Sloping inverted Ls for 160 and 80 meters

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 in MMANA. 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 things considered.

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 band.