Topband: 1/4 versus 1/2 wave vert.

[David Gilbert]

w4dlz at 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