I consider use of the words "takeoff angle" to be VERY misleading, and
cause a LOT of misconceptions. It is common to use the words "takeoff
angle" to define the vertical angle at which field strength is greatest,
and that the antenna only works at that angle. Nothing could be further
from the truth.
Several years ago, I presented a modeling study showing that the low
angle radiation (15 degrees and below) of a horizontal antenna for 80 or
40 M continues to increase as the antenna is raised above ground. I
showed this by ignoring where the peak of the vertical pattern was,
instead looking only at the low angle field strength as the antenna was
raised. I didn't look at the SHAPE of the pattern -- instead, I plotted
curves of field strength at 5, 10, and 15 degrees as the antenna was
raised. A similar study for 20M produces comparable results, taking
wavelength into account. That study is presented in the link below,
starting around page 10.
http://k9yc.com/AntennaPlanning.pdf
That study is, of course, for "flatland," where terrain is not a factor,
but the same concepts apply in irregular terrain, except that the
terrain shifts the lobes up and down.
73, Jim K9YC
On 7/23/2017 6:27 PM, Steve Maki wrote:
Err, we better keep straight whether we're speaking of heights above
ground or takeoff angle.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|