[CQ-Contest] Bandpass filter
Guy Olinger K2AV
olinger at bellsouth.net
Sat Jul 7 14:17:11 PDT 2012
The W3NJN QRO filters certainly do not have any dippy-dink caps, all
appropriately sized. Cost more money, but in line after the amps they
simply killed the problem. Worth every bit of the price. Definitely
not worth getting into the esoterics and trying to come up with
something that will do it for less. Just shoot the thing with the
filter equivalent of a double-barreled shotgun and be done with it.
On Sat, Jul 7, 2012 at 10:06 AM, Tom W8JI <w8ji at w8ji.com> wrote:
>> 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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