Question for Antenna Experts?????

[Wile E. Coyote]

I was wondering what would happen if someone put up a base antenna (Bandit/with groundplane) and put the ground plane up-side down. That is the ground plane is pointing up instead of down?????? What would this do to the signal being transmitted or receiving signal????????? Friend said that he did this so that he wouldn't bleed all over his neighbors TV's and radios. My neighbors haven't complained about my radio and I have the same set-up, but mine is hooked up per the instructions (ground plane pointing down). And not running any power and he is running about 500watts. Wouldn't this reflect his transmitting signal back into the radio???????? He lives in a rual area and I am in the middle of a residential area in a city.

[Circuit Breaker]

I fail to see how mounting the antenna upside down will prevent RFI issues. And how did he do this? The feed point would then be at the top, which means his coax would be running right beside his antenna on its way back down to the shack. Antennas causing RFI is a myth. All they do is radiate. Some better than others, but the antenna itself does not cause interference. If he has no interference problems with his amp off, then it means his neighbor's equipment is simply not shielded well enough. If putting his antenna upside down solves the problem, then what's happening his is radiation pattern has been changed and his signal isn't hitting the device in question as strongly as it used to. But, again, the antenna itself is not the problem.

[Double D]

Pointing the radials upward will make little or no difference. The earth is the ultimate ground plane and if you want to avoid rfi, you should mount the antenna higher.

[Red Warrior]

I fail to see how mounting the antenna upside down will prevent RFI issues. And how did he do this? The feed point would then be at the top, which means his coax would be running right beside his antenna on its way back down to the shack. Antennas causing RFI is a myth. All they do is radiate. Some better than others, but the antenna itself does not cause interference. If he has no interference problems with his amp off, then it means his neighbor's equipment is simply not shielded well enough. If putting his antenna upside down solves the problem, then what's happening his is radiation pattern has been changed and his signal isn't hitting the device in question as strongly as it used to. But, again, the antenna itself is not the problem.

This type of antenna has ground radials slanting downward at 45 degree angle from the bottom of the antenna. He is referring to mounting the spider upside down not the entire antenna. Mounting the spider upside down will have the radials pointing up at 45 degrees rather than down. This will tend to make the antenna very lossy.

[Texas Mobil 219]

Wouldn't shoot the signal into the gound?

['Doc]

The angle between the vertical element and the radials determines the input impedance of the antenna. It has nothing to do with 'where' the resulting signal goes, doesn't 'point' it up/down, side ways or whatever. I'm surprised your friend's SWR is anything approaching normal, if his antenna hasn't been modified in some other way.
Also, contrary to popular belief, those radials radiate too. They don't keep RF from getting 'under' them, they are not a shield of any kind. Got a field strength meter? Try it, see for your self.
- 'Doc

(Sorry, I'm not an 'expert' either.)

[Circuit Breaker]

The earth is the ultimate ground plane and if you want to avoid rfi, you should mount the antenna higher.

Not quite true. Earth is the biggest source of loss. Getting it higher keeps those losses down.

And Doc is right...in many antennas, the angle of the radials determines the input impedance. As the angle increases, so does the impedance.