On 9/7/2013 6:59 PM, Tom W8JI wrote:
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.
False premise alert. There are two different mechanisms involved. One is
the pattern distortion (gain) in the horizontal plane produced by current
distribution on the double zepp, and amounts to a couple of dB but a
narrower pattern as compared to a dipole. But the earth reflection is
still there, because it's a horizontal antenna at some height above earth.
No one said the earth was not there.
My point is exclusively about collinear gain, since that is the mechanism in
the topic.
The mechanism for around 3 dB gain is exactly the same for both except
without the collinear earth image the horizontal configuration, or the
configuration absent earth, requires a physical element. The second half of
the element, as explained, substitutes for the collinear earth image.
1.) When vertical and just over earth a reasonable amount, the collinear
effect is from earth reflection at a distance radially from the antenna.
This is true for the 5/8th wave as well as a stack of 1/2 wave over 1/4
wave.
2.) When horizontal, in the form of an extended double zepp, or vertically
when absent reasonably good earth, or when elevated far from earth (like VHF
antennas high in the air), the collinear effect must come from the second
half of the extended dipole or another element. The earth cannot be part of
collinear gain because phase is wrong, and it cannot be a part if it is not
there in the proper position to form the image.
With a horizontal antenna, the earth only contributes broadside gain. This
is because the fictitious "image antenna" is in-phase, parallel with, and
right below the horizontal antenna. If the antenna is a VERTICAL extended
double zepp far above earth, or 5/8th wave far above earth,
the second antenna half is required. Obviously if the earth is missing, or
far below the antenna, it has no way to form an image.
1.) The gain of a vertical 5/8th wave comes from the earth image.
2.) Nearly all of the gain of a vertical collinear stack of a 1/4 wave and
1/2 wave comes from the upper element.
That is just how this stuff works.
73 Tom
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Topband Reflector
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