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Antenna Ratings
On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing
"Changing the height of the antenna above the ground will change the current flow assuming that the power to the antenna is constant." 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? Later, Tom S. Woodstock, CT The method of feeding the antenna has nothing to do with the take off angle of an antenna. The stuff you are reading in the handbook about ground effect on vertical angle are discussing HF antennas where due to the low height compared to wavelength and the propagation method at hf the ground has a great effect. An antenna at VHF is usually several wavelengths above ground and ground has little effect on take off angle. Given a 1/4 wave ground plane antenna with radials at 90 degrees, the ground plane of that antenna will be the controlling factor. It does tilt the angle up a little from what a 1/2 wave dipole would be. If you take the ground radials and bend them down and folded over the coax feed line so that they are vertical you then have a half wave center fed vertical dipole antenna. This particular version would have the coax going up through the center of the folded down ground radials. A tube or pipe could be substituted for the folded over ground radials. In most cases that is exactly what is done. A metal tube is used in place of the radials. This is commonly referred to as a "coaxial sleeve antenna". Just about all of the fiberglass type VHF antennas have some form of this type of ground plane in them. Another type of 1/2 wave vertical antenna that does not need a ground plane is one that is fed at the end of the 1/2 wave length rather than at the center as above. The METZ type half wave is one such type. It uses a coil at the feed end to transform the low impedance of the coax to the high impedance feed end of the antenna. At a high impedance feed point the current is much less than it is at a low impedance feed point (center of 1/2 wave) so not much of a ground plane is needed. In this case the coax shield acts as the ground plane for the 1/2 wave antenna. It is not the best ground plane but again not much is needed in this case. The high current point is in the center (quarter wave point) of the antenna. Once you get a few wavelengths above ground additional height does not much effect the radiation lobes (pattern) of the vhf antenna. But height does effect the line of sight or in this case radio line of sight, (which is slightly greater than visual). What Larry is talking about with the donut shaped vertical pattern is a result of gain in the antenna. The higher the gain the flatter (sharper) the vertical pattern of the antenna. Any tilt of the high gain antenna will raise the pattern above the horizon or tilt it into the sea. It will not be at the horizon where it does most good. A lower gain antenna, like the 1/2 wave, has a much fatter pattern. Like a fat doughnut or a ball. Tilting it one way or the other still maintains about the same amount of radiation at the horizon. This is because it normally radiates in a wide vertical pattern. With a wide vertical pattern a great portion of the signal is wasted as it gets radiated at high and low angles that are not useful. But the advantage is that it can be moved (tilted) a long way and still maintain about the same amount of useful signal. To get gain, a gain antenna narrows the wide vertical pattern. It robs some of the power normally radiated at high and low angles and places more of it at the horizon. Thus the narrower vertical pattern. The advantage is a stronger signal. The disadvantage is that if it is tilted very far the signal drops off sharply because the vertical angle is very narrow. Regards Gary |
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