The Tee-Match: NEC Illusions and MININEC Realities
Many months ago, I attempted to apply NEC-4 to the modeling of a Tee-match
system. The original system modeled was based on a 5-element 48'-boom Yagi
design by K3LPL. The modeling was done at a height of 195' over medium
earth. Of all the variables investigated, the relative diameters of the
Tee-match bar to the main driven element had the greatest effect on the
reported antenna gain.
Antenna without Tee-match:
Gain: 16.1 dBi @ 5 degrees
Beamwidth: 52 degrees (does not change)
F-B: 23.0 dB
Feedpoint Z: 36.3 + 16.1 ohms
Antenna with 0.5" diameter 48" Tee match rod, spaced 6":
Gain: 15.31 dBi
F-B: 23.13
Feedpoint Z: 262 + 87.9 ohms
Antenna with 0.5" diameter 48" Tee match rod, spaced 7.5":
Gain: 15.5 dBi
F-B: 22.8 dB
Feedpoint Z: 215.7 + 87.55 ohms
Antenna with 2.0" diameter 48" Tee match rod, spaced 7.5":
Gain: 16.2 dBi
F-B: 23.0 dB
Feedpoint Z: 196.2 + 29.9 ohms
The variability of results, including the increases in gain with increasing
diameters of Tee-match rod, raised many questions about the adequacy of the
model and limitations of even NEC-4 to correctly model the T-match
situation. As subsequent investigation established, at 14 MHz, spacings of
elements under about 9" (with variation for changes in element diameters)
yield unreliable results in NEC-4. (Matters are worse in NEC-2.).
Therefore, the notes were withdrawn from circulation.
However, many questions remained. One was a nagging feeling that perhaps
the ratio of the diameter of the Tee rod to the main driven element might
have some effect on antenna gain, although perhaps not to the extent
suggested by the NEC-4 models. The second question concerned how to model
the Tee match adequately on MININEC.
I finally resolved the problem of remodeling everything in MININEC by using
a different model (5-elements, 40' boom) based on a design in YA by K6STI
and adapting it to use in ELNEC. There are a number of modeling issues
that I shall by pass here, but which will be described in a longer version
of the note at my site.
The only changes among models in the following sequence are alterations of
the driven element length, the Tee bar length and diameter, and the
segmentation of the driven element to maintain a constant segment length to
the degree possible. Therefore, a tabular presentation of the data seems
most efficient. All models are in free space and use constant 1" diameter
aluminum elements. Length dimensions are in feet. Except for the
reference model, all models are distinguished by the diameter of the Tee
bar.
Model D.E. Tee Gain F-B Feed Z
Length Length dBi dB R+/-jX
Reference 33.20' ---- 9.827 22.15 25.5 - j0.96
T-0.5 34.10' 8.40' 9.855 21.71 209.7 + j0.85
T-0.75 33.90' 7.50' 9.789 21.81 210.7 + j1.36
T-1.0 33.80' 7.00' 9.817 21.86 209.7 + j2.49
T-1.25 33.80' 6.80' 9.832 21.86 198.6 - j1.27
T-1.50 33.70' 6.50' 9.853 21.92 205.2 + j1.41
The gain reports are given to 3 decimal places to demonstrate how truly
insignificant the differences are among the models. Re-segmenting Tee bar
and the driven element at borderline cases (around 7') creates a higher
degree of change in reported gain--at the extremes no more than 0.066 dB in
the table. Compare this to the full dB of difference in the NEC models.
Likewise, front-to-back ratio changes by a maximum of 0.44 dB among the
models.
For the range of ratios of Tee bar to driven element (0.5 to 1.5), there is
no indication that any particular ratio is better or worse than any other.
Variations in the reported figures are insignificantly different in all
respects, especially given the limitations of the models and the program.
The net result is this: within reason, any size Tee bar may be used with
any size element with no significant change in antenna performance.
Just thought that those who showed some interest in the original
investigation might like to know the outcome of this one.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|