[Amps] placement of RF choke bypass cap.

[Roger]

Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
>
> On Fri, 19 Mar 2010 14:41:00 -0400, "Gary Schafer"
> <garyschafer at comcast.net> wrote:
>
>   
>> There is no way for current to enter the center part of a tube or rod or
>> flat sheet.
>>     
>
> REPLY:
>
> Imagine a large flat sheet of copper. You solder a wire in the center
> of it, and another wire on the other side, directly opposite the first
> one. Are you telling me that RF will not pass from one wire, through
> the sheet into the other wire?  Or how about if the two wires were not
> directly opposite each other, but spaced a few inches apart. Still no
> current between them? Don't be silly.
>   
Having spent the majority of my working years as an instrument tech 
(before quitting and going back to college)  and much of that doing 
calibration and development of standards I spent a good many hours 
working inside a screen room as my location was only a 100 to 300 yards 
from a lot of very high powered RF equipment.

I can say with certainty that an RF signal from VLF through UHF applied 
to the inside of that wall, did not make it to the outside of that wall.

We even set up transmitters inside and outside the room close enough 
that the operator and I could talk directly.  After all we were only 3' 
apart, but a sensitive receiver inside the screen room could not detect 
the signal from the outside antenna even though both antennas were 
parallel and about 2' apart nor could the one outside detect the signal 
from the one inside.
> If what you say was true, all our radios and amps would not work.
>   
The RF doesn't have to go through the metal to get to the other side. 
It's almost impossible to build a circuit that is totally shielded using 
standard construction techniques. There are many routes a signal can 
normally take to get to the other side without going directly through a 
metal plate.
> There are dozens if not hundreds of places where RF passes through
> sheet metal from one side to the other, such as through the shield
> side of a coax connector.
>   
The RF gets from one side to the other any where it finds a 
discontinuity.  IE, A hole, seam, screw, feed through, or it takes the 
long way around the edges. Normally for efficient transfer from one side 
to the other we need to provide a path other than the long way around.
This is the reason the handbook says to mount the bypass cap as close to 
the base of the plate choke as possible. The farther you move from the 
plate choke, the more inductance is added to the circuit and the lower 
the resonant frequency of the combination.  The caps on the back side of 
the chassis will work, they just aren't as efficient.  The RF will get 
there even if it has to go all the way around the chassis, to the screw 
holes, folded seam, tube socket, or anything that can carry a current.

At least that is my understanding of the situation.

73

Roger (K8RI)


> 73, Bill W6WRT
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