EI7BA wrote: Yes I am I am talking dipoles here..The dipoles have single
wire
capacity hats at either end, which is the most efficient way of loading
them. The bottom capacity hat wire is 10ft over ground, and the top hat
is at 55ft. all the details are at
http://www.iol.ie/~bravo/low_band_antennae.htm#My%20TX%20Antennae
The antenna you are describing on that page is commonly called a top
loaded T, with two elevated radials (not a vertical dipole), although
the radials described are only 1/8 wavelength each. There must be some
mistake on that diagram because when I put those dimensions into EZNEC I
get a resonant frequency of 2.43 MHz. I don't get any of the other
characteristics described on that page either. The antenna was
described as having a feedpoint impedance of 65 ohms and a 2:1 SWR of
170 KHz, and a gain of -0.25 dB. I get a feedpoint impedance at 1.85
MHz of 10.4 -j278 over average ground. Bandwidth at that frequency has
no meaning.
I tried to find the error. I can make the antenna resonate on 1.87 MHz
by lengthening the two radials to 133 ft each, but the feedpoint
impedance goes to approximately 12 ohms over average ground. It's even
less over salt water. I could make the two top wires 130 ft each
(instead of 65 ft each) and make the antenna resonate on 1.835 MHz, but
the feedpoint impedance goes to approximately 17 ohms over average
ground. I could simulated a feedline that is not decoupled from the
antenna and causes a lot of ground loss and that would raise the
impedance and widen the bandwidth. The gain is largely dependent on
whether you are talking about average ground or salt water. So does
that diagram accurately reflect what you built?
Jerry, K4SAV
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