Half-Length 80-Meter Vertical Monopoles:
the Best Method of Loading

L. B. Cebik, W4RNL (SK)

1. Goals and Methods of the Study

The development of a compact and efficient 80-meter monopole has yielded numerous schemes of loading. To the present, no published studies known to me have systematically addressed the relative merits of these schemes in terms of overall antenna performance. Obviously, a short monopole will perform less well than a full-length resonant monopole (using the same constant ground and ground plane conditions). First, the gain of the shortened monopole will be less than that of the full-length monopole. Second, the shorter monopole will have a lower feedpoint impedance, thus increasing the ratio of ohmic (loss) resistance to radiation resistance.

A. Goals of the Study

Within these constraints and with a target frequency of 3.6 MHz, one can establish the following goals for any project to yield a workable short 80-meter monopole:

1. The antenna should have the maximum gain achievable within the limits of the antenna type. In general, this gain is limited by the theoretical gain of the shortened monopole prior to the addition of any loading scheme to bring it to resonance. However, some schemes may in fact marginally (and perhaps insignificantly) increase the gain beyond that point.

2. The antenna should exhibit a true vertically-polarized circular radiation pattern. Any horizontally-polarized field component should be sufficiently small that it does not affect the overall antenna field shape or strength.

3. The antenna should have the highest possible feedpoint impedance at resonance for maximum efficiency. Given a constant set of ohmic losses, presumed for the sake of this study, the higher the radiation resistance, the higher the antenna efficiency.

4. The antenna should exhibit the flattest SWR curve possible between 3.5 and 3.7 MHz. A flat SWR curve indicates a wider operating bandwidth before adjustments to the antenna must be made.

5. The antenna should promise the most compact and mechanically practical assembly possible. An antenna must have mechanical properties that enable it to withstand reasonable handling and weather conditions. Moreover, it must be reproducible within the normal manufacturing and economic limits of current practice.

This preliminary study cannot assure that its final recommended antenna configurations can meet every goal, even if a model seems to promise that achievement. However, every effort has been made to formulate configurations that hold reasonable promise of being both good performers and practical manufacturing products.

B. Methods of the Study

The evaluation of the results of the modeling exercise require an understanding of the methods used and the rationale for selecting these methods. These considerations have two significant aspects: the selection of the modeling software and the selection of the modeling parameters.

1. Modeling Software

There are four major antenna modeling calculation engines potentially relevant to this study: NEC-4, NEC-2, MININEC 3.13, and Proprietary MININEC. Currently, these are implemented in the following commercially available packages: EZNEC-Pro (NEC-4 and NEC-2), ELNEC (MININEC 3.13), AO (MININEC 3.13), NEC-Wires (NEC-2), NEC-Win Pro (NEC-2, with GNEC-NEC-4 soon to appear), NEC4WIN (MININEC 3.13, with a NEC-2 version under beta test), and MININEC for Windows (Proprietary MININEC of Rockway and Logan). Other versions of the same software have more or less user features offered by the same sources are not here listed.

NEC-2 has the well-know limitation of being unable to handle with any reliable accuracy antenna elements whose diameter changes along the length. In addition, it also yields unreliable outputs for antenna elements of complex geometry, where the antenna diameter changes at a corner. NEC-2 is also limited in accuracy wherever it encounters very tight angles so that regions of presumed current impinge on each other. Since all of these factors are essential to the modeling of short, loaded vertical monopoles, NEC-2 is unusable.

I have recently discovered conclusive evidence that NEC-4, while an improvement in all these areas over NEC-2, is nevertheless unreliable in the same areas of concern to modelers of loaded vertical monopoles. In general, both gain and feedpoint impedance figures are inconsistent for the modeling requirements. Some of the limitations of NEC-4 are published at my personal website. (See http://funnelweb.utcc.utk.edu/~cebik/radio.html for an index of relevant notes.)

The proprietary revised FORTRAN MININEC recently issued by Rockway and Logan has yet to be tested for accuracy in these areas of concern. The present user interface has made testing a laborious task, and this software was set aside for the present modeling exercise.

MININEC 3.13, used by AO, ELNEC, and NEC4WIN, handles closely spaced wires of differential diameters without significant problem. Moreover, segments of different diameters meeting at angles are also routinely handled within the limits imposed by the scheme of placing pulses at segment ends. This requires the use of very short segments at junctures of this type to ensure minimal effective element length shortening. The modeler can achieve this goal by the use of sufficient equi-length segments or be the use of a segment-length tapering schedule. Tapering schedules are built into both AO and ELNEC.

