[CQ-Contest] Bandpass filter

Tom W8JI w8ji at w8ji.com
Sat Jul 7 07:06:44 PDT 2012

> 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.

Here is the problem, although appearently it is difficult to get across. :-)
Filters do not dissipate out-of-band power, they reflect it. Stress on the 
filter will vary with filter placement on the line, and distance (because of 
standing waves) from the source. The antenna is the source, the receiver end 
the load, in the case of split feedline antennas.

If the element is mistuned based on a good choice of impedance reflected 
back to the element, coupled power is reduced. If impedance reflected back 
to the out-of-band element enhances coupling, power can be far worse than 
the 50 ohm measurement case.

This is probably esoteric, but there will be bad spots and sweet spots in 
the system where a capacitor's voltage rating or current rating might be 

The REAL problem is, based on the cheap filters I have seen, is the 
capacitors are just sized far too small. In filters I have seen, capacitors 
are barely large enough to last into 50 ohm loads at rated power. They have 
no current headroom, or voltage, and often are types with low Q in the 

I added this section to a web page on coupled power:


look at levels down the page under:

Coupled Power, very close-spaced elements, different bands

Maybe that will help make my point that most (all that I have seen) articles 
and basic understandings are incomplete or incorrect, and some are outright 
wrong. Some say stub (or filter) location does not matter!

I would NOT put a reject stub or filter right at the output port of an 
amplifier, I'd put it just under 1/4 wave away **on the reject band** for 
most amplifiers. I'd put it at the antenna, and/or plan the impedance the 
antenna sees at the stub, if I wanted to protect gear.

We can brute force things, or we can plan and understand better. Either one 
solves the problem, but understanding how it really works and what really 
causes the issue is probably better for the community as a whole. Even if we 
can't have a universal recipe, we could at least stop propagating the silly 
ideas that all sources and loads are 50 j0 out-of-band, and filters absorb 
and/or dissipate a significant part of out-of-band energy.

73 Tom 

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