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Wimpie[_2_] · April 3rd 14, 01:11 PM posted to rec.radio.amateur.antenna

El 02-04-14 22:32, escribió:
wrote:
El 02-04-14 20:25, Ian Jackson escribió:
In ,
writes
Ian wrote:
In ,

writes

The ideal radial length for ANY ground plane antenna is slightly
longer
than 1/4 wavelength, no matter for what frequencey.

Why is this? I would have thought that a 1/4 wave would be best, as it
offers the lowest impedance.

First you have to define what "best" means.

Yebbut ........
You've just said "the ideal radial length for ANY ground plane antenna
is slightly longer than 1/4 wavelength, no matter for what frequency".
I assumed that "ideal" = "best".
.
All antennas are a trade off for impedance, bandwidth, gain and in most
cases physical ability to build the structure.

Changing the radial length will have a small effect on impdedance
and resonant
point but changing the radial angle will have a bigger effect on
impedance
and a very small effect on resonant point.

True - but what's the angle of the radials got to do with their length?

I would suggest downloading the demo version of EZNEC and modeling a
GP to
see what small changes in various parameters do.

I had presumed you had already do this (or something similar) in order
to say that slightly longer than a 1/4 wavelength was ideal. However,
I have always assumed that the steeper the angle of the radials, the
more the groundplane becomes like a vertical halfwave dipole - and the
lower becomes the angle of radiation.

You are right, very steep radials become the lower half of a half wave
dipole as the currents do not cancel eachother and contribute to the
field of the quarter wave monopole. The "ultimate" version is the
sleeve dipole.

Not really.

When they are in the horizontal plane, the contribution to the total
radiation pattern is very small, and the contribution from the radials
is even zero for the vertically polarized component at zero elevation.

The theoretical gain of a GP with horizontal radials, radials drooping
45 degrees and and drooping 85 degrees is 1.42, 2.22, and 3.67 dbi.

You may check your simulations, as in free space you will not exceed
the half wave dipole gain with near vertical radials (for the quarter
wave version).

A quarter wave monopole with near vertical radials has same current
distribution as a vertical half wave dipole (use sum of current in all
radials). Of course provided that you don't have significant common
mode current in the mast or coaxial cable, as this may increase or
decrease the free space gain.

When extending both sloping radials and radiator you can get more
gain, but you get significant increase in common mode current as the
radial ground no longer act as a floating ground point, and the input
impedance has a reactive part.

The "somewhat longer then 1/4 wavelength" I also noticed with radials
connected to a coaxial braid to form a narrow band common mode choke.
the choking effect (common mode insertion loss) is better when they
are somewhat longer then 0.25lambda (depending in thickness).

The effect of sloping angle on zero elevation gain is small, and you
get hardly measurable more gain when they are almost vertical. Sloping
radials have some other advantage: less birds.

Changing the angle of the radials has little to no effect on elevation gain
unless the radial ends are a very tiny fraction of a wavelength above ground.

I can't match this statement with your earlier gain figures, or did I
misunderstand you.

Elevation radiation angle is almost totally determined by the antenna
height above ground.

Agree, but now the number of variables increases as you need to take
into account nearby ground conductivity and far away ground conductivity.

--
Wim
PA3DJS
Please remove abc first in case of PM