Dear Dick, LF Group,
I am puzzled as to how the top-loaded
vertical antenna plus stub is supposed to operate. In Fig 1, the elevated
loading coil is in series between the vertical element and the top loading,
which is perfectly reasonable – but I can’t see how the stub in Fig
2 could produce an equivalent series inductive reactance, especially since one
terminal of the stub is unconnected.
I can see that Dex’s antenna works (I
presume the arrangement in Section 2.12 of ON7YD’s antennas page), but I
don’t think it is equivalent to an elevated loading coil. The vertical
wire from the hot end of the loading coil to the top loading section will be at
high voltage along its whole length, leading to much the same displacement currents
flowing to ground and in the mast, and therefore the same current distribution,
as would occur with a conventional inverted-L with a ground-level loading coil.
This contrasts with a top-loaded vertical with elevated loading coil at the top
of the vertical section, where all the vertical part of the antenna is at low
RF potential, and only the top loading section carries a high RF voltage. The main
effect of using the mast and the additional vertical wire from the top of the
mast to the cold end of the loading coil as part of the antenna system would
seem to be to add their distributed inductance to the cold end of the loading
coil. In other words, you would achieve an almost identical result by grounding
the cold end of the loading coil to the bottom of the mast, and adding some
extra turns to the cold end of the loading coil.
Cheers, Jim Moritz
73 de M0BMU
