On 1/17/2026 12:56 PM, Brian Beezley wrote:
I never worry about takeoff angle, which I take to mean the angle of the
maximum lobe in the elevation plane. Communication takes place at angles
determined by path distance, ionospheric height, ionospheric tilt, and
probably more. It does not necessarily occur where any particular lobe
is located. I generally pick what I think is a representative low angle,
usually 5 degrees, and compare gain for all antennas I'm considering there.
YES! Takeoff angle is a stupid concept. For those reading the mail,
look at the link I posted about antenna planning, which contains plots
of multiple conditions on the same scale, and compare the dB difference
between those conditions at the same vertical angle. While propagation
takes place at all angles, for the most part, low angles are most
important, especially on the lower bands.
All of these plots look similar, so looking only at the field strength
at max misses a lot. And both NEC and ARRL plots automatically scale
plots to that maximum value. When using NEC for designs, it's helpful to
save a plot for one condition, then import it into the screen for
variations we make. The cursor on the imported design will read out the
difference from the current design. VERY instructive.
Doing this can often point out that curves that look very similar and
have little difference in "takeoff angle" can have differences of 3-6 dB
at low angles! And look at the differences in how the height of HF
verticals changes both their vertical pattern and ground loss!
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|