Good and Bad Parallel Feedline Lengths

There are many rules-of-thumbs on 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 EXE program approximates the optimum feedline lengths (plus the feedline lengths to avoid) given the length of a horizontal dipole, the operating frequency, and the velocity factor of the feedline. 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 reality.

The theory behind the "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 are in the vicinity of the current maximum points on the ladder-line where the 50 ohm SWR is lower than 3:1. The "bad" lengths are in the vicinity of the voltage maximum points on the ladder- line where the 50 ohm SWR is very high. The unzipped, ready to run, 31kB DOS "IMAXMIN.EXE" file can be downloaded from: IMAXMIN.EXE

The BASIC source code for the above program can be downloaded from: IMAXMIN.BAS

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

A later version of this program shows the band edges but may be a little too busy for some tastes. For what it's worth, here it is: IMAXGRA2.EXE

These DOS programs 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 easy.