[RFI] 2nd harmonic help

Wed Feb 4 17:22:08 EST 2009

Before throwing money at the problem, be sure you have identified it.
Filters on the feedline can only stop harmonics that are generated on the
radio side of the filter.  If you have a rectifying joint outside, no amount
of filtering inside will get rid of it.  If you have receiver overload
problems, no amount of transmitter filtering will get rid of it either.
Some simple tests are to reduce power and make sure the harmonic drops off
smoothly with the tx power, if there is a sudden drop in it there is likely
something overloaded or rectifying.  Put your tx into a dummy load with a T
connector on it, add a 6" wire antenna to the T as an antenna (weren't these
sold commercially at one time?)... monitor on the receiving radio with a
small sniffer loop inside the shack and see if the harmonic is there in that
configuration, if it is then adding filters can help get rid of it... add
the filters in that configuration and make sure they have the desired effect
before trying on unpredictable antenna loads.

If you need cheap filters, here is the 'standard' contester stub design.

Note that these stubs are not the final word in filtering.  ICE 419 bandpass
filters have much better rejection characteristics than these stubs... but
the stubs can be used after an amplifier where its hard to put lumped
filters.  The w3lpl rx filters are also good, but can only be in the
receiver path: http://www.k1ttt.net/technote/w3lplfil.html so they are good
for preventing overload but not transmitted harmonics.

Next, be realistic.  I have been working on building a m/m station for 25
years or so now and have not gotten rid of the exact harmonics.  My goal is
to make it so you can work within maybe 5-10khz of the harmonic without it
being a real problem.  That isn't always possible on every antenna and band
combination, but we get pretty close... but these are all monoband antennas
separated by often hundreds of feet.

Also, adapt your frequencies... you say 7040 and 14080 are where you want to
be?  Try 7050 and 14065, or 7030 and 14095, or if 20m is busy we have had
great runs above 14125 that gives 40m lots of room to tx!  The more
separation the better.... this is one form of qrm where you are in complete

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

> -----Original Message-----
> From: Ken Eigsti [mailto:diverken at chaffee.net]
> Sent: Wednesday, February 04, 2009 01:40
> To: rfi at contesting.com
> Subject: [RFI] 2nd harmonic help
> Hi
> My first post to this reflector and if it is redundant, please forgive.
> Problem: I am running Two ICOM 756PRO3's in a SO2R configuration. I have
> 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.
> Just wondering which type? Use the "rods" and tape the coax parallel to
> the
> coax? Or the clamp on type that clamp around the coax? Or???.and best
> place
> to get them? Any suggestions will be helpful..
> 73 and thanks
> Ken W0LSD
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