On 9/10/14, 8:11 AM, Bill Turner wrote:
I am modeling a full size ground plane with four elevated radials for
40 meters using EZNEC v. 3.0. I have it six feet above ground.
so the soil is 2 meters away (or about 1/20th of a wavelength) from the
radials. That means that there is a significant effect from the soil
(it's lossy).
I noticed that changing the length of the radials has much less effect
on the resonant point than changing the vertical element.
For example, on 40 meters, changing the vertical element by one foot
moves the resonant frequency by about 200 kHz, but changing all four
radials by one foot moves the resonant frequency much less, only about
20 kHz or so.
Partly it's probably the soil, the other thing is that "elements" that
are broad have wider,shallower resonances than ones that are narrow. A
"fat dipole" made with a cage has broader bandwidth than a single wire.
Fan dipoles have broader bandwidth than wire dipoles.
What your radials are is sort of half of a biconical dipole, with the
cone squashed flat.
The theory for why this (fat dipoles have wider BW than skinny) is
straightforward, but complex. You'll find that the resistive component
of the impedance doesn't vary as much as the reactive component. Larger
diameter (or tapered) radiators have smaller reactive components, off
resonance, and it varies slower.
If you're interested, I can find a reference, but most antenna texts
sort of present the data for various a/l (radius/length) values, and may
even show how the equation for the feedpoint impedance is derived, but
ultimately that doesn't provide a good intuitive conceptual picture of
what's going on (aside from "fields from various parts of the antenna
interact", but hey, we already knew that). I don't have my copy of
Kraus here to see what he says, but that's where I would start.
I'm curious why this is. I would have thought the effect would be the
same.
All comments appreciated.
73, Bill W6WRT
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