Use of a quarter wave stub is common in fixed-frequency high power work
to introduce mechanical linkages or electrical circuits that need to be
inside the center conductor of a coaxial transmission line. As long as
it is designed to be very close to lambda/4 in length, it should be
transparent to the RF travelling in the coax. I have done this at 2
meters and at 462 Mhz using RG8 to put a DC short on radios to limit
lightning damage on the monopole antenna. This quarter wave is a half
wave for the second harmonic, which would introduce a short across the
coax for this frequency. So there is a benefit in that the even
harmonics get suppressed even more.
In actuality, for large diameter transmission lines where the diameter
is a large fraction of wavelength (like 9 3/16 inch OD) the measured
quarter wave and measured half wave do not agree exactly. We proved this
last summer with both network analyzer sweeps as well as modeling with
CST Microwave Studio. The result of this is that there is some small
modification of the plate current waveform by this stub, but it is still
small enough to ignore, a few kW out of MW. And of course, stubs are not
so good when freq must change, unless special care is taken to broadband
the stub with some matching at the Tee. This is covered in the
literature but not so easy to practice.
In the "olden" days, quarter wave stubs were referred to as 'metallic
insulators' for supporting the center conductor of coax or open wire
transmission lines. I am using the stub to introduce a tuner (paddle)
into my cavity amplifier, which allows me to adjust loading (coupling)
of the output circuit while under power. Two methods work, sliding
contacts and bellows for the center conductor extension. I chose
bellows, but am nervous enough to now install an RTD to the inside to
measure temperature when I am operating at > 300 kW average power. At
VHF the skin effect helps but there is also series inductance of the
bellows convolutions.
> Message: 3
> Date: Mon, 20 Dec 2010 23:16:00 -0500
> From: "Fuqua, Bill L"<wlfuqu00@uky.edu>
> Subject: Re: [Amps] wires through center of coax
> To: "Roger (K8RI)"<sub1@rogerhalstead.com>, "amps@contesting.com"
> <amps@contesting.com>
> Question, how do you place a wire in an hollow conductor with out creating a
> path for RF current on the outside to the inside?
>
> 73
> Bill wa4lav
>
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