TopBand: Shielding

[w8jitom@postoffice.worldnet.att.net]

> From:          Larry Higgins <n9dx@michiana.org>
> Date:          Sat, 8 Feb 97 02:46:38 +0000

> At 08:48 PM 7/8/97 +0000, W8JI wrote:
> >The reason they don't help is if you shield an antenna from the 
> >time varying electric field you also shield it for any time varying 
> >magnetic fields or radiation fields. All shielding does is make the 
> >loop's "shield conductor" become the actual antenna, instead of the 
> >stuff inside the shield. 
> 
> Tom, maybe you could shed further light on this subject.  I've been under the 
> impression that a Faraday shield on a loop antenna acts as an electric field 
> shield, but not a magnetic field shield, because the shield (usually) is not 
> made of a magnetic material. 

The type of material, magnetic or not, only affects the 
resistivity of the material at RF. Eddy currents prevent magnetic 
effects from taking place no mater what the material. That's why a 
steel core will DECREASE the inductance of a RF coil, unless it's 
powdered up and insulated in small sections or grains.

Physics tells us we can not have an electric field inside a conductor 
(steel, lead, copper or otherwise), and any time varying magnetic 
field is also accompanied by a time varying electric field. Since the 
time varying electric field reaches zero, the magnetic field does 
also. This happens at several times the skin depth.

So if the conductor is several skin depths thick, neither an electric 
field or a time varying magnetic field can pass through it.

The way the loop receives is the radiation and electric and magnetic 
induction fields set up currents on the outer conductor. None of 
these fields pass through the shield. Instead the INSIDE of the 
shield, isolated from the outside by the skin depth of the material 
(think of the inside the shield wall metal as perfect insulator for 
ALL fields at RF), appears as a separate conductor connected across 
the voltage source at the open ends of the shield. This causes a 
current flow of the OPPOSITE direction on the inside of the shield, 
with the current spilling over the edge at the open shield. The wire 
inside the shield, actually forming the loop, "sees" the electric and 
magnetic field from the shield's inside, and this drives the loop 
with signal that is back in phase with the shield's outside.

If you close the gap at the open end of the shield, nothing gets 
through to the center. You effectively short the voltage out that was 
driving the inside of the shield, and the inside of the shield no 
longer drives the loop inside.

The net effect of ALL of this is the shield actually becomes the 
conductor that picks up signal, and the rest of the system inside 
the shield is like a transformer. So the primary thing a shield does 
is change RF balance, it does NOTHING to stop the electric or 
magnetic field.. if it did, the antenna would be stone dead for 
receiving

The same thing applies to "Snake Antennas", and shields over the 
vertical end wires of Beverages. Any RF from more than a 
few shield diameters away  **effectively**  cuts through the shield 
like a knife through butter. The shield might as well not be there at 
all, since the same effect in the loop occurs. The only exception is 
if an electric induction field is originated near one small point of 
the shield, and the shield "moves" the open end away from that point. 
But that effect only works when the electric field source is VERY 
near the shield, certainly not more than one or two shield 
lengths away.

So if you have a five foot drop line from a Beverage, and place a 
grounded vertical shield over that drop wire, you change the response 
mostly near the grounded end of the shield. Nothing at all changes 
ten or twenty feet away. If you don't ground the shield nothing 
changes at all that you could measure, except system losses increase 
slightly because of circulating currents in the shield.

>Also, I've worked with industrial  instrumentation 
> for years, and in every installation chapter there is a caution to the user
> that all control lines should not be in the same conduit with power lines,
> and should be in their own STEEL conduit.  I've seen two instances where
> the wisdom of this advice was proven.  In both cases, instrumentation wires
> were in aluminum conduit.

Different effect, the skin depth at 60 Hz is not deep enough to 
isolate the inside and outside of the steel, and eddy currents do 
not fully remove the external magnetic effects.

> eliminated.  Also, isn't there a difference between a mu metal shield and
> an aluminum foil shield?

Depends on the thickness of the metal, the frequency, and the type of 
effect you are talking about. I was speaking of a shield on 160, 
where almost any good conductor is at least several skin depths 
thick and eddy currents eliminate external magnetic effects. This 
leaves the only effect of using steel conductors an increase in 
effective loss resistance. There is no "flux conduction" through the 
material because of eddy currents.

73, Tom W8JI 

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