On 6/15/20 7:49 AM, Robert Harmon wrote:
Interesting discussion on inverted V's. I have probably a typical inverted V
setup for 80M strung off of a tower. I have the center of the V strung off my
tower at 90 feet and the ends slope down to 35 feet to two poles on each side
of my property. Here's an idea I have been thinking about. Hanging a
vertical dipole from the tower with loading coils in each leg to compensate for
the shorter length. Better low angle radiation ? I know I would have reduced
bandwidth but that would be ok, I hang out in the very low end of 80 anyway
chasing CW DX. What do you think, improvement over the V ?
As Jim pointed out horizontal polarization beats vertical polarization
almost all the time. Your V is H-pol.
I've modeled dozens of antennas for restricted spaces (like suburban
lots) and, of course, verticals are attractive in that application
(inconspicuous, and they fit), but I've not found something that
consistently beats even an electrically short horizontal dipole (or a V
with shallow droop, up to 45 degrees).
Unless your house is in the middle of a particularly excellent soil
properties (salt marshes again).
There are specialized cases where the trade might push one way or the
other. Or you might value inconspicuous over performance (defined in a
TBD way).
A dipole parallel to a tower will likely couple to the tower, which will
become "part of the antenna". That's regardless of whether there's
loading coils, etc.
I wouldn't even hazard a guess as to the effect - that's definitely in
"try it in NEC" territory. The tower could "short" out the dipole, or
be a great radiator. Think of things like compact loops with a short
coupling loop.
Bob
K6UJ
On Jun 15, 2020, at 7:10 AM, jimlux <jimlux@earthlink.net> wrote:
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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