Coaxial cable depends on an electromagnetic field whose end point are on the
inside of the shield and the outside of the center conductor.
For that matter, it's the same effect that keeps a parallel conductor
transmission line from radiating.
This effect holds true at all frequencies, so that in theory transmission
line- per definition- have infinite bandwidth.
What does cause a cutoff effect is the loses- they increase as the square of
frequency- and the consist of dielectric losses and Ohmic losses due to skin
effect.
If you check at a low enough frequency, you will observe an increase in
leaked radiation, due partly to punch over of internal energy through the
too thin external shield. That's one of the reasons the make double shielded
cable, not only to reduce high frequency leakage.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Bill, W6WRT
Sent: Wednesday, March 24, 2010 12:46 AM
To: amps@contesting.com
Subject: Re: [Amps] placement of RF choke bypass cap.
ORIGINAL MESSAGE:
On Tue, 23 Mar 2010 16:09:52 -0400, "Gary Schafer"
<garyschafer@comcast.net> wrote:
>Perhaps you could explain to us why coax cable does not allow RF energy
>to move from the inside surface of the shield to the outside surface if
>not for skin effect?
>You would agree that the inside surface of a coax cable carries RF
>current would you not?
REPLY:
Second question first: Yes, I agree. Common knowledge.
First question: Because the electrons on the inside of the shield are
attracted toward the center conductor, which is of the opposite polarity.
You have both skin effect and electric field attraction going on at the same
time.
73, Bill W6WRT
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