[TowerTalk] Vertical dipoles

Steve Hunt steve at karinya.net
Wed Nov 18 13:25:11 PST 2009


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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