[Elecraft] ultimate rejection

[Merlarts at aol.com]

Sometime in 1977 I acquired a TS 820; this radio was supposed to be hot  
ticket; well made, highly sensitive, with fancy dual gate MOSFETs, and the  
crummiest filter performance imaginable.  
 
It was'nt the fault of the filter.  It was a reasonable 8 pole design  which 
probably produced pretty good numbers in the lab.  But in the radio  it was 
awful.  
 
The filter sat on a sort of Faraday shield, and the input and output talked  
to each other across that latticework of copper strips.  
 
Going first for the simple, dumb approach I began putting little RF  
"barriers" across the middle of the filter.  They consisted of a piece of  printed 
circuit material, connected to ground and soldered all along its  length.  It 
worked great!  Everything about the filter got  better:  opposite sideband 
rejection, ultimate rejection etc.  
 
Back in the not so sophisticated days filters always had some sort of  
metallic barrier keeping the input from "seeing" the output.  In the Drake  TR4C, 
which claimed a shape factor of 1.66, the radio was designed so that  the 
sideband switch was sitting right next to a piece of the chassis which  separated 
the input from the output.  
 
Enough reminiscing;  I just spent about a hour with my KSB2, making  sure 
that the solder connections near the 7 pole filter were filed down, or  clipped 
as much as possible, so that they don't act as little "antennas",  thus messing 
up the characteristics of the filter.  It worked!  The  "audio image" that 
one hears when tuning past zero beat got a lot weaker!  
 
Next, I went to local hobby shop and bought some tin.  I made little  tin 
"houses" which I placed around the matching toroids, ground them  carefully.  It 
worked too.  Next I added a little tin "fence" which  went on the bottom the 
board, shielding the input of the filter from the  output.  It worked.  Three 
times is a charm!
 
My point:  it ain't rocket science, but the old ideas of keeping RF  away 
from places where it shouldn't be work just as well as they ever did.  
 
I don't have means of measuring this stuff accurately.  Let's just say  that 
when tuning across really loud forty meter broadcast stations the filter  
sounds A LOT tighter then it did.  
 
Has anybody else tried this stuff.  There must be hundreds of you old  guys 
who have messed with such things.  
 
So, until Elecraft finds a way to put a bulletproof 8 pole filter in its  
magnificent radio it's time to improvise a little.  
 
73,
 
Merlin W3ICT