[CQ-Contest] 10/15 interaction

donovanf at starpower.net donovanf at starpower.net
Thu Nov 6 13:29:47 EST 2014

Hi Randy, 

High power bandpass filters are definitely NOT an appropriate 
solution to your 15 meter receiver overload problem. 

The optimum solution is probably a 200 watt W3NQN monoband 
bandpass filter inserted between your transceiver and your amplifier. 
W3NQN's filters are highly optimized for rejection of signals from 
adjacent bands. 

You can find the filter performance data here: 



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

From: "Randall K Martin" <rkmassoc at comcast.net> 
To: cq-contest at contesting.com 
Sent: Thursday, November 6, 2014 3:18:35 PM 
Subject: [CQ-Contest] 10/15 interaction 

Over the years, I've managed to clean up most of the cross band 
interference that occurs when I operate my SO2R station running high 
power. The one exception is that I cannot use my 10M monobander to 
transmit while receiving on my 15M monobander. This is not surprising, 
since they are only separated vertically by about 8 feet. Transmitting 
on 10M must be dumping significant power back into the 15M transmitter 
(an elecraft K-3) since it causes the COR relays to click and I get the 
"HI RFI" error. When this happens, I immediately stop transmitting and 
flip the 15M rig over to a tribander on a second tower. Normally, I'm 
able to remember not to do this, but in the heat of a contest, I forget. 
What I find odd is that I can transmit on the 15M monobander while the 
second radio is connected to the 10M monobander and there is no "HI RFI" 
problem. In fact, very little interference. 

Here's my question. I'm contemplating purchasing a high power band pass 
filter (4O3A) to prevent the interaction. I'd prefer not to buy both a 
10M and a 15M filter if I don't have to. My intuition tells me that I 
should be able to add the filter to the 10M transmitter. Any thoughts on 
this? Is there some other path in which the RF could be reaching the 15M 
rig that would render the band pass filters ineffective? 

I have dunestar 100W band pass filters on both rigs, placed between the 
rig and the amp. I know that adding separation between the monobanders 
would help, but my tower configuration isn't really amenable to adding 
more separation. 

Randy K0EU 

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