Given that he was starting with so few radials, I am suspicous that his
feedline shield was *not* sharply decoupled and by chance was presenting a
low enough impedance as a current sink that the (inferior) radial system was
simply not seen by the circuit overall. If the feedline is decoupled and the
impedance changes sharply, I think he will see your commentary in evidence
directly.
73, Guy.
On Mon, Sep 28, 2009 at 3:24 PM, w8av <w8av@aol.com> wrote:
> Dennis:
>
> You have not reached a point of diminishing returns here. Although you see
> no virtual change in the antenna VSWR, the radiation efficiency of the
> antenna system improves every time you add additional radials under the
> antenna. There is published data in the Proceedings of the IRE that show
> that once you reach 120 radials you see no additional improvements to the
> efficiency of the antenna by adding more radials. With regard to length,
> the magic cut-off number for no improvement to radiation efficiency is 0.4
> wavelengths. That is why when I worked in the broadcast industry, we always
> installed 120 radials of at least a quarter wavelength long under all AM
> broadcast towers.
>
> You have reached a point where the transmitter is happy with the antenna
> match but you can improve the antenna's signal radiating efficiency by
> adding still more radials under the antenna. You are trying to reduce the
> ground losses in the near field of the antenna with the ground system and
> when you finally reach 120 1/4 wavelength radials, the antenna will be about
> as good as it can be. You should also not see any VSWR change by adding the
> additional radials.
>
> If you can't get to a 1/4 wavelength in all directions, you can add more
> radials in the shortened areas to help improve the radiation efficiency in
> those directions where the ground system is shortened. If you can't get 120
> radials down, put as many down as you physically can and make them as long
> as you can.
>
> 73.................de Goose, W8AV
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