Ed,
here are some dimensions derived using EZNEC:
Height 90', 4" diameter
four #14 top hat wires 20'
Base coil 5 uH (make coil 10 uH and tap it)
Radiation resistance 22 ohms
This article by K3LC will help you chose radial lengths numbers.
http://www.ncjweb.com/k3lcmaxgainradials.pdf
A starting place is to use 500' of wire to make twelve 40' radials as shown
in Fig. 3. Being that your vertical is shorter than a 1/4 wavelength these
K3LC graphs are not optimum but I beleive they are more than good enough.
If we say the 32,000 radial system is close to 0 ohms, the 500' radial
system (12 radials) is close to 20 ohms (based on the gain figures). Given
that the radiation resistance your antenna is 22 ohms this gives an input
impedance of 42 ohms, a VSWR at resonance of 1.1:1, a 2:1 VSWR bandwidth of
200 kHz, and a radiation efficiency of 50%.
Install 2000' of radial wire to obtain a 10 ohm ground and a signal increase
of 1.2 dB.
Dave WX7G
On Sat, Aug 29, 2009 at 3:32 PM, Ed Stallman <n5dg@airmail.net> wrote:
> Hello Towertalkers
>
> My plan is to construct a 160m vertical using 4 inch irrigation
> tubing 90 ft tall, with 4 top loading wires spaced 90 degree and
> sloping down at 45 degrees . My target freq is 1.830 but would like
> to make the top-loading a bit short so that the antenna was resonant
> about 2 MHz. Then put a small (a few uH) loading coil at the base to
> bring the antenna to resonance at the desired frequency. I am
> placing the antenna 300ft away from any other structure , hoping for
> no interaction . I'll start with 32 ground radials and add more
> later, not sure on the length ? 90ft or 130ft ?
>
> two questions
> how long will the top-loading wires be for resonant at about 2 MHz
> using # 14 copper insulated wire?
> how long should I make the ground radials ?
>
> Thanks Ed N5DG
>
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