I notice the subject of "INTERMODULATION" is given significant coverage in
"THE MITIGATION OF RADIO NOISE AND INTERFERENCE FROM ON-SITE SOURCES
at RADIO RECEIVING SITES" (November 2009) published by the US Naval
Postgraduate School.
*Available at*:
http://www.arrl.org/files/file/Technology/RFI%20Main%20Page/Naval_RFI_Handbook.pdf
A search of the PDF document for "intermodulation" will bring up 32 uses of
the word throughout the report.
The authors' work tracking RFI & intermodulation at more than 20 US Navy
receiving sites worldwide is very instructive.
73,
~ Alan K0AV
On Wed, Dec 22, 2021 at 11:41 AM Don Kirk <wd8dsb@gmail.com> wrote:
> Hello Alan, Tim, Jim, and gang,
>
> Just thought I would provide a little update, and also a very interesting
> observation regarding the problem at Mikes (W9RE) in which his stubs or
> band pass filters for that matter have no impact when transmitting on 40
> meters while listening on 20 meters (same issue on other band combinations
> too). I will keep this somewhat brief for now as there is still a lot more
> work to be done to uncover all his underlying problems but we at least have
> a specific task that must be accomplished before we dig deeper.
>
> A month ago I had never dealt with stubs or PIM (passive intermodulation
> distortion) and only had limited experience working with active IMD in
> regards to overdriving mixers, nevertheless I now know much more about
> these subjects than I did a month ago.
>
> The biggest thing we discovered is that the Raspy sound everyone talks
> about when listening to their transmitter up on the harmonic band most of
> the time is probably the result of mixing with 60 hertz, and the only way
> we were able to figure this out was to use my cheap laptop based SDR
> receiver zoomed in on the RF spectrum display while using the minimum RBW
> (resolution bandwidth) available which in my case was 7.6 Hz.
>
> We have done too many experiments to discuss online (yes we shut off all
> power to the house including back up battery systems, yes we did transmit
> using battery power, yes we did place stubs at high impedance points
> relative to the antenna and transmitter, yes we tried bandpass filters on
> the transmitter, etc.), but so far what we have found is that if you have a
> receive antenna close to your house the chances of your signal sounding
> Raspy on the harmonic band is tremendous and if you look at the signal on
> each side of the center frequency signal on the harmonic band you will see
> what appears to be sidebands extending outward at intervals of 60, 120, ...
> Hz (I hate to call them sidebands in this case but that term provides a
> good picture of what you will see). Also the signal strength of these
> harmonic sidebands do not necessarily track the signal strength of the
> center frequency on the harmonic band when rotating portable direction
> finding antennas (sometimes the sidebands go up while the center frequency
> signal goes down, etc). This phenomena undoubtedly deals with passive as
> well as active mixing with all the components/contents of a home and it
> appears almost impossible to eliminate and I was even able to duplicate
> this problem running 2 watts in my mobile parked out in the street in front
> of my house with my portable SDR receiver located near my house, and
> duplicated this same issue at Mikes (it sure is not unique to just Mikes).
> Now the next question will be if we eliminate sources of mixing out around
> Mikes antenna farm can we reduce the mixing with 60 Hz that he detects when
> listening on the harmonic band using his tower mounted antennas, and we
> hope the answer is yes, and then stubs and bandpass filters will have a
> chance. We have to eliminate the mixing products, otherwise stubs and
> bandpass filters on his 40 meter transmitter will be useless for solving
> his local receive problem on the harmonic band (as others had already
> mentioned/speculated). I'm sure we still have a lot more to learn and
> uncover but seeing how pervasive the mixing with 60 Hz is has been eye
> opening.
>
> The immediate action item for Mike (W9RE) is to repair or remove his 40
> meter Moxon on one of his 5 towers as it's SWR jumps around a considerable
> amount when transmitting, and we also find 3rd order IMD products the
> strongest and also erratically changing amplitude a large amount when near
> this tower even when the 40 meter Moxon is not in use (not being used as a
> transmit antenna). We are hoping that some or all of the mixing issues are
> related to his ill Moxon.
>
> Just FYI,
> Don (wd8dsb)
>
> On Sun, Dec 5, 2021 at 7:50 PM Mike Wetzel <mjwetzel@comcast.net> wrote:
>
> > I have been fighting a bad 2nd harmonic problem for a number of years
> > mainly
> > from 40 to 20 meters. A few years ago a double stub on 40 only reduced
> the
> > 2nd harmonic about 15db so I thought something else was going on. This
> > year
> > the double stub has no effect at all. Others have suggested it is a
> > reradiation thing. The 2nd harmonic note is not pure but the raspy,
> growly
> > thing that Frank (W3LPL mentions). Don, WD8DSB has been helping me
> > remotely
> > on a weekly basis to try and figure out what is going on. This week he
> > came
> > over and used his SDR, portable radio, loops, portable flag (March 2021
> > cover QST) and initial data suggests strong 2nd harmonic content coming
> > from
> > the direction of my 20 meter tower.
> >
> >
> >
> > Setup: we temporarily used a low 40 meter dipole as a transmit antenna
> > with
> > feedline going directly to my transmitter and with Don in the field
> certain
> > orientations of his flag antenna showed some reduction in 2nd harmonic
> when
> > the stubs are inline but other orientations showed no reduction with the
> > stubs inline which appears to indicate the stubs really are working but
> > something else is generating strong harmonics on 20 meters when
> > transmitting
> > on 40 meters. My 20 meter tower is 170' of Rohn 45 with a 3 stack of 48'
> > boom 20 M OWA antennas. Antennas utilize grounded elements (except DE)
> and
> > use a 6 turn coax choke balun. Top one is at 170' on a mast with a prop
> > pitch in line rotator, middle one is at 110' on a swing arm with prop
> pitch
> > and the lower one is at 50' again on a swing arm with a prop pitch
> rotator.
> > At 90' on this tower is a W6NL/W2SC 40 M Moxon fixed SE. The Moxon right
> > now has an intermittent connection somewhere as the SWR will fluctuate
> > slightly during transmit. Right now there is no bonding between the mast
> > or
> > the side arms to the tower. It is on my list to do ASAP.
> >
> >
> >
> > N7WY has suggested to use higher harmonics and smaller yagis to try and
> > isolate a particular area. Later this week we plan on doing thorough
> > measurements of the 2nd harmonic out in the field looking for location of
> > maximum signal strength, etc. We also plan on using VHF direction
> finding
> > gear to see if we can hear the signal up on the higher harmonic
> frequencies
> > as others have suggested.
> >
> > I am wondering if anyone else has experienced a problem like this and has
> > any ideas?
> >
> >
> >
> > Also posting on Towertalk reflector.
> >
> >
> >
> > Thanks,
> >
> >
> >
> > Mike W9RE and Don WD8DSB
> >
> >
> >
> >
> >
> > ---
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