From G3PLX:
Jim said:
This is fine, and the same as Dick's Topload3 diagram, but with the
parallel
wire stub replaced by coax - but in Figure 2b of the original article, the
connections are different. In this case, if the open wires are replaced by
a
coax stub, the top hat would also connected to the coax braid as well as
the
top of the mast, and the coax inner at the top of the mast wouldn't be
connected to anything! In the description above, both terminals of the top
end of the transmission line are connected to something, but in Figure 2b,
one terminal isn't connected at all.
I mentioned coax only because I think it helps to think of one side of the
(twin wire) stub as being the 'inside' and the other as being the 'outside'
in the same sense as coax. The 'inside' and 'outside' legs of the stub are
tightly coupled to each other, but in addition the 'outside' leg doubles as
the antenna.
If you did it with coax 'inside out' as Jim describes above, then the inside
couldn't double as the antenna, so my analogy breaks-down, but with the
braid outside you can even leave the top of the braid unconnected and think
of the antenna current flowing up the outside of the braid, in at the top,
and down the inside of the braid, looking for all the world as if there is
no current at all.
If you then swap back from coax to twin wire line for the stub, you can't
tell which leg is which, and that's where Dick's Fig2b starts to look wierd.
His topload3 and topload4 diagrams help a lot. The antenna doesn't read
diagrams and radiates regardless!
Incidently, Dex, have you thought about removing the wire you have labelled
'tower grounding wire' and returning the feed to the base of the tower
instead? There should be no voltage between the bottom of the tower and the
bottom of this grounding wire, so you might as well use the tower to 'ground
itself'. Maybe you cannot rely on the tower to conduct current!
73
Peter
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