Dennis,
it appears that the near field losses are equivalent to 25 ohms at the
antenna base. Since adding more radials does not improve this I suspect the
nearby trees are the source of the loss.
Dave WX7G
On Tue, Sep 29, 2009 at 8:36 AM, Guy Olinger K2AV <olinger@bellsouth.net>wrote:
> 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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> >
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