Topband: Inverted L and the High Angle Field

[Jim Brown]

On 2/29/2020 9:56 AM, Guy Olinger K2AV wrote:
> Have to disagree on the no-use-for the horizontal leg.
> 
> It fills in the doughnut hole in the high angle radiation, which in
> contests is very good for keeping others from planting on your run
> frequency. Also there are high angle path opportunities that are gradually
> becoming more recognized.

This effect is not nearly as great as generally believed, and depends on 
the relative lengths of the horizontal and vertical sections. For 
example, consider an L with 70 ft and 70 ft sections, as compared to 100 
ft vertical and 35 ft horizontal. the difference in field at 70 degrees 
elevation between the two antennas is 4 dB over Sandy soil, 3 dB over 
Very Good soil. For 50 ft vertical and 80 ft horizontal, the difference 
is 4 dB over very good soil as compared to 100 vertical, 35 horizontal. 
The reason for the difference, of course, is the current distribution 
between the two sections depending on their electrical length.

Comparing a T with 50 ft vertical and 55 ft horizontal to an L with 50 
ft vertical and 85 ft horizontal over Very Good soil, the difference is 
also 4 dB.

These differences are less than an S-unit, hardly enough to prevent 
someone poaching a run frequency. Propagation for shorter distances 
corresponding to a higher wave angle are covered by ground wave on 160M.

The way ARRL and NEC plot vertical patterns can be deceiving, because 
they always set the peak of the pattern to full scale. To see the 
difference, one must plot both antennas with the same scale. In NEC, 
this is accomplished by modeling each antenna, displaying the vertical 
pattern and saving the traces, then recalling the saved trace for the 
other antenna and selecting the recalled trace. The cursor can then be 
moved along the vertical axis and will display the difference in dB 
between the traces.

There are many examples of this technique in
http://k9yc.com/AntennaPlanning.pdf

A shorter vertical sections reduces the radiation resistance, which 
increases losses in the counterpoise/radial system, which in turn 
strongly depend upon soil quality.

73, Jim K9YC