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

L. B. Cebik, W4RNL





5. Summary Comparisons and Conclusions

In order the clarify the basis upon which the conclusions of this study are drawn, the initial goals of this study are apt for review:

1. The antenna should have the maximum gain achievable within the limits of the antenna type.

2. The antenna should exhibit a true vertically-polarized circular radiation pattern.

3. The antenna should have the highest possible feedpoint impedance at resonance for maximum efficiency.

4. The antenna should exhibit the flattest SWR curve possible between 3.5 and 3.7 MHz.

5. The antenna should promise the most compact and mechanically practical assembly possible.

Goal 2. is achieved by every model studied, with all horizontal field components sufficiently suppressed to yield essentially true vertically polarized circular patterns. Goal 5. requires a separate discussion for each model after examining the results with respect to goals 1., 3., and 4.

For the sake of compactness, some of the data described in detail in the preceding chapters may be summarized in tabular form. "Mast" means the main element. All antenna are 37.5' tall. Except for the special case of base-loading with a Q=300 inductance, all models are listed in terms of lossless wire.

File Name   Antenna Description     Gain  Feedpoint Impedance 50 kHz SWR
                                    dBi   Z = R  jX in ohms
1.  Capacity-hat models
35VC10      2-wire hat, 1" mast        4.940 22.75 - 0.33           1.5 - 1.4
35VC15      2-wire hat, 1.5" mast      4.940 22.75 - 0.92           1.5 - 1.3
35VC20      2-wire hat, 2" mast        4.940 22.79 - 0.73           1.4 - 1.3
35VC10A     4-wire hat, 1" mast        4.947 22.83 + 0.24           1.4 - 1.4
35VC15A     4-wire hat, 1.5" mast      4.945 22.83 - 0.15           1.4 - 1.4
35VC20A     4-wire hat, 2" mast        4.943 22.86 - 0.11           1.3 - 1.3

2.  Inductive base-loaded models

2.1  Lossless inductive loads
35BL10      0-loss load, 1" mast       4.885  7.94 + 0.03           2.5 - 2.4
35BL15      0-loss load, 1.5" mast     4.886  7.89 + 0.38           2.2 - 2.3
35BL20      0-loss load, 2" mast       4.886  7.85 + 0.06           2.2 - 2.2

.1  Lossy inductive loads:  Q=300
35BL10      Q-300 load, 1" mast        4.390  8.90 + 0.03           2.3 - 2.2
35BL15      Q-300 load, 1.5" mast      4.422  8.78 + 0.38           2.1 - 2.2
35BL20      Q-300 load, 2" mast        4.445  8.69 + 0.06           2.1 - 2.1

3.  Linear base-loaded models

35LL1015    12.5' load, 1" mast        4.891  8.20 + 0.75           3.5 - 3.8
35LL1515    11.4' load, 1.5" mast      4.891  8.51 + 0.22           3.3 - 3.4
35LL2015    10.6' load, 2" mast        4.892  8.72 + 0.43           3.1 - 3.2

4.  Linear top-loaded models
35LH1030    17.6x1.5' load, 1" mast    4.911 16.71 - 0.46           2.3 - 2.1
35LH1530    17.6x1.25' load, 1.5" mast 4.913 16.84 - 0.26           2.3 - 2.1
35LH2030    17.6x1' load, 2" mast      4.913 16.94 - 0.38           2.3 - 2.1

5.  Zig-zag top-loaded models

5.1  1' mast-to-load spacing
35VT1010    16.55' load, 1" mast       4.914 16.55 + 0.24           2.7 - 2.8
35VT1510    16.9' load, 1.5" mast      4.913 16.47 - 0.10           3.0 - 3.0
35VT2010    17.1' load, 2" mast        4.913 16.42 - 0.70           3.3 - 3.2

5.2  2' mast-to-load spacing
35VT1020    13.25' load, 1" mast       4.924 17.78 + 0.41           2.1 - 2.1
35VT1520    13.75; load, 1.5" mast     4.923 17.66 + 0.13           2.2 - 2.2
35VT2030    14.1' load, 2" mast        4.923 17.57 + 0.32           2.2 - 2.2

