It models fine for me:
120ft vertical radiator
Bottom "capacity hat" 8ft off ground, 4 x 20ft horizontal wires + 4
perimeter wires
Two top loading wires at 45 degrees, each 28ft long.
All wires #14 copper over average ground
Fed in the centre of the vertical radiator, at 1.93MHz it has a
feedpoint impedance of 72+j2 ohms
Dimensions taken from the QEX article.
73,
Steve G3TXQ
K4SAV wrote:
> 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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