On 14 Oct 2006 08:20:07 -0700, "CW" wrote:


Of course I didn't assume that there is one "take off angle", but I did
realize that there is a theoretical optimum take-off angle, and that
the field strength diminishes as one moves away from that angle in a
generally smooth and continuous way. This is obvious on any antenna
radiation pattern chart. The point I was getting at was that the field
strength diminishes in both the horizontal AND vertical planes.

Uh huh. But I think there's an apples and oranges thing going on
here.

If you want to discuss antenna patterns v. height, that is one
discussion. If you want to discuss optimum propagation paths that is
another. "Theoretical optimum take-off angle" leaves me guessing but
I think you're referring to the former above and the latter below.

The
common antenna rotator allows directing the radiated field in the
horizontal plane, thereby "peaking" the signal in that plane.
There seems to be very little attention paid to peaking the signal in
the vertical plane, which can be readily accomplished by raising and
lowering the antenna height. I was curious as to the discrepency. The
mechanical complexities just don't seem to fully account for the
disparity in usage of these thechniques. I would estimate that
rotators are at least 2 orders of magnitude more common than variable
antenna height mechanisms. Since I'm assuming that both vertical and
horizontal components of feild strength are important in HF signal
propogation, I was wondering why so relatively little attention is paid
to peaking the vertical component. Is it because of mechanical
complexity, lack of understanding, or something else?

I personally know at least a couple of dozen serious hf DXers
(presumably the more interested in "optimizing" this sort of thing)
and know of a lot more and I don't know of one of them that tries to
peak a signal by "readily" cranking a tower up and down.

But let's say it's easy to have a free-standing, 100' high (the limit
in my county), motorized, positive pulldown, crank-up tower that we
are absolutely confident we can crank up and down while out of sight,
without worry of the coax getting jammed up or the winch cable wearing
out, etc.

Atop this tower we install a well-designed, 3-element, 20-meter Yagi.
The ground is unremarkably average.

Suppose that despite all of the propagation vagaries there is a DX
station we want to work and the "optimum takeoff angle" to his
location is 13 degrees.

Let's begin with our antenna at 50' above ground.

Using EZNEC, I modeled such an antenna (my design) and find that at
50' above ground, the gain (13 dBi) peaks at an elevation angle of 18
degrees; not "optimum" for this path, so we start cranking.

At 75' the gain (13.4 dBi) peaks at 13 degrees but we don't know that
so we keep cranking until we reach 100'. At 100' the peak gain (13.6
dBi) occurs at 10 degrees; again not "optimum."

At the "optimum" 13 degrees, the penalty for having the antenna at 50'
is 0.9 dB and for having it at 100' is 1.2 dB. Now the question has
to be, can you tell the difference?

(The math is correct BTW. Elevation patterns are not symmetrical)

After all of these heroics, for all practical purposes, we can't tell
the difference, but if we are wrong, we're better off at the *lower*
height.

Maybe this is why nobody varies their antenna height to peak the
signal.