On 12/28/11 7:50 AM, Roger Parsons wrote:
> Eddy and others
> I am a little unconvinced by some of the arguments that have been presented.
> Consider a Yagi in free space with nice copper elements. It would work quite
> well if one could find a way to connect the transmitter.
> Now consider replacing one of those copper elements with one made out of
> wood. Because wood is a poor conductor, very little current would be induced
> into it from the driven element, and to all intents and purposes the Yagi
> would become one having one less element. The pattern would (probably) be
> distorted because of the missing element, but the efficiency of the array
> would be almost unchanged.
> By extension, keeping all the original elements in place, and introducing an
> additional wooden element into the array (even if resonant), would have very
> little effect because the current induced into that wooden element would be
> negligible. This is a close parallel to having a tree near an antenna.
> The question of whether it is good to have an antenna in the middle of a
> forest is completely separate. If we put our nice Yagi into a big wooden box,
> it will not work so well because there is now a lossy medium through which
> the wave must pass. The thicker the walls of the box and the more conductive
> its material the greater its effect will be.
You're describing the difference between the near field (wooden element
in Yagi) and far field (propagating through a box). One would normally
use a somewhat different analytical technique for the two, even though
the underlying physics is the same. For the propagating wave situation,
we assume that the ratio of electric to magnetic field is constant (at
least within one medium), but in the near field case that's not
More to the point, the fields are much more intense and localized in the
"within the Yagi" case, particularly for superdirective arrays (which
almost all all Yagis are) where there's a lot of energy stored in the
fields around the elements. A lossy element potentially occupies a
larger fraction of that field space than a tree in the forest.
Also, bear in mind that the "average density" of that forest is probably
a lot lower than a solid box.
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