Carl wrote:
>
>
>>
>>
>> Bill, W6WRT wrote:
>>> ORIGINAL MESSAGE:
>>>
>>> On Fri, 19 Mar 2010 14:41:00 -0400, "Gary Schafer"
>>> <garyschafer@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)
>
>
> I agree Roger. Having worked in several screen rooms from National to
> fairly recent, if a signal got in or out it wasnt a screen room.
>
> Some of the rooms were used for the testing of the CIA's Tempest
> program which was concerned about leakage from computer monitors as
> well as the rest of the electronics and cabling.
And the signal levels, voltage levels, and current levels weren't
something you see measured with a normal VOM<:-)) Input (pencil) tubes
that we had to wear white gloves to handle and breathing on them would
change the input impedance of the volt meter. A finger print on one
meant the junk pile. Mostly Fluke equipment.
73
Roger (K8RI)
>
> Carl
> KM1H
>
>
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