In the dummy load case you might be seeing some leakage through the cases or
back through the power supply cable. try putting a dummy load on the 20m rx
also and maybe add some chokes to the power cables. If you are running both
of them off one power supply try using a separate supply.
On the very strong signal that isn't suppressed by adding 40m stubs, it is
likely you have a source outside of the shack. Anything outside that acts
as a rectifier. Any rusted joints, bad connections, or even diodes in other
equipment like swr meters or remote switches can create the second harmonic
that cannot be filtered out inside. There are odd things that can do this,
just recently I found some wall warts that were picking up rf from one band,
modulating it very wide, and reradiating on another band. I have had swr
meters where the 12v input for reading pep was rectifying. I have had just
a piece of unbonded hardline shield at the top of my 20m tower was creating
a 40m subharmonic and 15m 1.5*f harmonic. The possibilities are endless and
depend on what is near enough to pick up enough 40m power to create the
harmonic.
It is possible to clean it all up, but it may take some time. after years
of working at it here a 50w cw signal on 40m is only about s3 on 20m and
can't be heard 5khz from the exact frequency.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373
-----Original Message-----
From: RFI [mailto:rfi-bounces@contesting.com] On Behalf Of Luca NCJ
Sent: Wednesday, September 13, 2017 10:28
To: rfi@contesting.com
Subject: [RFI] SO2R and 2nd harmonic
Hi
I am strugling with a 2nd harmonic problem into a SO2R set-up.
Set-up is composed by 2 x K3 controlled by MK2R+ machine, W3NQN BP filters
on each radio and monobanders on each band.
Interference problems are from 40m to 20m (2nd H) and this rise up floor
noise level about 60 khz on each side from main harmonic frequency.
On elecraft panadapter the scope shows on 20m that signal is not clean and
it has some more peaks over the band.
I also changed one of the K3's with a different one but no difference.
I have added a double stub W2JVN design on 40M to suppress the 2nd H on 20M
so now I have BPF + stubs.
MiniVNA says the stubs suppression is -65db and the stubs are placed
directly on the 6x2 switch at the 40M output but no reduction of
interference is shown.
Looking at the 20M S-meter it shows 59+30 signal and disconnecting and
reconnecting the stubs there is no difference on the S-meter.
So my first impression is that the signal on 20M is not received from the
antenna outside.
At this point I disconnected all cables connected to radios (except 12V of
course) leaving 20m radio without antenna even.
Now I connected a dummy load to radio on 40M and started to TX with 50W.
Still 20m 2nd harmonic is audible and holding the dummy load by hands the
2nd H intensity is reduced.
What is your opinion on this situation and how to solve?
Tnx in advance.
**** Luca IK2NCJ ****
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