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Re: re: QRM 2m, question for sat builders



Higher frequencies are advantageous for downlinks as cosmic noise decreases
at 6 dB per octave and less power is needed at the satellite. For digital
amateur satellites licensed in the U.S. there is a bandwidth limitation is
20 kHz on 2 meters and 100 kHz on 70 cm. Downlinks usually need to be faster
than uplinks.

73,

John
KD6OZH

----- Original Message -----
From: "William Leijenaar" <pe1rah@yahoo.com>
To: <amsat-bb@amsat.org>
Sent: Monday, February 14, 2005 18:39 UTC
Subject: [amsat-bb] re: QRM 2m, question for sat builders


> Hi AMSATs,
>
> >I have a question for those "in the know" (this means
> I'm not interested in
> >speculation). Why are so many of the newer LEOs
> designed as Mode J? Is it due
> >to the commercialization of the Microsat design? More
> commercial downlinks
> >available in the UHF range? Harmonic suppression at
> the satellite end? I truly
> >wonder what the impetus towards J is....
>
> There are several reasons for this:
>
> - Bigest reason is that with MODE-J you have no
> problem with the 3rd harmonics of
>   the transmitter. This makes your receiver overloaded
> and less sensitive without
>   any good filter at the TX side. Its a real serious
> problem.
> - Then on 70cm there is much more bandwidth available
> than on 2m. An downlink on
>   70cm makes the change smaller of having downlink
> interference of 2 satellites.
> - For the groundstation you also need to have more ERP
> on 70cm compared with 2m.
>   This is because of more path losses. More power on
> 70cm is more complicated than
>   on 2m because of more cable losses, more expensive
> amplifiers etc...
>
> Ofcourse there are also disadvantages with MODE-J
> like:
>
> - Higher downlink doppler effect.
> - You need a good RX, often LNA is needed and more
> gain antennas
> - At the groundstation you need MODE-J filter to avoid
> TX to RX interference
>
> The disadvantages with MODE-J on the groundstation are
> actually the same as on the
> satellite with MODE-B.
>
> Just wait for P3e, it will have MODE-UV (the old
> MODE-B) :o)
>
> 73 de PE1RAH, William
> ---
> All your favorites on one personal page - Try My Yahoo!
> http://my.yahoo.com
> ----
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----
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