[RFI] 2nd harmonic help

Jim Brown jim at audiosystemsgroup.com
Tue Feb 3 22:58:41 EST 2009

On Tue, 3 Feb 2009 18:39:46 -0700, Ken Eigsti wrote:

>Problem: I am running Two ICOM 756PRO3's in a SO2R configuration. I have ICE
>filters in line with both rigs. When operating on 40 the 2nd harmonic is
>wiping out 20 meters on the second radio. I have had others suggest to use
>some ferrite chokes. I might add I am mainly in the RTTY contests and the
>2nd harmonic of 7040 is 14080,right in the rtty band on 20.

What are you using for antennas? How much separation? What kind of feedlines?

It's not at all surprising that you're having trouble with harmonics. Filters 
don't reduce out of band signals to nothing, they simply reduce them to the 
point that they are less likely to blow away the receiver. You can improve 
things in several ways:

1) Increased antenna separation  2) Antenna directivity  3) minimize feedline 
interaction with SERIOUS coax chokes at the feedpoint of each antenna 4) Buy 
a K3 to improve the overload capability. 5) Add resonant stubs to both 
feedlines. 6) Avoid twinlead as a feedline -- it's not practical to choke it. 

For #3, study the tutorial on my website http://audiosystemsgroup.com/RFI-
Ham.pdf.  For #5, I'm told that W2VJN's book is very good. 


Jim K9YC

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