On Tue,8/9/2016 9:48 AM, Ed Sawyer wrote:
HFTA tells you where to put antennas based on terrain and desired angle. It
doesn't tell you how to achieve the assumed gain of the selected antenna.
That's my understanding as well.
As I've said many times, and as any serious work with HFTA clearly
shows, the concept of "takeoff angle" is seriously flawed. Far better to
study and optimize the field strength at DESIRED vertical angles.
You may find that ideal stacking distance puts the combined lobe at the
wrong height. The 1dB or so of stacking variance for a normal range of
stacking distances me be quickly overtaken by the right height of the
combined effective takeoff angle.
What I have found useful with HFTA is to put dipoles every 5 ft up the tower
and see where the lobes max out in the desired angle.
That's a great way to use HFTA,, but we should also run it for every 5
degrees of azimuth, and compare the vertical pattern with data for
propagation to targets in those directions. When you do that, depending
on terrain, you may more than "sweet spot" for height. Those two heights
might be great to use one at a time, and also stacked.
Another point. N6BV advises that it's ALWAYS good to make multiple runs
with HFTA making small changes in the height or azimuth to expose errors
in the calculation that result from certain kinds of irregularity (or
regularity) of the terrain data. Without these errors, small changes in
height or azimuth should be pretty small.
73, Jim K9YC
TowerTalk mailing list