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Antenna Ratings
On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing
wrote: Well, you kind of danced around the answer, but I'd still like to know how the ground plane effects the radiation angle which logically would also have an effect on reception of a signal. On page 3-9 of the ARRL Antenna Handbook (16th addition - sorry, it's the latest I have at hand at the moment) states: On a vertical antenna that USES a ground plane, the radiation angle increases towards straight up as the ground plane becomes "smaller", electrically less efficient. On a half-wave, end-fed vertically polarized antenna, where no ground plane is used as part of the antenna design, I doubt you could measure any difference. "The total current in the antenna consists of two components. The amplitude of the first is determined by the power supplied by the transmitter and the free-space radiation resistance of the antenna. The second component is induced in the antenna by the wave reflected by the ground. This second component, while considerably smaller than the first at most usefull antenna heights, is by no means insignificant." Ham antennas, dipoles, beams, etc., used for HF communications are HORIZONTALLY polarized antennas. This is a whole new ball game when they are close to "ground" be it a sheet metal roof or the ground, itself. Unlike the radiation pattern of the vertical halfwave in question, the radiation pattern of a horizontal dipole, which is still perpendicular to the dipole wire, INTERSECTS the ground plane below it and the RF re-radiates or reflects off the ground plane. The radiation pattern of a horizontal dipole very near ground is straight up and has a hot-air-balloon shape straight up. As the antenna moves away from ground, a dimple forms in the "balloon pattern" which forms a null at zenith with the radiation now two "lobes", perpendicular to the dipole whos angle of radiation drops from zenith out towards the horizon as the dipole becomes 1/2 wavelength off ground. Beyond 1 wavelength off ground, the pattern becomes the familiar donut perpendicular to the horizontal wire radiating upward and outward, even down towards the ground plane, whos reflections and re-radiation phase angles caused the odd pattern in the first place. Vertically-polarized signals point the NULL in the radiation pattern off the ends of the dipole. One of these nulls is towards the ground plane so little re-radiation takes place. A 1/4 wavelength "ground plane antenna" has a radiation pattern elevated only slightly towards zenith, which isn't much of a problem at all. So it would seem that the "ground plane/wave" is not an umimportant consideration when considering antennas. Ground effect is VERY important in a horizontally polarized dipole or beam antenna. That's why we put the beams way up on towers so they radiate towards the horizon, not at high radiation angles. The phase shifted re-radiated patterns of the slightly longer reflector and slightly shorter directors (lagging and leading, respectively) "pull" the donut towards the directors and away from the reflector, pointing the beam's radiation pattern in the desired, narrow direction....and giving great gain.....if it's not too close to the ground, that is! Further on that same page, is the following: "Changing the height of the antenna above the ground will change the current flow assuming that the power to the antenna is constant." Again, this is for "ham antennas" which are generally horizontally polarized. The reflected wave from the ground back to the horizontal dipole GREATLY changes its impedance characteristics because that reflected wave causes a phase shifted current in the radiating element, itself. Close to ground, this creates a large REACTIVE component, which shows up as reflected power at the transmitter output, not good at all. Again, it would appear that the "ground plane/wave" is not insignificant. Now, as I understand it, at VHF frequencies, the methodology of providing energy to the antenna (loading/feed) is not as important to the generation/reception of the signal as is height. In fact, if I read the pattern charts correctly, the height of the antenna has more to do with the lobe pattern (the donut you were discussing) than the method of feeding the antenna. Yes/No? No, not on VERTICALLY polarized antennas. On VHF there is no replacement for POWER and ALTITUDE. VHF is line-of-sight communications. The higher the transmitter and receiver antennas are, the longer your range and better your signal at the longer range. It's why WCSC runs hundreds of kilowatts from a 2000' tower.....RANGE. They pay very dearly for both. You should see the WEEKLY electric bills and tower maintenance bills. On a skywave system, like AM radio at night or ham radio HF, altitude is not very important other than to make the antenna's radiation pattern and impedance what we want because it's horizontal polarization. |
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