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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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