[Fourlanders] adjusting takeoff angle

[Jim Worsham]

Personally, I would like the ability to select upper, lower or both beams.
While not optimal it is relatively simple to do with a few good RF relays
and is very commonly done in HF contest stations.  Adjusting the phase
between the antennas IS NOT going to be trivial.  I did that kind of stuff
with phased array antennas in a previous life.  We used ferrite type phase
shifters but that isn't going to be practical at 50 MHz.  Alternatively, we
could could use an IQ arrangement and vary the phase by controlling the
relative amplitude of the I and Q components.  As I said before, not
trivial.  I suggest we try the single yagi mounted at an angle as an
experiment in September if you guys want to.  It sounds like Paul has a
beam.  We just need someway to mount it at an angle and rotate it.  With
some of the mechanical gurus we have in the group we should be able to come
up with a way to do that.  Alternatively, I can talk to Bob and see if he
thinks he will have time to work with me between now and September on
putting together a switch box for the upper, lower or both beams approach.
I am certain I can come up with a design.  I will need help locating the
relays and getting it all in a waterproof box that can be mounted on the
mast.

73
Jim, W4KXY

-----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 1:28 AM
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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