[TowerTalk] Takeoff angle
Bob Shohet, KQ2M
kq2m at kq2m.com
Mon Jul 24 08:42:04 EDT 2017
Are you referring to the fact that at any given height over terrain for any antenna, there are multiple lobes, and that the angles of these lobes change as the height changes? The YO program that Brian, K6STI developed several decades ago showed this beautifully and you could model a given antenna (or stacks of antennas) at a given height over a given terrain (from your own terrain file) and when you changed the height you could see how the lobes and pre-dominant wave angles changed on a given band. At my very complex qth (hills, ravines, etc), some low height antennas also have a very low angle lobe and some high antennas have very high angle lobes in addition to low angle lobes. Many of these lobes are counter-intuitive and are the result of complex terrain qth’s, which is why modeling is invaluable to the goal of attempting to ensure that you have one antenna or combination of phased antennas to cover all the signal arrival angles on a given band.
From: Jim Brown
Sent: Monday, July 24, 2017 12:17 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Takeoff angle
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.
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.
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