[Fourlanders] adjusting takeoff angle

Fri Jun 15 09:34:14 EDT 2007

Not only is Az-El complicated, but for angles in the 15 to 40 degree range
it does not work well. In the early 80's we did some experiments with this
for very similar reasons.  Our station in Malibu, CA was on a peak of 3000'
with very steep drop offs in all direction.  When the MUF was high we were
getting killed, especially on 20M, by small stations down in the flat lands.
We mounted at great expense and work, our 20M 5el yagi (@100') on an Az-El
that we built.  After about a year of experimenting we concluded that only
at elevation angles greater that 45 degrees there was any noticeable change
in signal strength.  We (Fred, AD6C and I) discussed this and came to the
following conclusion:

As the elevation is increased, the image, which is inverted, diverges and
thus cancels out the effect.  At very high angles, the effect of the image
on the pattern diminishes as ground absorption takes over (the image points
down deeper into the earth).  Fred confirmed this by doing the calculations.
At very high angles we lost the gain due to the image but noise was reduced
and very high angle signals and S/N were improved.

Our final solution was to fix several antennas at 1/2 wl above ground in the
most useful directions (Europe!).

As you may recall, I often side mounted a small yagi at a low height aimed
to the SE.  Often the signals from Texas were significantly strong that the
long boom antenna on the top of the tower.

As to adjusting the phase difference between the two stacked antennas, this
can work well, but you will have to design a system which will allow you to
adjust the phase angle real time.  Not a simple task especially at higher
frequencies.

Good Luck.

Neal Sulmeyer, K4EA

-----Original Message-----
From: fourlanders-bounces at contesting.com
[mailto:fourlanders-bounces at contesting.com] On Behalf Of Paul Yeager,
ABR(R), REALTOR(R)
Sent: Wednesday, June 13, 2007 01:28
To: Fourlanders
Subject: [Fourlanders] adjusting takeoff angle

Ron WW8RR points out that our high-gain stacked beams are likely striking
the ionosphere quite some distance away, due to their low takeoff angle.
Ron further suggests that this is causing us to miss closer in stations that
we might be able to work, if could arrange a higher takeoff angle.  (Ron -
please let me know if I have misstated something here)

  I see two obvious ways to change the takeoff angle.

  1) Mount anntenna(s) on an az-el rotator.  This seems pretty simple, but
when one considers the details, it becomes a bit complex mechanically.  How
does one mount a horizontally-polarized yagi on an az-el rotator?  The only
way I can see is to mount it on a rather long "boom", rotated by the
elevation rotator.  The boom needs to be rather long to avoid interaction
between the antenna and the tower/rotator, and may need to be fiberglass or
other non-conductive material to avoid degrading the performance of the
antenna.   This will also place quite a side load on the rotator and tower,
unless the boom is extended to the side opposite the rotator with a
counterweight.

  Alternatively, we  might extend the antenna's boom, then add a right angle
to the rotator.  This increases the load on the elevation rotator
considerably, unless a counterweight is added.

  See what I mean about mechanical complexity?

  2) Using a similar antenna arrangement to the one we used for the contest,
adjust takeoff angle by adjusting the phase between the two antennas.
Mechanically, this is no more complex than the current arrangement.
Electrically, it's quite more complex.  Not rocket science, but certainly
more complex than a single feedline running to a splitter feeding both
antennas.

  We could do something as simple as selecting upper/lower/both yagis, or
get more complex and switch sections of feedline in and out to actually
change the phase.  Two switchable sections of feedline would give us 4
choices of takeoff angle - in-phase pluse three different phase delays.

  Anyone have any idea how to model phase vs. takeoff angle for out pair of
yagis?  Speaking of modeling...  how about takeoff angle vs. "skip
distance"?

  I suggest that we test the concept using a lightweight 6m yagi, like the 4
element Comet I have, mounted on an az-el rotator, either during the
September contest or at some time between now and then.  This would give us
an idea of whether or not the concept is viable, and what sort of
adjustments to takeoff angle are necessary.  Once the concept is validated,
we could construct a "phasing system" to use with our pair of yagis.

  So... what do y'all think?

  73 de W4SKI

Paul Yeager, ABRR, REALTORR/Broker
Accredited Buyer Representative
Mountain Fever Real Estate
828 400 9442 (cell)
828 926 2545
888 926 2545
828 926 3860 (fax)
paul at mtnlist.com
www.mtnlist.com

What is ABRR?  Find out at http://www.rebac.org
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