Tuesday, April 14, 2009

160 m Inverted L Antenna

The other antenna that W4BXI likes to use on 160 m is a center-fed inverted L using ladder line. The sketches below provide the configuration details. Click on the image to enlarge it.

I entered this data into the 4nec2 software as a new model for analysis. Since there is not a convenient way to depict the sloped terrain, I used a level ground terrain. The model structure is shown below as it appears in the program. White lines are the antenna elements, blue lines are the ladder line feeder, and green lines are the xyz axes.

The program's main output window is shown below. It indicates the impedance at the end of the feedline to be 131+j691 ohms (703.31 ohms @ 79.27o) at 1.8 MHz. The SWR for a 50-ohm source is calculated to be 75.7. For comparison, the impedance at the antenna feedpoint, with feedline removed, calculates to be 38.5-j40.5 ohms (55.88 ohms @ -46.45o).

The variation of SWR, for a 50-ohm source, is shown in the graph below.

The next set of graphs show the variation of impedance, etc. vs frequency at the end of the feedline. The values at 1.8 MHz are highlighted.

The program is able to calculate the component values for several types of networks to match the feedline impedance to a 50-ohm source. The values for each network type are shown below. Component values for a (C-L-C) high-pass T-network are highlighted in green. This gives an indication of the component ranges that may be used in an antenna tuner to achieve a match for this model approximation.

The next plot shows the far field horizontal radiation pattern for the modeled antenna. Major lobes are seen to be perpendicular to the 130 ft leg that is 60 ft above ground. It is not known how this pattern would differ for a sloped ground terrain.

The final figure shows a 2-dimensional slice of the radiation pattern, color-coded for intensity. The elevated 130 ft leg of the antenna appears as a white line emerging from the pattern. The other leg is seen as a white vertical line that connects to another white line extending to the right, close to ground. Blue lines represent the ladder line feeder.

The program enables one to rotate this pattern in all directions for other views.

1 comment:

Anonymous said...

Appreciate your spending some time to publish this kind of important blog, it's been very helpful.