[Icom] RE: Filtering at 9 MHz vs. 455 kHz

[icom@mailman.qth.net]

Hello all,
   One issue I've not seen anyone address in the issue is radiation of RF
around the filters.   There are traces into and out of the filters, plus
the pins on the filter.   There is a few pf of capacitance between these
leads & traces.   The higher you go in frequency, the more blow-by you
will get around the filter.   This limits the filtering capacity of the
filter, however good or bad it is.   You will see lot more leakage around
a 9 mhz filter than what you will a 455 khz (.455 mhz) filter.   That is
why the low frequency 455 khz filter gives much better ultimate
rejection.
   Another way to say this, you can place a couple leads about 3 inch
long about a 1/2 inch apart in-line in a box between an antenna and
transmitter, you could pass the rf at 900 mhz with about .05 db loss
(that's how Poly-Phasar makes part of their EMI lightening protects, I
have dozens of these on 930 mhz, 500 w paging transmitters).   If you
tried passing dc through the gap, you have ininity loss, absolutely
nothing would pass.   The higher the frequency, the more energy it will
pass.   That's why the FT100 and IC706 with the 9  mhz low IF's (or so)
does not perform well on a large antenna, with a crowded high power band
(contest, ect).  The blow-by on the filters becomes very evident.   You
can hear a S9 +60 or stronger signal over a hundred khz or more of the
band,  rf is blowing past the filters at this signal level at 9 mhz with
the few pf of capacitance around the filter.   An ultimate rejection of
-60 db is irrelevant, by the time you're down to -50 or less, it is
coupling past the filter.
   Hope this helps some understand what is going on.  

73's de Stephen, K0SD in NC

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