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Flying Horseman
| | Posted on Sunday, October 14, 2001 - 6:59 pm: | 
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Another question for anyone that knows more than me. Which should be about everyone. How much room is needed around a ground plane so as not to effect the SWR? Any good answer's out there? |
Tech181
| | Posted on Monday, October 15, 2001 - 12:01 am: |
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For CB antennas around 18 feet of freely radiating space around the antenna. And at least half that from the bottom of the ground plane radials to whatever is below them like your roof.
Steve
Tech181
Tech181@copperelectronics.com |
Hamcber
| | Posted on Monday, October 15, 2001 - 11:28 am: |
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181 is correct in a different way.
You see, small fluctuations in SWR are the least of your worries. Pattern distortion is the worst effect that near-field objects have on your antenna. I had a ground plane antenna side mounted on a tower on a bracket about 4 feet away from the tower face and the SWR was just fine. The effect on the pattern was the downside. However, with a 190 foot tower, I couldn't very well top mount the antenna legally, so side mount was the only option.
Later, I put up another (shorter) tower for the ground plane itself and the pattern is much more omni-directional now. Near-field objects not only effect the horizontal pattern, but can also effect the vertical (also known as 'take-off') angle in various directions. Also, objects can reflect signal at odd phase angles from the fundamental and cancel out signal in various directions, not just the direction of the object itself.
Half wave spacing is not a magic number, but a good general rule to keep in mind. However, the best rule is NO objects near the proximity magnetic field of the antenna. That means more than 2 wavelengths away.
That's all the info you get for free. If you need more, I am for hire! |
Flying Horseman
| | Posted on Tuesday, October 16, 2001 - 9:05 pm: |
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181 I would like to say thaks. I think I can handle that with no problem. Will see what happens. Hamcber, I work too hard for my money to give it away. Thanks for the input. |
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