I had this situation Mike (probably forever) but only noticed it when I
went to try SO2R some years back.
I don't run any bandpass filters, but rather do my isolation with
physical antenna distancing and opposite polarization of antennas.
(ie: all radio one are on verts/vertical arrays and all radio two are on
a single horizontal antenna), spread over 2.2ac
When I installed stubs to try to rid myself of the 'growl' it made
virtually no difference......I think that was my clue. Issue was in the
shack.
Turned out to be a switching power supply. (wall wart) I wasn't getting
any 'noise' from the power supply as my antennas are a fair distance
from
the shack, but evidentally something in the power supply was
re-radiating my 40m RF and making it sound like a growly bear on 20m.
When I swapped it out, all the growl from 40m>20m went away and I was
left with the "normal" T9 harmonics.
GL Mike VE9AA
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
</pre>
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