At 09:09 PM 4/2/2007, Jim Brown wrote:
>On Mon, 02 Apr 2007 20:42:17 -0700, Jim Lux wrote:
>
> >A more accurate statement would be that the performance starts to be
> >determined more by the far field ground properties, which, with a
> >vertically polarized antenna, are pretty important to overall
> >performance.
>
>Yes. It is VERY well known (that is, long ago proven science) that
>radials affect the EFFICIENCY of a vertical, while the ground in the far
>field determines the vertical radiation pattern, especially the strength
>of the low angle radiation. High conductivity => good low angle field
>strength, poor conductivity => poor low angle field strength. Again, the
>ARRL Antenna Book and ON4UN's book are very good on this topic.
That they are (frustratingly so, if you're trying to get good
performance from small vertically polarized antenns).
Having gone back and looked at them, what's kind of missing from the
discussion is the synthesis of the tradeoff between the close-in
efficiency effects vs the far-field and takeoff angle effects. Like
RK's comment about feet of vertical being a good trade for miles of
radials (to exaggerate a bit)..
Obviously, it depends a lot on the environment, but it would be
interesting to look at it and see if there is some sort of
generalized statement along the lines of (not that this is actually
correct.. it's an example): If you have conductivity <0.005 S/m, then
you are better served by a shorter vertical with a lot of radials
than by a taller vertical with fewer radials, or an elevated vertical
dipole (or sloper, etc.).
There's entirely too much "average soil" modeling which totally
obscures the reality that soil properties probably have an order of
magnitude larger effect than any moderate changes in antenna
configuration (until you get to the point where you put the antenna
so high, or with so many radials, that you become soil properties immune).
Jim, W6RMK
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