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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