[RFI] Reply to intermod question FYI

Blackburn Blackburn" <blackburn@qnet.com
Sun, 27 Dec 1998 21:38:32 -0800

> From: Blackburn <blackburn@qnet.com>
> To: steve godwin <kd6zz@juno.com>
> Cc: blackburn@qnet.com
> Subject: Re: [RFI] PC keyboard RF immunity?
> Date: Sunday, December 27, 1998 9:03 PM
> Steve,
> Don't have a lot of experience with solving intermod problems, but I'll
> give you a couple things to try.  My professional expertise is control of
> EMI/EMC, lightning, and related effects for an airframe manufacturer.
> From what you describe I presume you have an intermodulation problem. 
> Unfortunately, intermod problems can be extremely difficult to find. 
> in mind that mixing of the two frequencies does not necessarily occur in
> one of the involved systems.  Mixing can occur in any non-linear junction
> exposed to reasonably strong RF field, even down to things like rusty
> fittings on metal fences (the infamous "rusty bolt" effect).  Many years
> ago I was involved in a radio club in Virginia who's 2-meter repeater had
> donated space on a tower used by the local power company for a VHF load
> management system.  Occasionally, we would hear the local NOAA weather
> station on 162 MHz (located at an airport a couple miles from the
> coming through the 2-meter repeater  input.  Turns out the problem was a
> bad power amplifier in the power company load management transmitter - it
> was generating harmonics which mixed with some harmonic of the 162 MHz
> signal to produce a difference signal equal to the input frequency of our
> repeater.  In this case, the harmonic generation for both signals and the
> actual mixing was occuring in the power amplifier itself, but the
> system was not associated with either of the two which generated the
> intermod product.
> From what you describe, I doubt that the mixing is occuring in your 220
> repeater itself.  It sounds like you've done most of the right things to
> protect the repeater itself from RFI, and a 220 beam shouldn't pick up
> enough AM broadcast energy to cause problems in your output amplifier or
> receiver front end.  I rather suspect the problem is external to the
> repeater itself.
> My first suggestion would be to listen to the repeater input frequency
> a handheld connected to some kind of a directional antenna while someone
> else keys the repeater transmitter, and see if you can identify the
> physical location of the source of the offending signal.  At 3/4 mile
> a 500 watt AM transmitter, it wouldn't take a very big unintentional
> antenna to generate the few tenths of a volt necessary to bias an
> inadvertant diode on and off, which is all that's necessary to create a
> non-linear condition and hence mixing products.  Assuming the mixing
> product falls in the repeater input passband, only a very small amount of
> energy is required to cause interference.
> You might also try a "shotgun" approach.  Take a rubber mallet or block
> wood and walk around the area whacking every metal item you can find,
> particularly things like towers and guys, chain link fences, metal
> sheds, galvanized metal gutters & downspouts, etc.(not cars), while
> listening to the problem.  If you find the offending joint, you'll notice
> an immediate change in the interfereing signal level, either louder or
> weaker, or possibly a microphonic effect.  In any case, if you find the
> offending joint there will be a correlation between moving it and some
> change in the interfering signal.
> Assuming the problem is due to the rusty bolt effect, and you can find
> offending joint, you can fix it by either electrically bonding the metal
> parts or by electrically isolating them.  Either way eliminates the diode
> effect which causes harmonic generation and mixing.
> Intermod products can also occur in electronic items, particularly things
> like telephones which end up connected to long antennas.  Finding this
> of problem takes a good deal of perserverence and some measure of luck. 
> Having access to a spectrum analyzer and a set of close field probes
> the job a lot easier.
> If you have a portable shortwave receiver, you might also try listening
> the 3rd & 5th harmonics of the offending AM broadcast signal, and see if
> you can locate a source in the vicintity.  An inadvertant diode strong
> enough to make mixing products would also likely produce harmonics.  A 50
> ohm resistor soldered across the end of a piece of RG-58 coax makes a
> suitable (uncalibrated) close-field probe and can be useful in located
> specific source once you've narrowed the area down to something
> Finally, just to be sure, try disconnecting the feed lines to any other
> antennas in the vicintity and see if that affects the problem.  I've
> heard of a ham radio generating intermod products, but you never
> Let me know if you find anything.  You don't want to keep beating your
> against the wall trying to fix the repeater when the problem is most
> somewhere else.
> Good luck,
> Keith
> KB6B
> ----------
> > From: steve godwin <kd6zz@juno.com>
> > To: blackburn@qnet.com
> > Cc: W2YR@aol.com; rfi@contesting.com
> > Subject: Re: [RFI] PC keyboard RF immunity?
> > Date: Saturday, December 26, 1998 11:47 AM
> > 
> > Your suggestions given to W2YR regarding the problem with keyboard
> > immunity were impressive. I am hoping that you will be kind enough to
> > give me some suggestions on another type of problem I have just about
> > given up on solving. 
> > 
> > I operate the only 220 MHz repeater in Monroe county.  The coordinated
> > frequency is 224.26 minus 1.6 MHz (which is the standard offset for the
> > 220 band plan. )  We have a local 500 watt AM station here, WKWF, which
> > is about 3/4 of a mile from my QTH.  When the repeater comes up the
> > repeater output frequency beats with the signal from WKWF and the
> > negative component comes in on the input (1.6 MHz).  I have tried
> > by-passing the coax shield, open stubs, shorted stubs and a helical
> > resonator-- all to no avail. I am convinced that the problem is not
> > coming through the power supply or the autopatch phone line. I am using
> > 9913 coax up to a 14 element beam on a 45 foot tower.  I previously was
> > using an omni-directional Ringo Ranger and the problem was the same
> > the Ranger. I am using a set of WACOM duplexers which have been factory
> > tuned and checked for attenuation. 
> > 
> > This situation makes the repeater almost useless and I am not supposed
> > re-broadcast WKLY on the 220 ham band - HI!
> > 
> > Any suggestions would be greatly appreciated. 
> > 
> > Please reply to KD6ZZ@KWest.net.
> > 
> > Thanks and Happy New Year
> > 
> > 73,
> > Steve - KD6ZZ
> > 
> > ___________________________________________________________________
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