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July 22nd 08, 06:18 PM posted to rec.radio.amateur.antenna

Richard Clark Richard Clark is offline

Richard Clark is offline

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coax filter dilemma

On Tue, 22 Jul 2008 09:04:23 -0700 (PDT),
wrote:

Open circuit stubs behave very unpredictably due to their sensitivity to their
environment. They radiate.

Yes, I considered that, however shorted stubs mean 1/2 wl at 98 MHz or
so, and they become more than 1m long, making the filter not that easy
to fit under the antenna,

Hi Francesco,

The point made about unpredictable open ends is found in their being
prone to common mode currents. You fix this through choking. Choking
can be achieved through coiling the stub. One, in a classical sense
(usually arrived at through examples oriented in microwave), does not
think of stubs coiled up; but coiling is a suitable, physical and
electrical solution. However, the open end is still prone to
environmental issues (moisture, bugs....). A shorted stub is
self-shielding, keeps out the environment, and coiling it up removes
the "too long" objection.

73's
Richard Clark, KB7QHC

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