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[TowerTalk] RE: [CQ-Contest] dual bandpass filter experiment

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Subject: [TowerTalk] RE: [CQ-Contest] dual bandpass filter experiment
From: W4EF@pacbell.net (Michael Tope)
Date: Wed, 7 Jul 1999 15:02:03 +0100
A good example of what Scott is referring to can be seen
by imagining a pair of simple bandpass filters each of which
is nothing more than a single parallel LC tank. If you 
imagine that these two filters are cascaded with a very short
transmission line, then one can see that they form an equivalent
tank which has twice the C and 1/2 the L of the original tank.
This "equivalent" tank will have twice the loaded Q of the original
tank and hence about 1/2 the effective 3dB bandwidth, but it still
has only one pair of dominant poles.

Mike, W4EF....... 

----------
From:   Scott Ellington[SMTP:sdelling@facstaff.wisc.edu]
Sent:   Tuesday, July 06, 1999 9:18 PM
To:     cq-contest@contesting.com
Subject:        Re: [CQ-Contest] dual bandpass filter experiment



George Cutsogeorge wrote:
> 

> There is another factor which has not been mentioned.  While
> these filters may be rated for 100 or so watts in the passband
> where the attenuation is a fraction of a dB, they do not have a
> rating for the stop band.  So assume 1/2 dB loss at 100 watts and
> we get 11 watts in the filter.  I would kind of think this level
> would be the maximum one would want to put into the filter in the
> stopband as it all gets dissipated inside the unit. 
> 

Most filters, including the Dunestar and ICE, do not ABSORB power outside the
passband, they REFLECT it.  (Absorbtive filters do exist, but they are more
complicated.)  So, dissipation in the filter outside the passband usually
isn't an issue.  It's possible, though, for the impedance of the filter and
antenna to interact in a way that could result in a resonance, which might
produce large voltages and currents in some filter components.  Two cascaded
filters could also interact with each other in a similar way.  However, such a
resonance is pretty unlikely, and impossible to predict unless you know all
the filter component values plus the source and load impedances.  One could
simply measure the voltage at the filter input, however, and make sure it's 
reasonable.

By the way, cascading two filters is not the same as doubling the number of
poles in the filter.  This is, in part, because each filter is designed to
work with a 50 Ohm source and load, and neither looks like 50 Ohms outside the
passband where they are connected together.  Adding more poles gets to be a
problem, because the more poles the filter has, the higher the Q some of them
have.  This results in higher insertion loss and more critical tuning.  Using
receive-only filters may allow you to use more poles, and hence better
adjacent band rejection.

73,

Scott  K9MA

-- 


Scott Ellington
sdelling@facstaff.wisc.edu
Madison, Wisconsin    USA


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