Quite a bit of the radiation from 5/8 wave verticals is at relatively high
elevation angles - above 40 degrees elevation.
There is a tendency to look at patterns and think (quite wrongly) a lobe in
a useless place is robbing enough energy from a desirable place to noticably
degrade performance.
Except for receiving, a severe problem making an antenna a "dog" is almost
never where the high angle stuff or out-of-pattern stuff is. No one really
notices sidelobe power that might only "waste" a dB or two, or even 3 dB.
The real problem behind something being a profound "underacheiver" is
virtually always when a significant lack of signal appears in useful places.
It is the significant lack that really matters, not the perceived waste.
The 5/8th wave, near a large flat ground, is a good groundwave antenna
within the area where sky wave might appear. It can even be good, under the
right circumstances, on bands that have really low angles of propagation.
The loss of signal that appears at the more common wave angles just kills it
for all around 160 use. Almost everyone who uses a 5/8th on 160 (or to a
lesser extent on 80) recognizes that pretty fast.
Tom
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Topband Reflector
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