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I second that. Assuming it is a vertical with a ground plane/radials, then as long as you are above the roof it should be fine.
Of course, with VHF and up, higher is better. If you can get it over a peak you should be in good shape with regards to the roof messing with the radiation pattern or SWR. If mounting at the edge of a roof, then I'd try and go as high as possible in order to get the radiating portion above the peak, but as long as the radials are above the edge by a few feet (assuming it has radials) then I wouldn't worry about it. Worst case is that you have slightly diminished performance do to the roof shadowing everything on the side opposite of the antenna (think giant mirror, only in a very limited sense....).

With that said, I made a 1/4 wave vertical for 144.39 MHz which is constructed out of an SO-239 and some steel roof trim. Its screwed to my shop's peak, and that's the antenna for my APRS digi. Sure, the proximity to the ground plane will affect the radiation pattern, just like it would with something like a Comet GP-6. But, even my less than ideal vertical works--and quite well, in my opinion.

Now, a horizontal antenna or a beam are two very different situations. I believe there is a rule of thumb for separation from near by conductive objects to reduce pattern distortion for any antenna. Proximity and orientation will play a significant role in what happens to the pattern and electrical properties. Sadly, anything past the standard rules of separating conductors by (I'll assume) 1/2 wavelength and keeping things at a 90 degree angle to each other to reduce coupling will require testing to determine the acceptable outcomes. For instance, I have mounted GP-6 verticals on a 2x4 shoved underneath a steel railing within 3' from the vertical bars had great results--on one side of my house! I'd do it again, too. I've also hung Yagi's such that the elements were vertically oriented and less than a foot from the top of said railing. It worked, and wasn't worth complaining about.

The railing and roofs are different applications obviously, but I wanted to point out that "ideal," "prefect," and "acceptable" don't always point to the same thing, as I'm sure you know. Also, it was my primary reference for a personal use case with antennas near metal objects.

I hope that helps. If I find the separation figures I'll try and post them unless somebody finds them first.

73 de KG5ZSU
Tommy Brooks



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