MININEC also requires that the modeler avoid very tight angles, where required segmentation for accuracy would exceed program limits. This limitation can be overcome by flattening the apex of very sharp angles so that the resultant angles approach right angles. This yields no large problems in modeling or in extrapolating those models to real "pointed" structures. Whenever this is done, the text indicates the alternative procedure used. In the end, the inaccuracies are no greater than those imposed by straight-wire modeling, common to all forms of available modeling software. The errors yielded by meeting the requirements of MININEC 3.13 are systematic. Therefore, trends produced by the figures are reliable, even if frequency shifted.

Therefore, all modeling for this study was done on ELNEC 3, a version of MININEC 3.13 with a user interface permitting the rapid alteration of antenna configuration and reasonable calculation times. Segmentation, where possible, was held at a limit just below a total of 128 model segments in order to avoid additional time penalties imposed by calculation of larger matrices. Cross checks with larger models show no significant differences in absolute values generated or in the trends.

2. Modeling Parameters

The overall goal of this study is to compare general loading configurations for short vertical monopoles. The general properties of these schemes can be explored in simplified form by standardizing certain modeling parameters. Among the standardization features are the following items:

1. Use of perfect ground: All antennas a modeled over perfect ground so that gain figures become directly comparable. Real ground and modeled ground planes lie beyond the scope of the present study, but may be advisable in later studies.

2. Use of lossless wire: Initial studies suggest that the two most common materials used for antenna element components produce few changes in key output figures. Although some material selection is predictable (aluminum for tubing, copper for thin wire), these decision belong to an advanced stage of study. The use of lossless wire permits one to sort out the general configuration potentials from limitations later introduced by actual materials.

3. Restriction of "main" element size: Main element diameters of 1" and 2" are used throughout, with occasional reference to 1.5" diameter elements. These elements general encompass the range of manufacturing possibilities and suffice to indicate appropriate trends in gain and feedpoint impedance.

4. Restriction of main-element length: commercial half-size monopoles are general in the range of 30 to 40 feet long. One commercial short, loaded 80-meter monopole is 37.5' long. For mechanical reasons, this length is judged to be a practical maximum for most amateur radio purposes. Therefore, it has been adopted as the standard length for all models (except the full-size resonant quarter-wavelength model used for comparison) in this study. A single main element length makes it possible to compare directly the effectiveness of the loading schemes examined.

5. Restriction of output data taken: The key indicators of antenna performance within the limitations of this study are gain in dBi, feedpoint impedance in R+/-jX ohms, and SWR referenced always to the antenna impedance at 3.6 MHz and resonance. For the purposes of this study, resonance is defined as less than 1 ohm reactance, a necessary condition for producing reliable SWR curves at low resonant feedpoint impedances.

Restrictions specific to a given model will be reported when that model is given detailed examination.

The model coding used in this study deserves an explanation. All file extensions are .EN, indicating an ELNEC file. All such files are readable and convertible by EZNEC into .EZ files. All model file names begin with 35 to indicate an 80-meter vertical antenna. The following codings distinguish among antenna types:

     File name           Antenna Type

35Vxx. Unloaded vertical monopole
35VSxx. Short, unloaded vertical monopole
35VCxx(A). "Capacity" hat vertical monopole
35BLxxxx(A). Base lumped-constant loaded antenna
35LLxxxx. Base linear-loaded antenna
35LHxxxx. Top linear-loaded antenna
35VTxxxx. Top Zig-Zag loaded antenna
35VZxxxx. Top helically-loaded antenna

The "xxxx" in each file name is a group of four numbers. The first pair is restricted to 10, 15, or 20, indicating main element diameters of 1", 1.5", or 2" respectively. The second pair is used where appropriate to indicate the spacing of the loading structure from the main element, center-to-center in feet, where 30 indicates 3.0' spacing. Wherever spacing is variable, detailed explanations are provided. Each subsequent chapter of this report begins with a list of file names of models reported in the chapter.

Figures are provided for antenna structures that may be unclear from verbal descriptions. Since all patterns are visually similar, only one such pattern is provided for the reference antenna in the following chapter. Data presentation will largely be confined to tables, with occasional graphs to clarify trends and curves in the data.

An appendix of file descriptions is included for ease in replicating the models used in this study.

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