Re: Caged Di-Pole

Richard Clark wrote:
On Thu, 17 Aug 2006 21:33:17 -0400, Mike Coslo
<mremovethiscoslo@xxxxxxxxxxxx> wrote:

how does really thick wire compare to a cage of the same relative diameter?

Hi Mike,

Close, but no cigar. Actually, the degree of closeness depends on how
well the cage represents the solid. That degree is a function of the
number of wires that form the skeletal shape. Four is pretty lousy,
120 would be outstanding. Then there is something in the middle:

http://home.comcast.net/~kb7qhc/antennas/verticals/Cage/cage.htm
offers an example that may be suitable. This is a 4 meter tall
antenna that tunes all frequencies from 20M through 10M.

A single fat wire can be a reasonable approximation to a cage provided that the diameter is very small compared to a wavelength. (The NEC recommendation is around 0.02 wavelength maximum diameter, which you can find in the EZNEC manual in the Building The Model/Modeling The Antenna Structure/About Wires topic.) For a cage of only a few parallel wires, you can use an equivalent diameter as follows, where d = the wire diameter and s = ctr-ctr wire spacing, everything in the same units:

2 wires - Equiv. dia. = 1.414 * sqrt(d * s)
3 wires in a triangle - Equiv. dia. = 1.587 * cube root(d * s^2)
4 wires in a square - Equiv. dia. = 1.834 * fourth root(d * s^3)
N wires equally spaced on a circle with radius r -
Equiv. dia. = 2 * r * Nth root((N * d) / (2 * r))

-- derived from equations in Fundamentals of Coupled Lines and Multiwire Antennas, by Hidenari Uchida (Sasaki, 1967).

I've made myself a note to include this in a future EZNEC manual update.

For cages larger than about 0.02 wavelength diameter, you should model the individual wires.

Roy Lewallen, W7EL
.

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