80 m Loop Feedline Analysis

Recently I used NEC antenna modeling software to analyze the effects of different feedline lengths on SWR for an 80 m horizontal loop at 40 ft height above the "medium hills and forest terrain" ground model. The goal was to see the effects of various feedline lengths that either conformed with or departed from the lengths recommended by W8JI in his article Choosing the Correct Balun in table  3.

I collected the model's data output in increments of 0.2 MHz from 3 MHz to 29 MHz. (The modeling software permits a maximum of 256 data points.) The data were collected for the following lengths of 450-ohm feedline:  384, 400, 418, 467, 512 ft.

The graph below presents the calculated SWR at nine selected frequencies. All feedline cases show a high SWR peak in the 60 m band.  (Click on the image for a larger view.)

 All lengths have similar SWR values at the lower frequencies, and more variation at the higher frequencies. The recommended lengths of 384 and 512 ft (multiples of 127.9 ft) seem to have a bit less variation in SWR at the high frequencies, although 418 ft behaves pretty well. 400 and 467 ft look less favorable.  In the real world, experimentation will lead to an optimal or an acceptable feedline length.  Naturally, it is best to avoid combinations that give very high SWR in a frequency range of interest, since the high SWR will require the antenna tuner to compensate more and the overall antenna efficiency is reduced.

Below are the full calculated SWR plots from the modeling program for each of the feedline lengths. No other antenna parameters were changed. (Click on each image for a larger view. These are reduced-resolution copies, but should suffice for overall impressions.) I found that shorter feedlines, especially 128 and 256 ft (not presented here), gave smoother curves, but I don't have an explanation.  The plots start at 3 MHz, for nicer scaling on the frequency axis, but the antenna is designed for about 3.5 MHz, so the SWR is high at 3 MHz. 

384 ft feedline

400 ft feedline

418 ft feedline

467 ft feedline

512 ft feedline

The program's output numeric data files provide SWR, reflection coefficient, resistance, reactance impedance and phase for every frequency.  Radiation pattern data are also available and can be displayed graphically. For this analysis, the radiation patterns are all identical, since the model's transmission line element is specialized, compared with the wire elements, and does not contribute to the radiation pattern. If the radiation effects of a feedline must be considered in a model, then it must be included in the model as a pair of parallel wires.  For more information on this topic, see part 4 of the QST article series "A Beginner's Guide to Modeling with NEC" by L. B. Cebik.