"Denny" wrote
If we look at the ham antenna literature over the 50 years predating
the advent of the NEC engine, we do not see much championing of the 5/8
wave vertical for lower HF... Since these folks did not have the NEC
engine poisoning their thinking one has to ask why the lack of
interest... The explanation for that, in my mind, is because it does
not have significant propagation advantage at HF offsetting it's
greater mechanical and financial burden to install... And because the
high angle lobes that are beginning to bulge are disadvantageous with
an antenna that is intended for low angle receiving/transmitting...
__________________

A 5/8-wave vertical is more expensive than a 1/4-wave vertical for sure.
But the high-angle lobe that develops when the radiator height exceeds
1/2-wavelength maybe isn't as serious as thought. At HF, probably the
groundwave is gone before that high-angle radiation returns to the earth via
skip, so it won't cause self-interference even if you are trying to use the
groundwave.

Below is a link to a clip from the FCC website, showing the elevation
patterns for vertical radiators of several heights in wavelengths. For a
given applied tx power, the sidelobe of the 5/8-wave vertical is
significantly greater in radiated field than that of a 1/4-wave from about
60 to 80 degrees. But the 5/8-wave has higher radiated field than the
1/4-wave at elevation angles below ~18 degrees -- which would benefit HF
DX.

AM broadcast stations operating full time with 50 kW transmitters tend to
use radiators of about 195 degrees in height. This gives them much of the
gain advantage over a 1/4-wave in and near the horizontal plane, which helps
their groundwave and DX coverage. But radiators of that height have an
insignificant high-angle lobe to worsen their self-interference at the edge
of their groundwave coverage area. Of course because of the frequency,
their groundwave can cover an area having a large radius.

http://i62.photobucket.com/albums/h85/rfry-100/73.jpg

RF