5.3  3' mast-to-load spacing
35VT1030    10.7' load, 1" mast        4.933 18.71 + 0.32           1.9 - 1.9
35VT1530    11.26' load, 1.5" mast     4.932 18.56 + 0.38           1.9 - 1.9
35VT2030    11.62' load, 2" mast       4.932 18.43 - 0.45           2.0 - 1.9

6.  Helically top-loaded models:  3' spacing
35VZ1025    3.98 turn load, 1" mast    4.954 22.10 + 010            2.1 - 2.1
35VZ1525    4.05 turn load, 1.5" mast  4.953 22.07 - 0.80           2.2 - 2.1
35VZ2025    4.13 turn load, 2" mast    4.953 22.08 - 0.94           2.3 - 2.2
35VH1525    4.14 turn load, 1.5" mast  4.952 21.95 - 0.09           2.2 - 2.2

Without question, the capacity hat models represent the best performance with respect to gain, feedpoint impedance, and operating bandwidth. Unfortunately, they present mechanical problems for the construction of a single, self-supporting (with or without guys) vertical antenna for 80-meters. Hat wires are simply too long and require end supports suited to specialized installations. For more on this subject, see the special section at the end of this chapter.

Surpassing the capacity hat verticals (marginally) in gain and only slightly lower in feedpoint impedance are the helically top-loaded monopoles using 3' spacing between the main element and the coil. Although these antennas have an operating band width approximating that of the 2-foot spaced zig-zag top load and the linear top load, it is anticipated that, when translated to real materials and feedlines, the effective bandwidth would reach 100 kHz. However, the mechanical questions are significant: can a light, stiff 0.25" wire 6' diameter 4-turn coil assembly be developed that will withstand wind loads at the top of the antenna. It is likely that such an antenna will require an insulated coil-bracing spider and special consideration of the bending characteristics of the main mast at various wind levels.

Among non-hat top loads, the 3-foot spaced zig-zag models provide the widest operating band width: well over 100 kHz at the 2:1 SWR points. Additionally, antenna gain is quite good, only slightly less than the helical and hat models. However, feedpoint impedance drops to the 18.5 ohm range, considerably less than the 22+ ohms offered by the hat and helical models. Because these antennas require an 11' long 6' wide set of load wires, special care must be taken to maintain spacing along a wind-bent antenna. This problem may be less severe than with the helical load due to the fact that much lighter wire can be used for the load assembly.

Top linear loading offers greater mechanical simplicity in its two wire, 17.6' assembly. However, its gain is less, its feedpoint impedance is lower, and its operating bandwidth is narrower than the models noted above.

Base-loading, whether in the form of an inductor or a linear load offers a very low feedpoint impedance (under 10 ohms) with no compensating qualities, such as high gain or wide operating bandwidth. Therefore, despite the mechanical simplicity of such models, they have little else to recommend them, especially when compared to the potentials of top-loaded short vertical monopoles.

This phase of the study has reached the conclusion that the top-loaded models of short vertical monopoles deserve further investigation in terms of potential performance using the real materials (aluminum and copper) over real ground and ground planes. A viable standard against which such performance potentials can be measured is the capacity-hat model.

One advantage held by this preliminary study is that it could be conducted within the modeling limits of a single method-of- moments program, MININEC 3.13. Since further studies will require ground planes and real grounds, they must employ one or more versions of NEC, both to take advantage of the Sommerfeld-Norton ground modeling algorithms and to allow for the large number of wires and segments utilized by extensive ground planes and complex antenna geometries. However, NEC--in both versions 2 and 4--is limited with respect to handling nonlinear geometries with wires of different diameters will require. This will require the construction of substitute models. Although absolute reported values will therefore become unreliable, models can be evaluated against suitable standards. In the present case, top-loaded short vertical monopole models may be measured against a combination of the full size quarter-wavelength vertical and the capacity-hat shorter monopole to permit correlations that cross modeling program lines.

Special Section: The Top-Hat Revisited

Because modeling top hats with MININEC is limited by the restriction on the number of segments allowed in any one model, a fuller exploration of possibilities was conducted in NEC-4. The figures given for gain and feedpoint impedance are not directly comparable to those appearing for MININEC models. Therefore, this section is separated for consideration on its own.

