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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
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