[CQ-Contest] Bandpass filter

Guy Olinger K2AV olinger at bellsouth.net
Fri Jul 6 22:53:58 PDT 2012

W8JI wrote:

>> Sorry Keith, the measured isolation between 15m TX and 20m RX on the C31XR
>> with physical separation of only 18" is legitimate.

>It could be much more than that, actually.


Yes, indeed.  Could be and IS.

I've had a C31 for years.  The separate feedline "option" for the C31
is QUITE the stretch. The numbers some previous posters have quoted
are conservative to my mind.  IMHO, the only station-safe
configuration to operate triple feedline C31XR at QRO is to also
spring for the high power NQN bandpass filters for 10/15/20.  That's
because, as others have posted, the typical exciter level bandpass
filter is not designed for 200+ watts out of band, and WILL go bad on
heat buildup in a contest. Only a matter of time before some cap dies.
 You will always lose your run frequency.  We do cause ourselves so
much trouble going on the cheep.  I'm willing to spend money to keep a
run frequency.

We have a set of the QRO NQN filters at NY4A.  We have used them
mainly for multiple large single band multi-element wire antennas
*interlaced* on a single catenary fixed at Europe.  We do wires on
catenaries because of frequent visiting tropical weather in eastern
North Carolina, and wire is oh, so much easier to replace than
aluminum monsters.  The filters, inserted after the amplifiers, simply
made cross coupling (of BOTH fundamental and harmonics) a non-issue.
The 2nd harmonic of 40m on 20 is just another moderately loud signal,
and no crud up or down from it, allowing us to operate up or down 500
Hz from our own harmonic.

We also use the ICE transceiver level filters on the K3's for the
usual needs with antennas that do not have the close coupling.

That's just one problem avoided. Who wants to burn up a friend's K3.
How much is peace of mind worth?

73, Guy.

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