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Re: [TowerTalk] Using Stubs to Reject Harmonics

To:, "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
From: "Rick Karlquist" <>
Reply-to:, "Tower and HF antenna construction topics." <>
Date: Tue, 2 Feb 2010 12:04:26 -0800
List-post: <">>
Tim Duffy K3LR wrote:
> Hi Jim:
> I always wanted to test the 1/4 wave "insertion" theory, but never got
> around to it. I would love to hear what the difference is - if you can
> experiment.
> 73,
> Tim K3LR
>> W8JI:  The best place to put a hi-reject stub is exactly a 1/4 wl from the
> source, if the source has a low pass filter in the output (like an
> ampolifier).
> Thereason is a shorted stub is a low impedance, if you just place it
> across
> the
> amp output the low shunt Z of the stub barely improves the bypassing. If
> the

What Tom says is right on the mark, as long as the last component in
your amplifier is a capacitor to ground, which is probably the case
90+% of the time.  In Jim's case, the last component is a series
inductor, in which case Tom's advice needs to be modified to
placing the stub at the amp, or a multiple of a halfwave away.
All of this confusion is yet another one of many reasons, IMHO, to build
filters out of lumped components instead of coax stubs.  And
the filters should be elliptic function low pass filters, not bandstop
filters.  For examples of these filters, study the output filters
in any solid state transmitter.  For example, see pages 3 and 4 of:

I have been using this linear on the air for the last 10 years.
It is so clean harmonic wise, I didn't need a low pass filter on
it for TVI purposes back in the analog TV era (we have no low VHF
stations in our area now).

Rick N6RK


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