The above modeling results just don't support that contention/posit so I'm
wondering what else comes in to play that could lead folks to love the 5/8
wavelength vertical over a shorter version, regardless of frequency? I
don't see one performance comparison that supports that claim. I'm not
saying the "claiming person" isn't correct but....I don't see how!
Help - what am I missing here?
72, Jim Rodenkirch K9JWV
The 5/8th wave obtains the small amount of gain it has through effects of
ground reflection. The current maximum is elevated, and that elevation
causes additional phase shift with the illumination of earth out some
distance from the antenna. The re-radiation of earth is sometimes explained
by an "image antenna". The image antenna is a fictitious antenna directly
below the real antenna, and this "image antenna" simply represents what the
earth at a distance does from the illumination from the main antenna.
The 5/8th wave moves the current maximum slightly higher than a quarter wave
above earth, so the fictitious image moves slightly lower than a 1/4 wave
below surface.
If you move the 5/8th wave above earth, such as in a groundplane well above
the earth, the extra length no longer provides gain. Instead, it actually
reduces gain at low angles.
Another effect is the extra height above ground of the high current area can
help get the antenna's main radiating area a little bit higher above ground
clutter.
This would also apply to an extended double zepp, where the second antenna
half makes the image unnecessary. Each half of the double zepp is the image
of the other side, so we don't need earth.
So it is a very specific benefit from the 5/8th wave caused by moving the
current height up above a reflecting surface, or in the case of a double
zepp moving current away from a second identical element while still having
a common center feedpoint.
One of the biggest antenna hoaxes played on people was the 5/8th wave CB
groundplane antenna. Two meter 5/8th wave groundplanes are the same. The
work on the broadcast band to increase groundwave signal because soil is
often reasonably low loss on the AM BCB. If the soil out some distance from
the antenna is lossy, or if it does not exist, the 5/8th length causes
increased low angle loss.
This is why when you look at models, outside of specific cases like some
cases of low broadcast band use, we have a tough time seeing the gain
imagined or claimed. A field strength meter has an equally difficult time.
:)
73 Tom
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Topband Reflector
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