[RFI] 2nd harmonic help

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Wed Feb 4 17:49:08 EST 2009

Dave and Ken,

Good points presented, Dave, but I'd like to look at the problem form a 
slightly different perspective:  dynamic range.

Here are the direct numbers: 

100 watts out of 746 Pro = +50 dBm

Spur & harmonic reduction = -60 dBc (typical)

So, (+50) - 60 = -10 dBm, or 0.1 milliwatt output at the harmonic 

Typical receiver sensitivity is down to at least -120 dBm, and often -130 
dBm (keeping in mind that -127 dBm = 0.1 microvolt in 50 ohms)

Therefore, Ken is attempting to solve a problem which may (key word) 
require upwards of 120 dB isolation between source (the rig) and victim 
(receiver of second rig) at the second harmonic.  That's the main reason 
Ken can "see" the harmonic blip on the band scope even with no antenna on 
the second rig.  There's plenty of harmonic RF to go around.

Dave, your suggestion to work a band around the harmonic is a good one, 
and the one most likely to produce useful results at reasonable cost. 
Since the QRPp folks have been known to cover some significant distance at 
power levels of 1 milliwatt and below, I think Ken will find that there is 
a point of diminshing returns at which throwing more effort at the problem 
will accomplish little.  However, he shoudl pursue all the various avenues 
of field reduction that have been presented, since the more the harmonic 
field is reduced, the less work the filters inside the second rig receiver 
have to do.

73, Dale

Sent by: rfi-bounces at contesting.com
02/04/2009 04:22 PM

<rfi at contesting.com>

Re: [RFI] 2nd harmonic help

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 
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 
something overloaded or rectifying.  Put your tx into a dummy load with a 
connector on it, add a 6" wire antenna to the T as an antenna (weren't 
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 
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 
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 
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 
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 
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 
combination, but we get pretty close... but these are all monoband 
separated by often hundreds of feet.

Also, adapt your frequencies... you say 7040 and 14080 are where you want 
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 

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 
> some ferrite chokes. I might add I am mainly in the RTTY contests and 
> 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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