Thinking bout building a double groundplane, with both sets of radials facing each other.
Does one the verticle elemants facing down affect the preformance ?
Does one the verticle elemants facing down affect the preformance ?
Upside Down Ground Plane
- Thread starter trimmerj
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He won't be the first to know. Upside down ground planes are an old trick. I'm wondering what you hope to accomplish, though. Ground planes have a fairly high angle of radiation, and a broad main lobe. Mounted right side up, a good portion of the radiation pattern is directed at the sky above the horizon.
here's where the trick comes in... many a repeater operator has mounted ground plane antennas upside down on mountaintop radio sites so that the main lobe is now aimed BELOW the horizon, where the desired coverage area is. In cases where the coverage area is near the base of the mountain, it often works MUCH better than a higher gain antenna mounted right side up.
At ground level, you may actually notice that the upside down antenna doesn't hear as good.
here's where the trick comes in... many a repeater operator has mounted ground plane antennas upside down on mountaintop radio sites so that the main lobe is now aimed BELOW the horizon, where the desired coverage area is. In cases where the coverage area is near the base of the mountain, it often works MUCH better than a higher gain antenna mounted right side up.
At ground level, you may actually notice that the upside down antenna doesn't hear as good.
N1BHH
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trimmerj said:
Double ground plane? Are you talking about a ground plane upside down? Radials on top, radiator facing down? I don't quite picture what you want. I have used a mag mount on a metal plate mounted to the top of window blinds when there was no height to put the vertical facing up, so it was hung downward and that worked fine. You only need one set of radials. You should not have radials in the upward plane or the downward plane when you have a radiating element in the upward plane and one in the downward plane. Radials must go in the opposite direction of the radiating element,
In other words if you want to have your vertical element facing downward from a feed point, your radials should face the opposite direction, upward. A dipole has two elements, thus the "DI" in dipole. One element is connected to one side of the feed line, presumably the center conductor and the other side to the shield, presenting a feed point impedance of 50~75 ohms, making it easy to use 50~75 ohm feed line. In a ground plane, one part of the antenna, the vertical radiator is connected to the center conductor, the other part is connected to the shield, presenting a 50~75 ohm impedance.
Placing radials and vertical radiating element(s) in the same general plane will cause wider fluctuations in feed point impedance and also severely distort signal patterns.
Placing two dipoles on one feed line is a practice I have used in HF antennas many times and it worked fine, one cut for resonance at one frequency and the other for another frequency. This was done in an "X" pattern, in the horizontal plane. I have done this in the vertical plane at VHF frequencies as well. If you want to, for example, cross dipoles, that is fine and if they are both to be used in the same band, you can have one longer and one shorter to cover a wider range of frequencies. You can also try a VHF dipole crossed with a UHF dipole and that would work, too. With this arrangement, no radials are needed, since the elements are connected across a feedline, one pair cut for one band and the other cut for another band. Both are fed at the same feed point and should work out no problem.
Take a look at this for design ideas: http://www.hamuniverse.com/multidipole.html
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