[TowerTalk] Vertical dipoles

[K4SAV]

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