Re: HF and the elevation advantage

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From: Bill Coleman (aa4lr@arrl.net)
Date: Wed Jun 06 2001 - 11:25:51 CDT


On 5/31/01 3:46 PM, Garie Halstead at k8kfj@ntelos.net wrote:

>A question for you propagation gurus out there....
>
>I regularly work the VHF contests every year where elevation is
>extremely advantageous. On the HF bands, I understand it would
>also be an advantage on 10m and probably to a certain extent on 15m.
>
>***But what about 20m and down?*** (Other than having an unobstructed
>shot in all directions).

The answer (as always) is, "it depends."

Generally speaking, however, HF propagation results in certain apparent
angles of arrival. For distant stations, these angles are generally
within a few degrees of the horizon. (However, there are exceptions -
multi-hop propagation can tend to raise arrival angles, as can operating
well below the MUF)

If we accept that most DX is going to arrive at low angles, then we
should construct our antenna system to respond to those low angles. For a
horizontal antenna (dipole, yagi), the antenna response at low angles is
directly related to the height in wavelengths above ground. This is why
height is the signal most important attribute of a horizontal antenna.

Generally speaking, we want a horizontal antenna to be at least 1/2
wavelength above ground, but there are diminishing returns beyond 2 or 3
wavelengths. As the antenna is raised, the pattern of the antenna
changes. Get the antenna too high, and you'll find that the antenna
becomes LESS responsive to certain arrival angles. (The guys with 10m
yagis at 200 feet can tell you there are times when this antenna is too
high)

Vertical antennas have patterns that aren't as affected by height.
Generally, a vertical has a low angle of radiation, which makes it good
for DX, regardless how high it is off the ground. (Ground losses will
tend to raise the angle the radiation, so good ground coupling is usually
important)

As for terrain, being on a peak, or partway up a slow affects the
radiation patter of antennas as well. If the terrain slopes away from the
antenna, that has the same effect as having the antenna be higher
(sloping toward looks like lower).

The guys who are really into this stuff (mostly contesters and DXers) go
to great lengths to figure this out. There are programs that will tell
you what arrival angles to expect signals from various parts of the world
(with varying solar conditions), programs that analyse the affect of
terrain on antenna patterns, programs to determine the antenna patterns
at various heights.

Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901


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