[TowerTalk] halfwave verticals -Elevated

[i4jmy@iol.it]

>From the efficiency viewpoint an half wave vertical against ground, no 
matter where the fed is, has a maximum current density way off the base.
The maximum radials density should be in that area where the current is 
maximum and not close to the base like with a classical 1/4 Wl.

The outstanding performances of a vertical over a very good 
conductivity area like sea water can't be related only to inherent 
efficiency because of a nearly perfect ground, since an artificial 
ground can easily built a GP whose basical efficiency is over 90%.
Incidentally, the gain of a 100% eff. over a 90% eff is less than half 
dB. 
What happens is that over a perfect ground that extends omogeneously  
for several WL all around there is nearly no diffraction of waves and 
the antenna gain stay very high at low angles, beeing also enforced by 
the so called *image antennna* that's stay in phase.
This advantage is mostly evident on bands where the useable angles are 
minimal, 20-15 and 10, but also the lower bands often benefit of quite 
low angles that reduces the required path hops and their big 
attenuation.

Opposite to the horizontally polarized antennas where more height from 
ground enhances the radiation at lower elevations, and may get up to 6 
dB gain enforcement at some angles (and nulls at others), other than 
overcoming an existing obstacle there is no advantage to elevate a 
vertical from ground.
What's gained in term of less ground losses and fair artificial earths 
when the antenna is high enough from the ground (and a fraction of wl 
is not enough) is nearly always lost in terms of keeping an efficient 
radiation at low angles, expecially when the area below and around is 
fair or poor in ground constants.

73,
Mauri I4JMY



> I don't know what program you used, but I ran
> a bunch of simulations on NEC3-GS.  The results
> are fairly discouraging.  With 10 WL long radials,
> you need thousands to be effective.  And even
> then, you don't get all that much advantage over
> an FCC ground (120 1/2 wave radials).  It never
> approaches the proverbial desert island surrounded
> by salt water.
> 
> I also built a 20 meter vertical with closely
> spaced radials 4 WL long.  It didn't do anything
> amazing in terms of DX performance.  It is
> documented at:  http://www.karlquist.com/ground.pdf
> 
> Let me know if you get results that either agree or
> disagree with mine.
> 
> BTW, I can make you a real good deal on 90 lbs
> of magnet wire on a single spool if you want
> to put out "disposable" radials!
> 
> Rick N6RK



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