[Amps] wires through center of coax

Gary Schafer garyschafer at comcast.net
Mon Dec 20 16:53:33 PST 2010


A hollow tube (the inside of the center conductor) will act as a waveguide
beyond cutoff and will not propagate energy. But the minute you place a wire
inside that tube it then acts like a transmission line. 

If that wire in the center and the inside of the tube can be isolated from
the energy on the outside of the tube at each end then no energy will
propagate inside the center. 
Either way it should have no effect on the intended transmission line
impedance (outside of center tube and shield of the coax) as long as its
input and output are isolated.

There can be some coupling to the inside wire in the center of the tube via
the transfer impedance of the coax center conductor. This is the result of
the resistance over its length. But that coupling should normally be very
small but maybe not so small in John's case with the super high power.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Fuqua, Bill L
> Sent: Monday, December 20, 2010 5:21 PM
> To: Jim Thomson; amps at contesting.com
> Subject: Re: [Amps] wires through center of coax
> 
> It just occurred:
> 
>      The characteristic impedance of a transmission line is equal the
> square root of the unit length inductance divided by the unit length
> capacitance.
> By adding a conductor on the inside of the hollow center conductor that is
> not at the same potential as the holllow center conductor you change the
> unit length capacitance.
>      If the wire is at the same potential as the shield the character
> impedance will be lowered. That does not necessary make it a bad
> transmission line but
> one of a different impedance.
> 
> 73
> Bill wa4lav
> 
> ________________________________________
> From: Fuqua, Bill L
> Sent: Monday, December 20, 2010 5:07 PM
> To: Fuqua, Bill L; Jim Thomson; amps at contesting.com
> Subject: RE: [Amps] wires through center of coax
> 
> If the potentials on the wire as it enters and exits the hollow center
> conductor maintained the same as that on the center conductor and it may
> also be necessary to have the same velocity factor for the wire going thru
> the center conductor as well.
>   Use of chokes on both ends of the wires and having the came dielectric
> constant between the wire and inside of the center conductor as you have
> between the center conductor and outer one.
> 73
> bill wa4lav
> 
> ________________________________________
> From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf
> Of Fuqua, Bill L [wlfuqu00 at uky.edu]
> Sent: Monday, December 20, 2010 4:53 PM
> To: Jim Thomson; amps at contesting.com
> Subject: Re: [Amps] wires through center of coax
> 
> Under the right conditions it can be done.
> ________________________________________
> From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf
> Of Jim Thomson [jim.thom at telus.net]
> Sent: Monday, December 20, 2010 11:04 AM
> To: amps at contesting.com
> Subject: [Amps] wires through center of coax
> 
> Date: Mon, 20 Dec 2010 00:12:23 -0700
> From: John Lyles <jtml at losalamos.com>
> Subject: [Amps] wires through center of coax
> 
> Bill - WA4LAV
> I hope that running wires through coax center conductor like this won't
> cause problems because I am doing it with the 9 3/16 inch diameter
> output feeder of a new amplifier I have been working on. I have a
> mechanical linkage going through the middler of the center conductor to
> the tuning paddle in the cavity amplifier. Also, have a RTD applied to
> the back of the paddle (inside) to measure temperature when it is
> running. The linkage and wires run through a 1/4 wave stub into the 3
> megawatt PA.
> 
> 73
> John
> K5PRO
> 
> ###  Running compressed, dry air, < 5% RH  [ typ 2-3 psi]   up through the
> center conductor has been done for a long time..and ditto  with the cavity
> between the outer and inner conductor's.  U have to..or u will get
> condensation inside em...
> esp  when used outdoors.. for broadcast use  up tall tower's.     Running
> wires inside the center conductor should not be a problem... Until   the
> wire exits out either end... or through the sides  via a special fitting.
> 
> Jim  VE7RF
> 
> 
> 
> >      I give up. I think most everyone else gets it. It is like a
> conversation I had with some old hams, not much older than I am aboiut
> some large coax.
> > They were convinced that since the center conductor was hollow you could
> run wires up thru it to carry current to lights and rotators without
> affecting the
> > impedance of the coax. Their argument was that current only  flows on
> the outside of the conductors. But that is not always true.
> > Current flows on the surfaces and in the case of a hollow conductor it
> is true it will flow on the outside as long there is nothing to electric
> > create fields on the inside. There are electric/magnetic fields between
> the outside of the inner conductor and the inside of the outer conductor.
> But once you put a conductor on the inside of the hollow inner conductor
> you have created a new bit of transmission line, there are fields now
> between the new conductors and the inside on the inner conductor. That
> changes everything.
> >
> > 73
> > Bill wa4lav
> >
> 
> 
> 
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