[RFI] Tower/Antenna Reradiation

[Steve London]

I have had a few instances of this problem lately. One was caused by an old
thrust bearing. The solution was a stout jumper from the mast to the tower,
as suggested by K3LR. The other was caused by a MinkaAire ceiling fan/light
controller .Solved by turning the on/off switch to off.

73,
Steve, N2IC

On Mon, Dec 6, 2021 at 8:15 AM Alan Higbie <alan.higbie at gmail.com> wrote:

> A few other troubleshooting ideas:
>
> (1) When the condition is happening . . . one-by-one, disconnect (and
> ground) each control line or device that is present on that tower.
>
> (2) Similarly, and again one-by-one, switch off or disconnect each
> non-essential device in the area.
>
> This process is what eventually led a local M/M station to find that he had
> inter-mod products being generated in one of his garage door openers.
>
> (3) The placement of the stubs may also be a factor.
> See K9YC's presentation on that topic:
> http://audiosystemsgroup.com/StubPlacement.pdf
>
> Please keep us posted on the progress you and Don make on this problem.
>
> 73,
>
> ~Alan K0AV
>
>
>
> On Sun, Dec 5, 2021 at 6:34 PM Tim Duffy <k3lr at k3lr.com> wrote:
>
> > Hi Mike,
> >
> > Several ideas.
> >
> > Your coax chokes (known as ugly baluns - two reasons - #1 they look ugly
> -
> > #2 they don't work very well) - have very small CMI - I suggest much
> better
> > chokes
> >
> > You need bypass low Z conductors (thick and short) across your rotation
> > systems (all of them). Bearings have dissimilar metals - a breeding
> ground
> > for harmonics.
> >
> > I have been down these same roads and I have solved these issues with
> > bonding and high CMI chokes.
> >
> > 73
> > Tim K3LR
> >
> > -----Original Message-----
> > From: RFI [mailto:rfi-bounces+k3lr=k3lr.com at contesting.com] On Behalf Of
> > Mike Wetzel
> > Sent: Sunday, December 5, 2021 7:47 PM
> > To: rfi at contesting.com
> > Subject: [RFI] Tower/Antenna Reradiation
> >
> > 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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