At 10:29 PM 5/7/2006, Jon M. Knodel wrote:
>Thanks to all those that have given me feedback in my tower project. I am
>getting ready to install it this summer and am again, looking for some
>help before I do. My house has a metal roof that I am afraid is going to
>interact with the antennas.
You bet it will..
>I will be able to completely load the tower with all the antennas that I
>want to at the 40' height. This would put the top of the tower only about
>25' above the metal roof. Is this close distance going to affect the
>antenna performance (3 element SteppIR yagi)? I can also sacrifice some
>antenna load and put the tower up to 60'. This would be the path I would
>take if the roof would interfere with the antenna at 40' (trading off
>antennas for additional height and less interaction with the metal roof).
>So, my question is: how much would the roof interact with my antenna?
One rule of thumb is that things within a 1/2 wavelength interact with the
antenna. 1/2 wavelength on 20m is 30 ft. But it's not like a magic wall,
where "inside the wall" bad things happen, and "outside the wall" nothing
> Because it is a SteppIR and fully adjustable, is this a concern at all
> due to the fact that it can be retuned easily?
Yes, but. Don't forget you're not concerned only with match, but with the
pattern. Somehow, I doubt that the SteppIR folks happen to have optimized
element lengths figured out for your situation.
However, they, or someone else, might have modeled/measured/figured out
element lengths that give good performance when the antenna is closer to
the ground, which at least would give you a direction to try.
> Or would the radiation pattern be adversely affected? I appreciate all
> the help I can get. Thanks.
Kind of depends on what you mean by "adverse".
There's two effects going on here.
One is just the height of the antenna above the ground (the kind of thing
that HFTA models very nicely) affecting the vertical pattern.
The other is the effects on the element currents from the interaction with
the roof. Those will potentially reduce the F/B ratio, as well as changing
feedpoint impedance and the gain. Tough to figure out the exact effects.
At least with a SteppIR, it's fairly straightforward to figure out what the
F/B is, with the 180 flip feature.
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