[CQ-Contest] Twisting a Stack of Yagis

[Dan Maguire]

I recently sent this off as a letter to the editor of the National Contest
Journal.  Maybe it would be of interest to folks on this list as well.

# # #

In recent issues of the National Contest Journal, Eric Scace K3NA presents a
unique way of showing the radiation pattern for an antenna.  He runs a model
through NEC to generate far field data across a "sky hemisphere" of
directions (similar to a 3D pattern) and then uses some post-processor tools
to project the far field dBi values onto a flat sheet for display and
statistical analysis.  Full details are included in the article, which is
available on the NCJ web site.  (See references below.)

One of the examples included in Part II of the article is a stack of two 20m
Yagis, mounted at heights of 50 and 100 feet and separately rotatable.  The
initial configuration has both Yagis pointed towards Europe, compass bearing
46° from the Washington DC area.  Then the upper Yagi is rotated
counterclockwise and the sky hemisphere pattern is captured at intervals
along the way.  The final position of the upper Yagi is 90° counterclockwise
from the lower, compass bearing 316° towards Japan.

I thought this example was very interesting and I exchanged several emails
with Eric concerning a follow-up.  My idea was to twist the stack as Eric
had done and show the results in a variety of different ways including 2D
plots, 3D plots, and VOACAP area coverage maps.

The original example output is available for downloading but you must have
Microsoft PowerPoint to show it, so the first thing I did was to extract a
subset of the images and assemble them into an animated gif.  The animation
shows reduced-size versions of Eric's "Eckert IV" flat projections of the
sky hemisphere, representing the top Yagi twisted by amounts of 0°, 15°,
30°, 45°, 60°, 75°, and 90°.  See
  www.qsl.net/ac6la/adhoc/twist_scace.html
(You may wish to open this link and the following links in separate browser
windows so that you can easily switch from one to another.  Note that some
browsers do not let you stop a gif animation and some "ad blocker" software
will prevent animations from being shown at all.)

Then I imported Eric's NEC format antenna model into the MultiNEC program.
MultiNEC has a "Rotate Wire(s)" function and the amount of rotation can be
controlled by a variable.  I created a series of test cases with the
rotation amount for the top Yagi set to the 0° through 90° steps mentioned
above, and plotted a set of 2D elevation patterns at the azimuth angle
representing the initial bearing towards Europe.  This shows how the
original main beam is degraded as the top Yagi is twisted away.  See
  www.qsl.net/ac6la/adhoc/twist_2d.html

Next I used the MultiNEC interface to Arie Voor's very nice 4nec2 program.
I requested a 3D plot at each twist interval and used the 4nec2 DirectX
viewer to show the results, taking care to insure that all the 3D plots used
the same scale for color coding and had the same XYZ orientation.  In this
series of plots the initial bearing of 46° towards Europe is towards the
lower left corner of the screen.  See
  www.qsl.net/ac6la/adhoc/twist_3d.html

Finally I used the MultiNEC interface to the VOACAP program to automatically
create a series of area coverage maps representing a different twist amount.
These maps, like Eric's originals, show results that are projected onto a 2D
surface.  But in this case the surface is the Earth and the parameter shown
is not transmitted signal strength but received signal strength.  See
  www.qsl.net/ac6la/adhoc/twist_voa.html

All of these animations are just different ways of looking at the same
scenario.  Each offers different insights.

Dan Maguire, AC6LA
ac6la at arrl.net

References:
1. Eric Scace K3NA, "Antenna Interactions, Parts I and II," National Contest
Journal, Jul/Aug and Sep/Oct 2003.  Available in pdf format at
www.ncjweb.com/bonus.php.  The example discussed above is available as a
PowerPoint slide show in file 'twisting_open_2_yagis.ppt' (5.92 MB).
2. 4nec2 program, see www.qsl.net/wb6tpu/swindex.html.
3. MultiNEC program, see www.qsl.net/ac6la/.  File
www.qsl.net/ac6la/adhoc/2stack_Europe.zip contains the 'weq' format
(MultiNEC) model for the above scenario.  This format preserves all Excel
formulas, variable definitions, and test case parameters as well as the wire
specifications and other details for a model.