[Amps] wires through center of coax

[Fuqua, Bill L]

    For the same reasons but in revers it works for 1/4 wave resonators in amplifiers and such but it is turned around. It works as a choke to balance currents on antennas.     This is a case where there is no RF voltage between the wire and the center conductor. If there is a RF voltage difference at the entry point or exit point you at best create parallel coax cables which will change the characteristic impedance of seen by both the load and the source. 
    Worst case there will be an abrupt discontinuity in impedance at the ends or depending on the dielectric of the wire and if it stays centered or wanders around inside the hollow center conductor have something that absorbs RF energy, changing impedance along the length, arcing etc.
     Speaking of lightning protection
      When I was young (around 1952) my father had a mechanics shop a few hundred feet from our house. The house and shop were on the same electric service entrance. He got a licensed radio system (around 40MHz) to contact his guys in the tow trucks.  He had 2 trucks at that time.  The tower was quite tall and had a folded vertical antenna on top.  Lightning hit the tower one night and I remember just a flash and bang.  The shop and house wiring was fried. There was a burnt spot in the wiring every few feet in all the house wiring. He had to replace all the outlets, switches and wires in the house. It seems that there was an resonance somewhere in the system that sent a wave down the wires and created standing waves that did all this damage. Sounds kind of like a old spark transmitter with a hell of a spark gap.
73
Bill wa4lav


________________________________________
From: John Lyles [jtml at losalamos.com]
Sent: Wednesday, December 22, 2010 3:17 AM
To: amps at contesting.com; Fuqua, Bill L; sub1 at rogerhalstead.com
Subject: wires through center of coax

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 at uky.edu>
> Subject: Re: [Amps] wires through center of coax
> To: "Roger (K8RI)"<sub1 at rogerhalstead.com>, "amps at contesting.com"
>       <amps at 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

>