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RF on pipes

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Clint-2

RF on pipes

This is a bit OT but it does concern my K3. Hopefully this will not open too big of a can-o-worms.

As an experiment to try and achieve a very low impedance ground system, I am using 1 inch rigid copper tubing for my station and Vertical Antenna grounds and ½" copper refer tubing to tie together my ground rods and the mains ground.

ANYWAY, I know that RF "travels" on the outside skin of a conductor. My question is: Does it also travel on the inside skin? In other words, would the impedance be the same on a round hollow tube as it would be if the tube was slit and laid out flat?

Thanks,

Clint KI6SSN
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k6dgw

Re: RF on pipes

On 7/9/2011 10:02 AM, Clint wrote:
>
> ANYWAY, I know that RF "travels" on the outside skin of a conductor.
> My question is: Does it also travel on the inside skin?

Only if you "inject it there [e.g. a copper plug into the end of the
pipe that connects with the inside only]. Similar to coax which
actually exhibits 3 conductors ... the center, the inside of the shield,
and the outside of the shield. We suppress currents on the outside with
baluns at the antenna feedpoint.

A small amount will of current will appear on the inside of the pipe due
to inevitable stray couplings even if you "inject" the RF onto the
outside only.

> In other
> words, would the impedance be the same on a round hollow tube as it
> would be if the tube was slit and laid out flat?

No. A flat strip has it's own impedance characteristics.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2011 Cal QSO Party 1-2 Oct 2011
- www.cqp.org
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Geoffrey Mackenzie-Kennedy

Re: RF on pipes

Evening Fred,

If the pipe is OPEN at its ends, would not the current flowing on the
outside of the pipe "fold over" at the end of the pipe and continue to flow
on the inside skin? The reverse, if you like, of the situation where a coax
feeder is used to feed a dipole / doublet.

73,
Geoff
GM4ESD

On July 09, 2011 at 18:17 Z, Fred Jensen wrote:

> On 7/9/2011 10:02 AM, Clint wrote:
>>
>> ANYWAY, I know that RF "travels" on the outside skin of a conductor.
>> My question is: Does it also travel on the inside skin?
>
> Only if you "inject it there [e.g. a copper plug into the end of the
> pipe that connects with the inside only]. Similar to coax which
> actually exhibits 3 conductors ... the center, the inside of the shield,
> and the outside of the shield. We suppress currents on the outside with
> baluns at the antenna feedpoint.
>
> A small amount will of current will appear on the inside of the pipe due
> to inevitable stray couplings even if you "inject" the RF onto the
> outside only.

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gm3sek

Re: RF on pipes

Geoffrey Mackenzie-Kennedy wrote:
>Evening Fred,
>
>If the pipe is OPEN at its ends, would not the current flowing on the
>outside of the pipe "fold over" at the end of the pipe and continue to
>flow on the inside skin? The reverse, if you like, of the situation
>where a coax feeder is used to feed a dipole / doublet.
>

No - HF fields cannot propagate inside an open tube because it behaves
as like a 'waveguide beyond cutoff'. Any HF EM field injected into the
open end will only penetrate a very short distance inside before it
decays away.

Coax is different because it contains *two* conductors: the inner skin
of the tube and the outer skin of the centre conductor. The internal
field is created between these two conductors and is thus able to
propagate down the line.

So the answer to the OP's question is that tubing will only give him the
benefit of the larger outside skin.

--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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Geoffrey Mackenzie-Kennedy

Re: RF on pipes

Point taken Ian, grey cells in reverse gear here :-)

73,
Geoff
GM4ESD

On July 09, 2011 at 19:17Z, Ian White GM3SEK wrote:

> No - HF fields cannot propagate inside an open tube because it behaves
> as like a 'waveguide beyond cutoff'. Any HF EM field injected into the
> open end will only penetrate a very short distance inside before it
> decays away.
>
> Coax is different because it contains *two* conductors: the inner skin
> of the tube and the outer skin of the centre conductor. The internal
> field is created between these two conductors and is thus able to
> propagate down the line.
>
> So the answer to the OP's question is that tubing will only give him the
> benefit of the larger outside skin.

Jim Dunstan-2

Re: RF on pipes

In reply to this post by Clint-2

Yes, as in coax for an example. when transmitting eg via coaxial cable which is properly installed the rf currents flow on the inside surface and not on the outside surface. I mention proper installation .. In which case steps are taken to keep the inside and outside surfaces separate (at rf). In your example, plain copper pipe wil have a total surface of the inside + outside. If you solder on caps then you will have just the outside.

Jim
VE3CI

Sent from my iPod

On Jul 9, 2011, at 1:02 PM, "Clint" <[hidden email]> wrote:

> This is a bit OT but it does concern my K3. Hopefully this will not open too big of a can-o-worms.
>
> As an experiment to try and achieve a very low impedance ground system, I am using 1 inch rigid copper tubing for my station and Vertical Antenna grounds and ½" copper refer tubing to tie together my ground rods and the mains ground.
>
> ANYWAY, I know that RF "travels" on the outside skin of a conductor. My question is: Does it also travel on the inside skin? In other words, would the impedance be the same on a round hollow tube as it would be if the tube was slit and laid out flat?
>
>
>
> Thanks,
>
>
>
> Clint KI6SSN
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

Nick Kennedy

Re: RF on pipes

A hollow conductor is not analogous to a coaxial cable. It has no
center conductor. In the case of the hollow conductor (copper pipe),
current will flow on the outside surface only. Skin effect still
applies and works exactly as if the conductor were solid.

73-

Nick, WA5BDU

On 7/9/2011 4:51 PM, Jim Dunstan wrote:
> Yes, as in coax for an example. when transmitting eg via coaxial cable which is properly installed the rf currents flow on the inside surface and not on the outside surface. I mention proper installation .. In which case steps are taken to keep the inside and outside surfaces separate (at rf). In your example, plain copper pipe wil have a total surface of the inside + outside. If you solder on caps then you will have just the outside.
>
> Jim
> VE3CI
>
> Sent from my iPod
>

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