[RFI] SO2R and 2nd harmonic

Thu Sep 14 13:13:14 EDT 2017

Hi Luca, 

Dave's reply is the only one that correctly addresses the problem 
you described. You described your problem as " rise up floor noise 
level about 60 khz on each side from main harmonic frequency." 

Many contest stations -- including mine -- experience the exact 
problem you describe, it has a name: "RF Flooding" Its caused 
when a powerful RF signal penetrates a non-linear device that has: 
- AC power or digital signals flowing though it, and 
- is connected to wires that receive the powerful fundamental RF 
signal and re-radiate broadband modulated harmonic RFI. 

The solution is similar to the process for locating and correcting 
any other RFI problem, except you must provide a strong RF 
signal to excite the harmonic RFI from the non-linear devices 
(its likely to be more than one device). Wall warts and ham 
radio accessories (rotator controllers and any other device with 
a small power supply especially cell phone chargers) are very likely 
RF Flooding RFI sources in your own home 

You'll find that broadband harmonic RFI varies in intensity as you 
rotate your directive transmitting and receiving antennas. I've found 
it very helpful to use an Elecraft K3 with P3 panadapter to document 
the broadband harmonic RFI and to determine with direction(s) the 
RFI comes from. 

A portable HF receiver such as an FT-817 can be very helpful in isolating 
the RFI source. As with any other RFI hunting, start in your own home 
by turning off all of the circuit breakers except to your transceiver and 
amplifier. 

As Dave said, this is likely to take some time, especially if the RFI 
sources are outside your own home. 

73 
Frank 
W3LPL 

----- Original Message -----

From: "David Robbins" <k1ttt at verizon.net> 
To: rfi at contesting.com 
Sent: Wednesday, September 13, 2017 11:27:44 AM 
Subject: Re: [RFI] SO2R and 2nd harmonic 

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 at arrl.net 
web: http://wiki.k1ttt.net 
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373 

-----Original Message----- 
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Luca NCJ 
Sent: Wednesday, September 13, 2017 10:28 
To: rfi at 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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