, much greater than the EME pathloss at 144
mhz.  The improvement in reflection adds about 11dB of gain but the decrease
in size adds about 124 dB of loss from the equations that I have seen.
Obviously the transponder would have to amplify the signal as well.

An EME station with 20dB (4 x 2.5 wavelength antennas) of antenna gain and
1500 watts experiences an echo to noise of about  8 dB assuming the moon is
in front of "quiet" sky.  THis is really the bare minimum to hear signals
off the moon often.

A station using a single yagi with  14dB of gain  and running 200 watts will
deliver 10 exp -13 watts to the transponder if the gain of the receive
antenna is 10dB.  If the transponder then amplifies this to 20mW a signal
will be delivered back to the station with the same signal to noise as the
bigger EME station.

In addition, a one watt transmitter on the moon with a 2dB gain antenna will
deliver a signal to a station with a single yagi with 14 dB of gain with a
signal to noise of about 17 dB.

I am interested if others would come up with similar numbers.  What type of
amplification can a transponder provide?  If it is limited, this would make
this unusable to even large EME stations.  Would a one watt beacon be a
better idea?


-----Original Message-----
From: Tony Langdon <tlangdon@atctraining.com.au>
To: 'Scott E. Olitsky' <solitsky@acsu.buffalo.edu>; Dan Schultz
<n8fgv@AMSAT.Org>; amsat-bb@AMSAT.Org <amsat-bb@AMSAT.Org>
Date: Thursday, June 22, 2000 7:40 PM
Subject: RE: [amsat-bb] Oscar 0

>> It would be interesting to do the calculations on this.  The
>> path loss for
>> EME is somewhere on the order of -250 dB.  Much of this loss
>> is related to
>> the fact that the moon is a very poor reflector and only 6.5%
>> of the signal
>> that hits the moon is returned to us.  A transponder should
>> would return
>> more than that so the path loss would be significantly less.
>> I had worked a
>> couple of very large stations with my satellite antenna and
>> 300 watts.  With
>> an extra 11dB of gain in a new amplifier and antenna system I
>> can hear my
>> own echos fairly often.  If you assume that the transponder
>> had the ability
>> to return 100% of the signal that it hears (i.e. "reflects
>> 100% rather that
>> 6.5%) The loss would decrease by that same 11 dB.  A ham
>> using a single long
>> boom yagi and a small amplifier, should be able to hear a CW signal
>> returning from the moon fairly often.  A huge antenna array
>> like those used
>> in EME would not be needed.
>This would seem to match with the guesstimate I put in a previous message.
>I'd estimate 100W and a long Yagi would do the trick.

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