<<<<I used several short bogs back in the 80's on a small urban lot where
their length was about 120'. I got the information on them from John K9DX.
The interesting thing was that the bog wire was coax (I used RG-174)where
the far end was shorted shield to center conductor and at the transformer
end, only the center conductor was connected to the transformer. The bogs
were the best RX antenna I had and they appeared to work well at times.
I have not tried this arrangement since then but would be interested in any
comments about the use of coax as described instead of wire for the bog.>>>>
Assuming the shield is several skin depths thick, the outside of the shield
of any "shielded" antenna is always the actual antenna. The conductor inside
the shield does nothing more than act as a transmission line with the inside
of the shield.
This includes so-called shielded loop antennas, where the wire inside the
shield is not the antenna. The thing on the outside people look at and think
is a shield (preventing electric field penetration) is actually the antenna,
and it responds to an electromagnetic field.
The outside of a shield is activated or fed at the unshorted gap, wherever
that gap is located.
So in this case (and all others with coaxial conductors) the ungrounded or
unshorted shield point of the coaxial system is the real system feedpoint.
The outside of the shield is the antenna, and it isn't shielded at all (or
it would not be an antenna). The center and the inside of the shield are
nothing but a transmission line with out-of-phase currents. Nothing of any
importance or significant level goes through the shield, neither magnetic
nor electric, because of skin depth.
Shielded loops are not shielded, and neither are shielded ground leads or
even BOG's. The outside is always the naked antenna, just like a single
unshielded conductor.
73 Tom
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Stew Perry Topband Distance Challenge coming on December 29th.
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