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Re: Communication thru P5-A



Thanks Cliff,

My range was an outright guess, so please be my guest and plug-in the more
accurate numbers.  Of course this will only be good at closest approach to
earth...the remainder of the time the distance will be greater, even
crossing the solar system (several hundred million km) when at conjunction.

The reason the formula does not ariive at my number is that it is not the
link formula for eme.  You have to factor in additional loss for scattering
(from a rough surface) and spherical dispersion  (the moon is round not a
flat reflector)...the numbers come to about another 50 dB!  In an attempt
to keep the analysis simple, I avoided getting into the detail of eme
path-loss and just gave the result.

I suspect 2400 MHz is the wrong example as well...I would expect maybe 5.7
or 10 GHz.  If so, the path loss increases by the square of frequency.
Fortunately, dish gain also increases by the square of frequency, so the
numbers come about the same for the simple point-to-point path-loss (not so
for eme since the signal must go both directions...thus eme antenna gain
requirements increase with freq^2).

This is simplistic, though.  Receiver NF and freq stability performance is
harder to achieve as you head higher into the microwave frequencies (it
simply gets harder to do).

Without all the math, it stands to reason that it will take a big dish to
receive a small signal over such long distances.  My est. of a 30-foot dish
is just a crude first guess!  Other factors may increase the antenna size
needed.

Now there are maybe a dozen moon bouncers out there with such large dishes.
 They can have fun trying to detect a carrier from P5A.  Note I don't say
anything about communicating thru a transponder.  EME usually implies the
use of narrow BW in the neighborhood of 100 Hz...only CW and digital
techniques like WSJT/JT-44 will work in such a narrow environment.

BTW I'm not saying hams can't do it...just it will be supremely
difficult...an effort only a dedicated few will attempt (I think).  I would
suggest that everyone read the account of the first eme contact on 24 GHz
as an example (CQ-VHF Magazine-this quarter).

Anyway, thanks everyone, for giving me math problems to solve...:-)

Ed, AL7EB


At 05:37 PM 8/26/02 +0000, Cliff Buttschardt wrote:
>Ed, Chris and the group.   There is something wrong here in these
>calculations.  The distance to the moon is 238,000 miles average or
>397,000 kM.  Two way path length would be 794,000 kM.  That would
>result in a 46 db less loss to the moon unless I misunderstood 
>something in the original assumption. 
>     If you are referring to Mars, the closest Earth/Mars distance is
>usually quoted as 56 million kM.  Mars has an average distance to the
>Sun of 228 million kM while the Earth 150 million kM.  Respectively,
>78 kM and 378 kM would be the worse case nearest and farthest distance
>depending on the oblateness of each planets orbit.  Sounds like we
>better refine the path loss calculations a bit!!   Cliff K7RR
>
>"Edward R. Cole" wrote:
>> 
>> Chris,
>> 
>> There was discussion about this a couple weeks back.  I wonder if you
>> remember what the paramters are for P5A?  What freq. will be used to link
>> with earth, transmitter power, antenna (gain).
>> 
>> With those facts we could calculate expected signal levels (i.e. path
>> loss).  I need to look up the distance of Mars, but to memory it is roughly
>> twice as far from the Sun, so would be on the order of 100 Million miles at
>> closest approach (which only occurs about every two years).
>> 
>> I have a spreadsheet that calculates the path-link for eme and it is around
>> 280 dB path loss at 2400!
>> The formula of simple path loss is
>> L = 32.4 + 20Log(Freq. in MHz) + 20Log(Distance in km)
>> Lmars(min) = 32.4 + 20Log(2400) + 20Log(160,000,000)
>> Lmars(min) = 32.4 + 67.6 + 164
>> Lmars(min) = 264 dB
>> 
>> so that looks like a 16 dB stronger signal!
>> 
>> Now at 2304 eme stations run around 100w and a ten-foot (36 dBi) or larger
>> dish!  EiRP = 400,000w
>> How does this compare with the ERP of P5A?
>> Lets say 10w and 20 dBi; EiRP = 1,000w
>> 
>> That is 1/400 the signal (-26 dB!)....hmmm 10 dB bigger dish than ten-foot
>> needed (about 30-foot).
>> 
>> Of course I'm going on assumptions, what are the real parameters?  Anyone?
>> 
>> Ed, AL7EB
>> 
>> At 11:10 PM 8/25/2002 -0400, CLVANCIL@aol.com wrote:
>> >Hi Everyone,
>> >
>> >At the Mars Society Convention Gary Synder (N7QAM) made a presentation on
>> EME
>> >as an analog for Deep Space Communication.  He said that moonbounce
>> >communications is about ten times as difficult as Communications with a
>> >spacecraft at Mars.  If this is true then anyone doing EME communication
>> >should be able to do P5-A communication.
>> >
>> >--Chris Vancil
>> >----
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>> >
>> 
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