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@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@contesting.com] On Behalf Of
> Mike Wetzel
> Sent: Sunday, December 5, 2021 7:47 PM
> To: rfi@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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