From G3PLX:
Dick's diagrams do look puzzling if you look at them literally, so I can
sympathise with Jim, but try it this way...
The capacitance hat is on the top. In stage 1 of the thought experiment,
there is a loading coil between the hat and the top of the antenna. Nobody
should have any problems so far.
Now I remove the two terminals of the coil and in it's place at the top of
the antenna I connect one end of a length of coax, with the outer braid to
the top of the antenna and the inner to the top hat. I run this coax down
the antenna. Maybe I thread it down INSIDE the antenna so that Jim doesn't
know it's there! At the bottom of the coax I connect my loading coil.
The voltage across the loading coil will be high of course, so I need to be
careful about the voltage rating of the coax, but there will be no RF on the
outside of the braid, so I can fasten it to the antenna all the way down.
I could even ground the bottom of the antenna (and the bottom of the coax),
and feed the transmitter into a link winding on the coil. One end of the
coil is grounded and so is the coax. You only need to redraw it slightly
and you have Dexter's antenna.
To reduce the voltage that needs to be handled by the coax. I could put most
of the loading coil at the top where I started, and then all I need at the
bottom is some variable inductance to tune down the band. This variable
inductance is still effectively at the top, and there is NO high voltage at
the bottom of the antenna.
It looks like I have a loading coil and a hat (a very small antenna?)
mounted on the top of a tower, but of course it's really a top-loaded tower.
Read all the controversy about the EH antennas on the antenna reflectors.
Read G3LNP's article about top loaded towers.
73
Peter
|
