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V antenna paterns
Jim Lux wrote:
In free space, there is not much difference in pattern between a dipole that is perfectly horizontal and one that is drooped or sags (or for that matter one that is bent horizontally). What changes is the feedpoint impedance (72 ohms for perfectly flat, getting towards 50 ohms with a 120 degree included angle). The other thing that changes is the apparent height above ground. If you model a drooped dipole and fiddle with the height to try and match the vertical pattern for a perfectly flat dipole, you find that the "effective center" of the drooped dipole is around 1/3 of the droop distance. That is, if you had a dipole where the center were at 100ft, and it drooped 30 feet on either end, the pattern is about the same as a flat dipole at 90 ft. The height above ground has a HUGE effect on the dipole pattern (especially at low elevation angles) and probably dominates any small changes. After all, a perfect dipole has a gain of 2.15dBi and a infinitely short dipole has a gain of 1.6dBi, and a drooped or sagging dipole pretty much has to be somewhere in between. Those gains are for dipoles in free space. A dipole over ground typically has several dB greater gain. The absolutely worst gain any efficient antenna can have in free space in its best direction is 0 dBi. So nothing you can possibly do to the pattern can reduce it below this value in free space, as long as you don't introduce loss. But as Jim says, you can mess with a dipole a fair amount without changing the pattern much -- except for filling in or moving the nulls, which can be very sensitive to small changes. Roy Lewallen, W7EL |
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