On 6/14/20 8:23 PM, Jim Brown wrote:
On 6/14/2020 7:03 PM, wesattaway wrote:
However, as overall height is raised then best performance occurs
when the wires are level. I think Jim Briwn may have some data on this.
Hi Wes,
My study was on the effect of height on horizontal and vertical
antennas, and I developed a figure of merit in dB for height of
horizontal antennas. The executive summary is that for 30M and below,
higher is better. :)
<snip>
3) Soil quality STRONGLY affects vertically polarized antennas -- the
better the soil conductivity, the better they work.
4) HF verticals work better on the roof than on the ground.
<snip>
There's two separate factors at work in #3
a) a "near field" effect - for a monopole vertical, the ground (or
radial field) is half the antenna. Hence the "120 radials" for FCC proof
of performance exemption. Not so much effect for a vertical dipole.
b) a "far field" effect - H-pol is reflected well almost at any
incidence angle and with any soil properties. Not so with V-pol which is
strongly affected by soil properties and incidence angle.
The difference in these two effects (in broad strokes) is that (a) is a
big deal close in (dimensions comparable to antenna height) and (b) is
about the soil properties farther away.
Consider a 50 foot tall monopole. You can think about the ray from the
antenna hitting a spot at some distance and then reflecting. And each
point on the antenna hits a different spot.
For a low elevation angle, say, 10 degrees, the spot for the top of the
antenna is 50/tan(elev) = 283 feet away. And it gets way farther out
very rapidly. For 3 degree elevation, the "reflection spot" is 1000 ft
away. Of course, for a spot on the antenna that is 25 ft high, the
"spot" is half as far away.
So for really low angle radiation (like 3 degrees), everything within 20
times the height of the antenna contributes.
Hence the popularity of verticals at the beach, or in the middle of the
proverbial salt marsh.
As Jim points out in #4, raising the antenna is good (reduces losses
from near field (a)) but does extend the far field issue. For a 50 foot
elevated dipole at 100 ft the radiation at 3 degrees is reflecting from
spots at 1500-2500 ft away.
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