Good and Bad Parallel Feedline Lengths


There are many rules-of-thumbs for lengths of dipole to avoid and lengths of parallel feedline to avoid. W5DXP has attempted to remove the myths and guessing games associated with this subject. The following DOS IMAXGRAF.EXE program approximates the optimum feedline lengths given the length of a horizontal dipole and the velocity factor of the feedline for the HF bands. A 1:1 current-balun is assumed and the program does not apply to any balun where N:1 is not 1:1. The results are only approximations based on EZNEC and must be fine-tuned to perfection in the real world.

The theory behind W5DXP's "good" and "bad" length designations are based on impedances that are friendly to built-in autotuners vs impedances that are unfriendly to built-in autotuners. The "good" lengths, indicated by the dots, are in the vicinity of the current maximum points on the ladder-line where the 50 ohm SWR is lower than 3:1, acceptable to most autotuners. The "bad" lengths, halfway between any two dots on the same line, are in the vicinity of the voltage maximum points on the ladder-line where the 50 ohm SWR is very high and much greater than 3:1.

A DOS graphic presentation of the current maximum points can be downloaded from: IMAXGRAF.EXE

The dots on the display indicate the lengths of ladder-line that are friendly to built-in autotuners, i.e. the current maximum points. Halfway between any two dots on the same line are the lengths of ladder-line that are unfriendly to built-in autotuners, i.e. the voltage maximum points. Where the dots line up vertically are the lengths that are good for multi-band operation. For instance, if a VF of 0.85 and a dipole length of 90 feet are selected, one will observe five dots aligning vertically at a ladder-line length of 40 feet. That's the ZS6BKW antenna that works well on 40m, 20m, 17m, 12m, and 10m. Here's what the display looks like for the ZS6BKW antenna.

The next longest length of ladder-line that would be good for 40m would be 98 feet. This is, of course, 1/2WL added to the 40 feet used by the ZS6BKW. Note that only three bands line up with a ladder-line length of 98 feet, 40m, 20m, and 10m. A ladder- line length to avoid for 40m operation would be 69 feet, which is halfway between 40 feet and 98 feet and would result in a very high impedance at the 1:1 choke, unacceptable to an autotuner. Note that the red dots on the 80m line indicate that a 90 foot dipole is too short for 80m operation because it would also present an SWR unacceptable to an autotuner.

This DOS program will run under XP and earlier versions of Windows. For later versions of Windows, an X-86 simulator program called DOSBox is available for download on the Web that allows a DOS program to run in a window. One advantage of DOSBox is that screen capture of the graphic is an easy Ctrl-PrintScreen keystroke.