Topband: Short Bogs

[Tom W8JI]

<<<<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