This thread has gotten a bit convoluted, so I'm changing the subject
to reflect the content.
>A small amount of lengthening would put the current max in the middle
>of the vertical section, but enough to get you to 50 ohms gets a lot
>of current flowing in the horizontal section.
That depends on how tall the vertical section is. It also depends on
how much lengthening is required to hit 50 ohms resistance, which
depends the loss in the circuit -- the wire and the radial system. My
vertical section is 86 ft, with horizontal legs on the order of 60 ft
(the ground slopes under this antenna, so my top loading is
asymetrical). For a base current of 1A, the current at the top of the
vertical section is 1.07A and the current maxima (1.17A) is at
roughly 50 ft. So in my vertical, the current in each of the
horizontal wires is just slightly half of the base current. I have 60
radials, average length about 80 ft.
NEC shows a very small effect on field strength or vertical pattern as
a result of the lengthening. As I see it, the principal advantage is
simplicity of tuning and easy matching to 50 ohm coax.
As I recall, the cap in my system is about 330pF. This value is NOT
difficult to obtain -- 10-15 ft or so of coax would do it, and would
certainly handle the voltage. I'm using some 3kV ceramic discs from a
local electronics surplus store. I've determined experimentally that
their loss is quite low -- I put 1.5kW into the antenna for a while
and then go out and feel them to see if they've gotten hot. You can't
use just any caps here, of course, and some I tried got quite hot.
To summarize: In my antenna, at least, the current maxima has been
moved up about 50 ft from the feedpoint, current in the vertical
section is quite uniform, varying by less than 2 dB, there is a slight
improvement in the vertical pattern, and a slight increase in computed
field strength. It matches 50 ohm coax, doesn't require a tuner
between 1800 kHz and 1850 kHz, and is easily tuned by my Ten Tec 238
above 1850. Yes, there is signficant current near the center of the
toploading wires, but they're #10, so loss is small. I would certainly
recommend this technique for antennas as short as 1/8 wavelength (70
ft on 160M), and wouldn't rule it out for shorter ones.
Jim Brown K9YC
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