I modeled a number of top-hat configurations, including those using only spokes and those using spokes plus a perimeter wire. All models use aluminum as the antenna and top-hat material, so comparison with lossless wire model shown above is inappropriate. In all cases, the antenna diameter is 1", and radials are 0.25" in diameter.

Antenna     Number      Radial          Gain        Feedpoint Impedance
Filename    of radials  length, feet    in dBi            in R  jX ohms

35SC1025    16           7.2'            4.95              23.02 - 0.75
35RC1025    32           6.1'            4.95              23.07 - 0.35
35EC1025    8 w/perim    6.7'            4.94              23.08 + 0.50

All of these antennas had SWR curves like those shown for the hatted models developed with MININEC. NEC-4 returns very slightly higher values at the +/-100 kHz extremes: 2.0 to 2.1 at 3.5 MHz and 1.9 at 3.7 MHz.

Increasing the number of radials or adding a perimeter wire permits a very significant reduction in the radius of the hat required. However, such hats a structurally very restrictive. A first consideration is weight, which is proportional to the total wire required by a given hat structure. The 32-wire hat required more than 190' of hat wire. The 16-radial version cut that to about 115' at a cost of a larger overall radius. The 8-spoke with perimeter wire model used the least wire (92').

A second consideration is ease of adjustment for the constructor. A perimeter model hat is normally fixed in construction. Large numbers of hat wires are not cost-effectively pruned. Hence, most builders simply change the antenna height until it matches the chosen hat.

There is an alternative structure for hats that verges on falling into the category of top loads that are not true hats. One may spiral as few as 4 wires for several equi-spaced turns in a flat plane. The turns may be curves or straight lines with regular expansions in distance from the main element with no change of performance. Figure 5-1 shows a squared turn model.

The squared-turn model used 1+ spiral turn per wire with a 6.4' maximum distance from the main element at the corners. Using shorter wires to simulate curved spirals yielded no change in ultimate radius of wire length. The final product reported here required about 112' of wire, with 4 easily trimmed or extended ends.

The aluminum antenna produced a modeled gain of 4.94 dBi over perfect ground, suggesting true capacity-hat performance an superiority over the best of the top-loaded models when using other than lossless wire. Figure 5-2 shows an elevation pattern, including the remnant -40 dB horizontal component created by the fact that the spiral hat does not achieve absolutely perfect symmetry.

The SWR curve suggests a 2:1 SWR operating bandwidth of at least 200 kHz:

Antenna     Feed  3.5 MHz     3.55 MHz   3.6 MHz     3.65 MHz    3.7 MHz

35HC1025    SWR   2.1         1.5        1.0         1.4         1.9
            Z     21.3-16.8   22.2-8.7   23.1-0.5    24.0+7.5    25.0+15.7

It is quite likely that operational models of an antenna such as this might be designed for 3 ranges: 3.5-3.65; 3.65-3.8; and 3.8-4.0 with only a height adjustment in the main element. Using the same hat required a mast length of 32.8' to cover the upper 200 kHz of the band. Alternatively, hat adjustment can be easily made. A third alternative would be set the antenna for the lowest range and to switch in series capacitors (between the antenna and a 2:1 matching network or device) to cancel the inductive reactance as the operating frequency is increased.

Mechanically, a spiral offers another advantage in addition to the limited number of wires. An insulated 4-spoke structure might be developed for assembly at the top of the main element. Each spoke might be physically independent but interlocking at the main mast with the adjacent spoke support(s). The hat wires might fall into grooves in each spoke to assure long term alignment. However, the disassembled support system would lie in a flat stack of 4 pieces for ease of transport. I suspect a main element large than 1" would be required to handle such an assemblage.

Just because I have not previously seen hats designed either as spirals or as any of the non-standard configurations noted in the last chapter does not mean that they have not been used or that they have not appeared in the lengthy literature on short verticals. nonetheless, I thought it worth reporting as one more way to break preconceptions about what hats must look like to do their job of producing relatively efficient and wide-band short verticals for 80 meters.



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.



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