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@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@contesting.com [amps-bounces@contesting.com] On Behalf Of
Fuqua, Bill L [wlfuqu00@uky.edu]
Sent: Monday, December 20, 2010 4:53 PM
To: Jim Thomson; amps@contesting.com
Subject: Re: [Amps] wires through center of coax
Under the right conditions it can be done.
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Jim Thomson [jim.thom@telus.net]
Sent: Monday, December 20, 2010 11:04 AM
To: amps@contesting.com
Subject: [Amps] wires through center of coax
Date: Mon, 20 Dec 2010 00:12:23 -0700
From: John Lyles <jtml@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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