Whether or not a radial density is enough by count depends on the
"aperture" that exists between adjacent radials at the ends, as
expressed in a fraction of a wavelength at the frequency in use. If
the aperture is small enough, then the ground begins to appear as if
it is a metallic disk and is highly reflective. This is related to the
phenomenon that a wire screen will appear solid to RF if the frequency
is low enough relative to the gap between the wires in the screen.
The most commonly quoted count for a quarter wavelength at a SINGLE
frequency is 60 radials. This is the point at which, by experiment,
adding additional 1/4 WL radials produces insignificant improvement.
This is a somewhat fuzzy number, in that one probably cannot make a
significant distinction between 55, 60, or 65. The difference between
30 and 60 IS reported as significant.
The aperture related to this 60 radial count is (pi * .5)/60 WL or
.026 WL spacing between the ends of the radials.
When your radial are confined to 1/10 of a wavelength you can reduce
the separation that exists at the end of the radials to less than that
aperture with a lot fewer radials.
The formula that turns this around and gives the number of radials to
achieve a .026 minimum aperture from the wavelength is (obviously)
count = 240 * length in WL,
where WL is the wavelength of the HIGHEST frequency in use.
This would mean that if you put down a 1/4 radial screen for 80
meters, for use by a 80/40 vertical, that you want the radial screen
to be useful as a 1/2 wave radial field at 40 meters. This would be a
120 x 70' radial screen. The extra copper would not do anything for 80
meters over 60 radials, but would give you the benefit of lower angle
gain on 40.
The greater count would also give the benefit of being insensitive to
deterioration of some number of radials over time.
The formula is made imprecise because at the shorter lengths, ohmic
losses in the ground and the loss of low angle reflection at a given
angle of measurement do not have a linear relationship with radial
length. But the length is the major factor and just considering the
aperture will be a good approximation.
Whether or not one should consider a Hy-Gain construction manual as
authoritative physics is another matter.
73, Guy.
----- Original Message -----
From: "Chris BONDE" <ve7hcb@rac.ca>
To: <n4kg@juno.com>
Cc: <TOWERTALK@contesting.com>
Sent: Saturday, September 28, 2002 7:51 PM
Subject: Re: [Towertalk] Can my 40' tower be a 160m vertical?
> At 04:59 PM 2002-09-26 -0600, n4kg@juno.com wrote:
> >YES. IF you can get 1/4 WL (or more) then 30 to 60 radials is
> >beneficial. IF you are limited to 1/8 WL long radials, there is
> >virtually NO benefit above 15 radials.
> >
> >Tom N4KG
> OK then please explain why hy-gain in their manual for the 14AVQ has
a
> table for optimum ground system configurations as
> radials 16 24 36 60
90 120
> lengthin wl 0.1 0.125 0.15 0.2 0,25
0.4
> feed point impedance 52 46 43 40 37 35
> radial ends buried (?) Y Y Y N N
N
>
> Which to me states, if longer radials, then need more, impedance
goes down
> and power goes up or if more radials need more etc.
>
> Just trying to figure out these things.
>
> Chris opr VE7HCB
>
>
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