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[aprssig] SAW filters/di(du)plexer

Stephen H. Smith wa8lmf2 at aol.com
Wed Sep 7 17:15:47 UTC 2005

> You're right... it would be a nightmare for a retrofit.  Furthermore, 
> you'd never get the isolation you'd need, just sticking them down to a 
> board (at least, not without a lot of cut and try or development 
> time.. Cellphones get 100dB+ isolation but they also spend millions of 
> dollars of engineer time to get there.
The real advantage cellphones have is that the TX/RX split is 45 MHz at 
800 MHz.   This is the equivalent of a 8 MHz split on two meters!  Even 
simple lumped-constant L/C hipass/lopass diplexers can almost achieve 
the required isolation at this kind of separation.  Especially when you 
consider that the max TX power of a cell handset is only a few hundred 

The main reason for using SAW filters is compactness and a flat passband 
that will pass 800+ adjacent channels uniformly; not extraordinary 
levels of isolation.

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

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