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

2002年10月07日 19:50:49 -0400

Well, guys, I got no responses to my queries, so I am sending this
follow-up to the Antennas Usenet group also.
I went ahead and put up the antenna today. The model was accurate. It
predicted a minimum SWR at 7.12 MHz and it was actually 7.07 as checked
with an MFJ-269 at the bottom of the antenna. So, it appears that the
Leeson corrections aren't necessary for a tapered vertical antenna.
Why?
After thinking it through, I answered my second question, and smaller
segment lengths do indeed result in the penetration of the segments of
the vertical radiator into those of the radials, which explains the
deterioration of the model's reliability (average gain test) with
increasing segmentation.
Apparently, the feed point placement in the model is not critical.
I merely eye-balled the radial angles and they turned out to be
guestimates at best. Apparently, based on the agreement with the model,
my eye was close enough.
If anyone has any comments, I'd be interested in hearing them.
73, Joe
"Joe Giacobello, K2XX" wrote:
> 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
>> - - -
>> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
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