THREE reasons for a ground screen underneath a vertically oriented antenna.
1) to give a high conductivity path for a current sink in case you are
feeding the vertical portion at the bottom against ground.
2) to put a shield between lossy ground and the E field radiation.
3) to give as much of the conductor's "first bounce" radiation as possible,
a high efficiency first bounce.
If it's a true vertical dipole you don't need it for 1) because it's not fed
against ground. But two and three are still with you. Two and three get
forgotten all the time. Maybe that's because reason number 1 losses are so
incredibly bad if you don't have the good screen for a base-fed 1/4 wave
vertical.
1) might be essentially satisfied with an 1/8 wave radial field. 2) is
effectively taken care of by 1/4 wave. 3) is an economic bang for the buck
kind of thing. If you inherit 100 spools, 20,000 feet each of #14 copper and
have a square mile meadow you own, go for it, put it all down, a copper
equivalent of your own private salt water swamp. Your low angle radiation
will be incredible. A 4-square antenna over salt water (or a half mile
radius copper plate) is a killer antenna. Then again you could sell the
copper, buy a whole station, and put up a tower and a yagi.
1) and 2) keep you from losses. 3) is where you get the increasing gain at
low angles, substituting a high efficiency bounce for a lossy one at lower
and lower angles as you extend the screen.
How much 3) is enough? Assume a 1/4 wl vertical in the center of a circular
salt-water pool, or dense circular radial screen, reverting to "normal" dirt
at the edge. What happens as the pool/screen size increases? Assumes we're
in a BIG meadow and big buildings, trees, etc, not a factor. Also we're not
attempting to estimate ground wave effects, just sky wave.
Screen Z Res Max gain @deg @15 @10 @5
1/8 37 +1.42 26 +0.18 -1.26 -5.55
1/4 37 +2.57 25 +1.36 +0.02 -3.83
1/2 37 +3.69 22 +3.07 +1.72 -2.41
3/4 37 +4.15 17 +4.09 +3.14 -0.68
4/4 37 +4.30 13 +4.25 +3.90 +0.68
Note there's not that much difference between 1/4 and 1/2 at max gain, but
look at the differences between sizes at the lower angles. Remember how
verticals are killer antennas at salt-water beaches? Also that 3/4 for forty
meters is only 100 foot radials, and that 1/4 on 160 is 4/4 on 40. Note that
the feed is as efficient at 1/8 wave (37 ohms) as it is at longer lengths.
Oh yes, and what if the antenna is a full-size vertical half wave, bottom
end say 6 inches above ground? Not worried about Z Res, but otherwise same
columns.
1/8 -0.05 @ 18 -0.28 -1.55 -5.12
1/4 +0.42 @ 21 +0.03 -1.33 -5.05
1/2 +1.84 @ 23 +1.13 -0.55 -4.65
3/4 +3.09 @ 22 +2.30 +0.33 -4.04
4/4 +4.02 @ 20 +3.52 +1.47 -3.27
8/4 +5.51 @ 13 +5.30 +4.89 -0.07
8/2 +5.93 @ 7 +5.30 +5.81 +4.67
This is all about the center of radiation being much higher, and needing a
much wider circle to get an efficient first bounce at a given angle. It does
explain the very long radials at WWVH, and why vertical halfwaves are
disappointing everywhere except at the beach, or over metal roofs on top of
apartment buildings, where they really shine.
73, Guy.
----- Original Message -----
From: "Jim Reid" <jimr.reid@verizon.net>
To: "Bill Coleman" <aa4lr@arrl.net>; <K7GCO@aol.com>;
<towertalk@contesting.com>
Sent: Thursday, November 15, 2001 3:59 PM
Subject: Re: [TowerTalk] vertical antennas math & 1/2 waves
>
>
>
> > >DXer screws around with 1/4 waves. A 1/2 wave is already
> > > resonant and doesn't need any more help in that department.
> >
> > Maybe it is because 1/4 waves are half as tall?
>
> And, because half wave verticals have very INTENSE
> E fields at the base of the vertical, they still need LOTS
> of radials! As WWVH learned out here on Kauai. See:
> http://www.boulder.nist.gov/timefreq/images/radiostations/
> wwvh-large/wwvh5.jpg .
>
> 15 MHz antenna array. Two half-wave vertical dipoles separated by a
> quarter-wave length and driven 90 degrees out of phase to direct
> the signal in a cardioid's pattern to the West. Note the large number
> of radials on the ground surface; several easily seen passing under
> the fence in the foreground.
>
> 73, Jim KH7M
>
>
>
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