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Old February 3rd 06, 07:17 PM posted to rec.radio.amateur.antenna
Richard Harrison
 
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Default Verticals versus Horizontal Dipoles

Reg wrote:
"If you can`t be heard at 1000 miles or more using a dipole, you are
more likely to be heard using a vertical regardless of what the other
fellow is using to receive."

That must not always be the case. Otherwise all the shortwave broadcast
stations I`ve worked in, and seen for that matter, would not use
horizontal antennas. They have no way of knowing what their audience
will use for antennas, and it does not make much difference as following
ionospheric reflection, all wave polarizations are available and may be
received.

At the equator, a time zone is about 1000 miles wide. at the poles (a
bad place for shortwave propagation) the width of a time zone is
insignificant. All the stations I refer to are in the temperate zone and
their targets are likely 1000 miles or so away, though some targets of
some stations are only a few hundred miles away.

Antennas at these shortwave broadcast stations are a product of studying
successful antennas and carefully designing new antennas anf testing
their performance in and around their intended targets. They are proved
to be effective.

Why would a vrtical antenna be better?

From Arnold B. Bailey`s giant antenna catalog in his "TV and Other
Receiving Antennas", the free-space gain is the same for a ground plane
as it is for a center-fed 1/2-wave dipole. An antenna`s proximity to the
earth may change the balance between horizontal and vertical antennas.

Terman writes on page 886 of his 1955 edition:
"Consider an antenna that is far enough from ground so that the total
power radiated by a given set of antenna currents is independent of the
presence of the ground. Then a ground reflection that reinforces the
main lobe will double the field strength of the main lobe, and so will
increase directive gain of the antenna system by a factor of 4. This
condition corresponds to an antenna height great enough to make the
mutual impedance between the antenna and its image small (see page
894).With horizontally polarized systems this will be the case if the
center of the antenna is at least one wavelength above ground; with
vertically polarized systems it is true even at lower heights.

However. when the antenna is sufficiently close to the ground the effect
of the ground reflection is to cause the directive gain to differ from
4. Thus , for a vertical doublet close to the ground, the directive gain
is twice the free-space value, since the presence of the ground does not
alter the directional pattern and there is no energy radiated in the
direction of the hemisphere occupied by the ground. In contrast, the
directive gain of a horizontal antenna very close to the ground can be
more than 4 as compared with the same antenna in free space, as
discussed below in connection with Fig. 23-36."

Seems horizontal antenna users are not fools after all.

Best wishes, Richard Harrison, KB5WZI

 
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