[CQ-Contest] 10/15 interaction

[Dave Hachadorian]

The problem is that too much 10M energy is coming down the 15M 
feedline.  Adding additional filtering to the 10M transmitter 
won't solve that problem.  What you need is more filtering on the 
15M feedline to reject the 10M RF.  The first thing I would try 
is a coax stub on the 15M feedline.  Try a 1/4 wave 40M shorted 
stub, about 23' long, made from RG-213.  A stub like that will 
pass 40 and 15, and will reject 20 and 10.  Tune it with a GDO, 
optimizing it for 10M.

I have a Ten-Tec amplifier whose QSK circuit trips when another 
amp is transmitting on 20, and that exact stub cured the problem. 
That amp is only used on 15 and 40, so the stub works out fine.

It is also possible that the length of the feedline on the 15 
just happens to tune the 15 driven element to 10 meters. Try 
adding 1/8 wavelength (at 28MHz) of feedline to the 15M feedline. 
That's about 4 feet.

You could also buy another 15 meter receive filter, and cascade 
it with the other one.  I conducted such an experiment one time, 
and the additional rejection was significant, but not 2X.  As I 
remember, it was about 1.5X.

Dave Hachadorian, K6LL
Yuma, AZ


-----Original Message----- 
From: Randall K Martin
Sent: Thursday, November 6, 2014 8:18 AM
To: cq-contest at contesting.com
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.

73
Randy K0EU



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