During an E-mail exchange with someone about transformers, we got on the
subject of Faraday shields. Building a Beverage with a Faraday shield (or
using a "shielded drop wire" from the antenna to ground) probably won't hurt
anything, but it almost with 100% certainty won't improve things.
A Faraday shield is really an just extra plate of a capacitor placed between
the primary and secondary. It makes the original small capacitance between
the windings look like two larger values (more than double the original
value). The junction of these "new" capacitors can be connected to ground.
This allows us to "shunt" capacitively coupled currents to ground at the
common point.
A Faraday shield is most effective in multiple turn or resonant transformers
that have very high primary and secondary impedances. That's because the very
small amount of primary to secondary capacitance can only couple a noticable
amount of signal if both windings have very high impedances.
A Faraday shield, in the case of a Beverage, will do nothing to eliminate
common mode excitation of the antenna by the feedline. It makes the problem
worse since it more than doubles unwanted capacitance to the common ground.
Using coax from the ground up to the Beverages feedpoint also offers little,
if any, advantage. It only helps if the noise is capacitive coupled from a
noise source within inches of the bottom of the vertical end wire. For
magnetic or electromagnetic fields the center conductor excites the shield
and the shield acts like the antenna instead of the center conductor. The
shield just moves the very same problem 1/4 inch further out. If a ground
wire is connected to the top, the shield just excites the vertical ground
wire and it radiates, and so on.
73 Tom
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