TopBand: Elevation Angle of Maximum Radiation from Verticals

Wed, 30 Jul 1997 10:48:36 +0000

Hi Earl,

Good to hear you again! I was thinking of the early W5RTQ articles 
about shunt feeding towers when the topic came up.

> Date:          Wed, 30 Jul 97 04:05:12 +0000

> Hi, Top-Banders:
> 
> After I posted a message comparing gains and radiation angles of
> various-size verticals, I received a message from John, ON4UN, pointing
> out that the radiation angle is largely affected by the type of ground at
> the antenna site, and he is quite right.
> 
> I modeled those antennas using W7EL's EZNEC v1.0 and using "average"
> ground, with no radials.  I also checked all models using ELNEC and
> K6STI's MN.  Results were the same (+/- one degree) using any of the
> software packages.

Be careful using NEC based programs to determine ground loss Earl. 
They show a large discrepancy (several dB) with the real world when 
FS measurements are made.

There are many documented cases of error, and the errors I've seen   
approach six dB in a single element system (either a vertical or 
dipole) mounted close to earth.

Another problem is ELNEC and other programs "look at" the antenna 
from a very large distance away. Radiation fields propagated through 
"space", with the total lack of any form of power loss, continue 
on to infinite distances. Any reduction is field strength at high 
angles is due to geometry, rather than losses. Your  program is 
really considering the FS after propagating  along the lossy earth 
for a near infinite distance, so even a small amount of loss causes 
severe gain reduction when viewed from an extremely large distance.

To get a "feel" for what this means look at the field intensity at 
zero degrees, and now consider the measured groundwave field 
intensity of a BC station at 20 miles. According to these programs, a 
BC station would have virtually NO groundwave signal, but would be 
heard quite well on skywave at large distances.

This is obviously not the case in the real world.

To get another feel for the error, model a 1/4 wl high dipole and 
compare it to a 1/4 wl vertical. Now consider how many 1/4 wl 
high dipoles are "hot performers" on 160m at distances over a 
few hundred miles. Considering real world performance, you'll 
probably conclude the models work much better on ten meters then 
they do on 160.

In the shortwave region, where groundwave propagation distance is 
very small compared to the virtual height of the ionosphere, 
programs calculating FS at large distances can work. 

In cases where groundwave attenuation is such that the signal along 
the earth remains strong at distances that approach the virtual 
height of the ionosphere, models using FS estimated at very 
great distances become almost useless.

Models have boundaries where they become inaccurate or useless, and 
on 160 meters with ground mounted or nearfield coupled to ground 
antennas we cross the boundaries where nearfield ground loss and 
farfield measurement geometry can  become problematic.

You'll find plenty of systems that don't work as the models would 
lead us to believe. They are good tools, but only if we know the 
limits.

73, Tom W8JI 

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