Fellows,

I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.
It seems to me that raising and lowering the height of a Yagi affects
the take-off angle by at least several degrees, meaning that the signal
delivery (target area) would be moved by at least many hundreds of
miles.
It would also affect the average wave incidence angle with the
ionosphere, so one could tune (or peak) the signal significantly with a
few feet of height adjustment. I am aware of stacked Yagi's being used
by some contest stations, where variable phasing feed techniques
between the upper and lower Yagi can (and does) affect take-off angle.
I'm just suprised at how little literature and practical use of this
technique exists. It can't be all that difficult to build a Yagi that
can be winched, or slid up and down the side of the tower by several
feet.
Does anyone have a better theoretical understanding of the possible
signal strength change from an ideal one-bounce propagation when a 20m
Yagi is varied in height from .9 to 1.0 wavelength in height?
How would a 0.1 wavelenght height change compare to a 30 degree
rotation angle change?
I know that some of the difficulty in quantifying the benefits of such
a scheme is that the refractive layers of the atmosphere change in
altitude regularly. So take-off/refraction/range calculations become
cumbersome. But perhaps some fixed assumptions would allow some general
statements about the typical gains vs. antenna heights for a fixed
range.

Thanks for any input on this apparently unusual technique.

73,
CW-AI4MI

On 14 Oct 2006 06:32:21 -0700, "CW" wrote:

Fellows,

I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.
[snip]

You make the all-too-common assumption that there is one "take off
angle" and apparently believe that no usable radiation occurs at any
other angle.

You wouldn't (I hope) believe that there is one azimuth angle and that
if your antenna isn't pointed *exactly* at the target you're SOL, so
why the concern about height?

CW wrote:
I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.

Many do, using motor driven towers. In addition to lowering
their arrays when a storm hits, some raise and lower their
towers during marginal conditions to maximize signal strength.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
t...
CW wrote:
I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.

Many do, using motor driven towers. In addition to lowering
their arrays when a storm hits, some raise and lower their
towers during marginal conditions to maximize signal strength.
--

its really only practical on crank up type towers, for those with guyed
towers its usually not possible.

10' change on 20m would likely not be very useful though. my hf stacks for
10/15/20 are all spaced 30' apart, 40m is spaced about 80'. even with those
height changes (which i can select instantly so i can make direct
comparisons without worrying about fading) there is often little difference
between antennas... though sometims there is a lot of difference. This
highlights the fact that often the signals arrive with a wide range of
angles, though at some times they must be in a relatively narrow range. so
having multiple antennas at different heights that can be selected in
various combinations is a handy thing to do.

Wes,


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. 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?

73,
CW-AI4MI




Wes Stewart wrote:
On 14 Oct 2006 06:32:21 -0700, "CW" wrote:

Fellows,

I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.
[snip]

You make the all-too-common assumption that there is one "take off
angle" and apparently believe that no usable radiation occurs at any
other angle.

You wouldn't (I hope) believe that there is one azimuth angle and that
if your antenna isn't pointed *exactly* at the target you're SOL, so
why the concern about height?

Dave,

Thanks for the reply. Multiple switched Yagi's at various heights is a
practical approach, and I'm delighted to hear that "sometimes there is
a lot of difference". What you are describing is a coarse grained
approach to the problem, which is also commonly done in the horizontal
plane by switching vertical antenna arrays, etc.
A fellow ham in the area has a Yagi mounted about half way up his guyed
tower, on a swing arm. It can rotate, but is limited to about 300
degrees of rotation. A similar setup could be used on a side mounted
tower trolly, where the height could be continuously varied by 30 feet
or more, AND rotated through about 300 degrees.
73,
CW-AI4MI



its really only practical on crank up type towers, for those with guyed
towers its usually not possible.

10' change on 20m would likely not be very useful though. my hf stacks for
10/15/20 are all spaced 30' apart, 40m is spaced about 80'. even with those
height changes (which i can select instantly so i can make direct
comparisons without worrying about fading) there is often little difference
between antennas... though sometims there is a lot of difference. This
highlights the fact that often the signals arrive with a wide range of
angles, though at some times they must be in a relatively narrow range. so
having multiple antennas at different heights that can be selected in
various combinations is a handy thing to do.

ORIGINAL MESSAGE:

On 14 Oct 2006 06:32:21 -0700, "CW" wrote:


I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.

------------ REPLY FOLLOWS ------------

The answer is because it isn't really necessary in most cases. If you
optimize your antenna for low-angle DX signals where you really need
the gain, there will still be significant radiation at high angles,
which are generally closer to you and will have stronger signals as a
result anyway.

This might be significant enough for a QRP operator to want to do it,
but at the 100 watt or higher level, you wouldn't gain much. Pun
intended. :-)

Bill, W6WRT

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

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?

Hi OM,

The single biggest factor is cost. You should be able to appreciate
the implication there.

The second (and related to cost) biggest factor would be wind load. A
taller tower is easier to push over.

Another factor would be the requirement for a very stout (cost again)
mast (more cost to lift more weight too); unless this is a telescoping
tower (costs more than a conventional one, doesn't it?).

About the cheapest consideration, the last one in this list, is what
you call "lack of understanding." The better question is:

What is the optimal angle for contact, not for launch?

Art recently came aboard here to seek validation for a secret design
that aimed "all" his power to England. When I did the modeling, that
optimal angle, depending upon
Frequency
Time of Day
Season
Sun spot cycle
varied from less than 6 degrees to as high as 12 degrees. FYI his
secret design is still secret.

Anyway, you can fulfill this last requirement (the cheapest) by using
two free software packages:
EZNEC for the launch characteristic of the NBS yagi vs. height;
VOAWIN (VOACAP VOAAREA) for propagation of that same antenna.

73's
Richard Clark, KB7QHC

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.

On 14 Oct 2006 06:32:21 -0700, "CW" wrote:

I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.
It seems to me that raising and lowering the height of a Yagi affects
the take-off angle by at least several degrees, meaning that the signal
delivery (target area) would be moved by at least many hundreds of
miles.

Or hows about raising and lowering the angle of the Yagi? Tilting it
somewhat? Maybe a winch and cable to the end of the Yagi and some
kind of rotatable joint at the tower end. That would also be complex
and cost some.

Tony