[Antennas] Modeling GP with tapered tubular vertical element and wire radials

2002年10月04日 13:31:41 -0400

Dear List:
I decided to put together a 40 meter vertical using tubing from a
defunct Yagi that I had around. The vertical radiator is composed of
four lengths of telescoping tubing of four different diameters, and the
radials are made of #12 wire angled at 103 degrees relative to the
vertical radiator, i.e. 13 degrees below horizontal.
I modeled the antenna using EZNEC with the expectation that I would have
to invoke the (Leeson) corrections for a tapered element. However, I
was unable to do so because the elements are (1) non-linear and (2),
even if I remove the radials to apply the Leeson correction to only the
vertical section, the tubing sections are of unequal lengths.
Nevertheless, when I run the average gain test on the as-is model, I get
a very respectable 1.017, strongly suggesting that the model is valid.
It also turns out that the average gain test value deteriorates as one
increases the number of segments in the bottom section of the vertical
radiator and the number of segments in each of the radials. (Thus one
cannot use the convergence test as a second criterion for model
reliability.) I believe that this anomaly occurs because there is
penetration of the lowest tubing segment into the adjacent segments of
the attached radials.
My questions are:
1. Given the satisfactory result from the average gain test, is the
model indeed valid?
2. Is the penetration explanation for the deterioration of the average
gain test with increasing segmentation reasonable?
3. With so few segments in the bottom section of the vertical radiator,
the modeled feed point is necessarily about 1 foot above the bottom of
the vertical. Since the actual feed point of the real antenna will be
at the very bottom, does this difference affect the reliability of the
model?
4. From a practical standpoint, the feed point impedance of the antenna
is quite sensitive to the angle of the radials. How does one accurately
adjust the angle of the radials when one is constructing the antenna?
Are there tools available to make this measurement?
I would appreciate any comments and input. Adjustment of the vertical
length of the antenna will be a real chore once it's in place. So an
accurate model would be a real favor.
Thanks for your help.
73, Joe