Preliminary work is best done in MININEC, because (with due caution) it is best capable of modeling nonlinear geometries employing wires of different diameters, a necessary condition of a compact 80-meter monopole. A monopole 37.5' long, corresponding to a common commercial height, is the constant main element used, with other parameters varied to achieve the following goals:
The range of models compared covered the following types of monopoles: full-length, 37.5' unloaded, lumped constant base- loaded, linear-base-loaded, "capacity" hat loaded; top linear loaded; zig-zag fold-back loaded, and helically loaded. Figures are provided on gain and feedpoint impedance at 3.6 MHz, as well as on feedpoint impedance and SWR at 0.05 MHz intervals from 3.5 to 3.7 MHz. Other data can be obtained from these models by rerunning them using the descriptions provided in an Appendix. Especially recommended is a study of current levels along the antenna wires.
No single model antenna achieves all of the goals listed above. However, the helical fold-back element extension model achieves goals a. through c., and perhaps holds promise of achieving e. The zig-zag fold-back element extension model excels in achievement of goals b, c. and d., with only slightly less gain than the helix and with some promise of meeting goal e. The capacity hat model shows excellent gan, bandwidth, and feedpoint impedance, but may be mechanically problematical except in a Marconi configuration. All other models show lesser performance in one or more categories. Unless mechanical constraints preclude further work on them, the helical and the zig-zag foldback models appear to be the best candidates for further study and testing. However, before hats are discarded as too large or too fixed or too unwieldy, the spiral hat noted in the last segment (Part 5) should be examined. It was modeled in NEC-4 because the model was too large of standard MININEC.
1. Goals, and Methods of the Study
2. Baseline Data: Full-Size and Capacity-Hat Verticals
3. Base-Loading: Lumped-Constant and Linear Loading
4. Top (Element-Extension) Loading: Linear, Zig-Zag, and Helical Loading
5. Summary Comparisons and Conclusions; With an Alternative Suggestion
6. Descriptions of Models Reported
Updated 5-12-97. © L. B. Cebik, W4RNL. Data may be used for
personal purposes, but may not be reproduced for publication in print or
any other medium without permission of the author.