w4dlz@arrl.net wrote:
>A 1/2 wave vertical has a high impedance feed at the base.
>The ground currents on a half wave vertical are very very low
>thereby ground losses are minimal. You are at a high voltage
>feed point . As far as I'm concerned all you probably need
>is one cooper rod at the base..
>
>Considerating this how can a quarter wave vertical be better...?
>Also the current is max at 135 feet up....
>
>Frank W4DLZ
>
>
>
What you say is true only for conductive ground losses at the feedpoint.
The radiated field also must contend with lossy ground. A couple of
other recent postings here have postulated why a quarter-wave vertical,
especially with a decent set of radials, might very well outperform a
vertical half-wave due to the effects of far-field ground absorption.
I'm well aware of the accuracy limitations of EZNEC regarding proximity
to real ground, but just for the heck of it I compared a quarter-wave
vertical with four elevated radials (10 feet high) and a vertical
half-wave, both with a base at the same height above ground (10 feet).
The radiated lobe was lower for the half-wave but at its maximum (15
degrees) was only 0.5 db better than the quarter-wave at the same
takeoff angle. At 40 degrees the quarter-wave was 5 db stronger than
the half-wave. Diminished high angle performance is great for a
receiving antenna but no advantage for transmitting unless it is
compensated by better strength at low angles. In fact, most of us have
several times experienced 160m conditions where the only usable antennas
were those with predominantly high angle lobes.
By the way, there was virtually no impact on the modeled performance for
the half-wave whether I left the radials under the antenna or not, which
of course is how the erroneous impression of ground independence for a
vertical half-wave came about in the first place. A vertical half-wave
may not need a low-resistance ground path for an efficient feedpoint,
but the radiation pattern is still strongly affected by ground. Even
EZNEC shows a huge difference in performance for the half-wave vertical
(with or without the radials in place) when the model is toggled between
"real" ground and perfect ground. Ground matters.
Again, the EZNEC results are probably not very accurate on an absolute
basis but as a comparison they may be more valid. If so, they don't
suggest that it is worth the extra hassle of hanging twice the wire from
the sky unless adequate radials for a quarter-wave vertical are simply
out of the question.
Dave AB7E
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