[Amps] placement of RF choke bypass cap.

[Gary Schafer]

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Bill, W6WRT
> Sent: Friday, March 19, 2010 10:20 PM
> To: amps at contesting.com
> Subject: Re: [Amps] placement of RF choke bypass cap.
> 
> ORIGINAL MESSAGE:
> 
> On Fri, 19 Mar 2010 20:10:48 -0400, "Gary Schafer"
> <garyschafer at comcast.net> wrote:
> 
> >
> >Yes that's right. You can solder a wire to each side of a sheet of
> copper
> >and there will be no current passed thru the copper sheet to the wire
> on the
> >other side. You can even have a solid piece of wire going thru the
> copper
> >sheet and if you solder it to the sheet on each side, nothing gets
> thru.
> >Same for a screw going thru the sheet. If it is grounded all around the
> >head, as it usually is, then nothing gets thru. The surface currents
> act on
> >the head of the screw just like they do on the flat surface. They
> cancel
> >currents deeper below the surface. This is "classic skin effect" at
> work.
> 
> REPLY:
> 
> Then how does a coax connector pass RF through a chassis? We can agree
> the center (insulated) conductor is no problem, but how does the
> grounded shield side pass RF through the chassis? According to your
> theory, the skin effect should stop it dead but we all know it does
> not.
> 
> And what about the ground lug on the amplifier chassis? Some guys run
> a long wire directly to center pin of the RF output connector and
> connect a ground wire to the ground lug for the other side of the
> antenna. According to your theory, no RF should be getting through the
> chassis to the ground lug, but it does. How?
> 
> 73, Bill W6WRT

Any time you run an insulated wire thru a chassis wall/plate you have
created a coaxial line.
Just like when you stick an insulated wire thru the wall of a screen room
you get all kinds of coupling. 
The wire or center of the coax is one lead and the inner side of the coax is
the other lead. If you don't have a shield but just the center conductor the
outside of the chassis becomes the return path and that path follows thru
the hole where the wire came out of the chassis and around to the inside of
the chassis.

When you mount your padder caps below the chassis you run a wire thru the
chassis to connect to the hot side of them. The return current follows back
thru the hole where the wire came thru.

Without the wire thru the hole there is little coupling thru the small hole
as it is such a small part of a wavelength. But with the wire thru it acts
like a coax line would.

73
Gary  K4FMX