Re: NEC-LIST: J-pole modeling

To all,
Just some feedback on my attempts at using NEC to model a J-pole.

To model the antenna and come up with an initial design, I first tried
using two excitations, one on the 1/4-wave section, and one on the
3/4-wave section, to avoid any small-loop problems. The copper-pipe
antenna built using an initial design based on this model was nowhere
close. After experimenting with conductor lengths, I got a version to
work, with an SWR of about 1.2 at the design frequency (146
MHz). Going back to model the antenna, I excited it using one wire
segment connecting the long and short conductors. I found that the
"NEC frequency shift" phenomenon still holds. In order to get
resonance, I had to model the antenna at a frequency that was 1.8%
higher than the design frequency. The computed impedance at resonance
was one that would result in an SWR of 1.2 (I couldn't measure the
actual impedance).

John Belrose, VE2CV, commented that every J-pole version is
differently dimensioned. I found, by playing with the dimensions in
the computer model, that there are many combinations of dimensions
that can be resonated. However, some combinations are better than
others. In some, the major lobe is tilted upward from the
horizon. Assuming the computed patterns to be true, my antenna
as-built has this type of pattern. (This is not a problem for general
base station use, but may be if you're trying to make every last dB
count).

The separation between conductors does not have a significant impact
on the antenna characteristics, and the max gain is relatively
constant and the azimuth pattern reasonably uniform for separations of
2 to 4 inches.

John Belrose indicated there are three dimensions to trim and adjust:
the length of the 1/2 wave radiator; the length of the 1/4 wavelength
stub; and the tap point on the stub for a 50-ohm match. I would
suggest that an initial optimum design be used, so that any trimming
and adjusting is minimal. In the published designs I looked at, the
general tendency was to not state the separation that was used (for a
design based on twinlead or ladder line, the separation can be
determined). The implication is that the required conductor lengths
don't depend on the separation. There is a dependence, however, with a
difference of 1/2 inch in separation resulting in a 1-inch change in a
dimension.

Lessons learned:

1. while dual-excitations work OK for a dipole or inverted Vee, they
*do not* work on the J-pole. There may be a mutual coupling effect
(the presence of one excited segment modifies the impedance of another
excited segment) with this approach.

2. start with a published design for which all details of the design
(e.g., dimensions, conductor separation, approximate tap point, feed
details) are defined.

Thanks again to everyone who responded and provided helpful comments.
Received on Fri Jan 09 1998 - 10:29:24 EST