Two single-band Ls near 70 ft tower
A single band inverted L installed in free space
away from any other objects would be ideal. However, in the real
world, amateur radio operators must frequently contend with limited
height, metal support structures (in my case, an ANWireless tower
topped with antennas) and interaction with other ham radio antennas.
Modeling the tower would be a challenge
The shortcut that I've chosen is to model a pole with a radius of 500mm and
a height of 21.33m. The mast and Cushcraft XM-240 were modeled above
the top of the tower as well. The 160 meter inverted L goes from the base to
the 19m level then slopes with a 30m wire to a height of 11.5m. The 80
meter inverted L goes from the base to the 14m level and then slopes with a
16.3m wire to a height of 9.2m. Each antenna is separated from the tower and
each other by 1 meter. This pair of 3/8th wave inverted Ls were selected for
the best match across the two bands.
The model also includes a grounded tower that has 16
radials. There is a variable capacitor in the model that is used to
adjust the match for each band.
SWR & Bandwidth
SWR plots on each band show reasonable bandwidth all
Far Field Plots
80 meters shows pronounced interaction in the far field plots. A cardioid
pattern develops creating a significant null when used near 3.75 MHz.
Additionally, the vertical pattern also changes dramatically across the
band. Note, when looking at the MMANA far field plots, you can
ignore the values displayed in the matrix as they assume only the
capacitance value of the center point.
Unlike 80 meters, the 160 meter far field plots do not vary across the
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