Hi Alan,
Thanks, and I looked through this document. The document is very good like
others we have come across regarding best practices to avoid PIM, but like
other documents it does not provide the "silver bullet" in regards to
techniques for locating the source of PIM. There are also numerous
documents from the Cellular industry that have also been very helpful in
understanding PIM, but again there is not much information regarding best
techniques to use for locating the source of the PIM. There is some test
equipment designed for use in the Cellular industry for testing of PIM, and
we have actually used some ideas gained from what the Cellular Industry
uses (like using 2 different transmitters at the 20 watt level at two
different frequencies and then observing the 3rd order product on a local
receiver, and using directional antennas to track down the source of the
PIM, but this is very difficult based on the the complexity of Mikes
antenna farm. One of his towers did show up as a problem using the above
mentioned technique, and Mike is now focusing his attention to that tower.
Thanks, and 73,
Don (wd8dsb)
On Wed, Dec 29, 2021 at 6:47 PM Alan Higbie <alan.higbie@gmail.com> wrote:
> 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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>> > https://www.avast.com/antivirus
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>> >
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