The 5/8-Wavelength Mystique
The 5/8 wavelength vertical monopole has long held the reputation of providing about a 3-dB gain advantage over the 1/4 wavelength vertical monopole. The foundation of that reputation rests upon theoretical calculations that show the longer monopole to have the derived gain increase when both monopoles are set over a perfect ground. Fig. 1 shows the modeled elevation patterns of the two subject antennas under the prescribed conditions. Both antennas are set up as 0.1" diameter copper elements, which have negligible losses. At the 3.75 MHz test frequency, the 1/4 wavelength monopole showed a gain of 5.08 dBi, compared to the 8.01 dBi figure for the 5/8 wavelength monopole.
For an example of the claim or illustrations of the theoretical gain of the 5/8 wavelength monopole over the 1/4 wavelength monopole, see Terman's Radio Engineers' Handbook (McGraw-Hill, 1943), pp. 793-795. Recent college antenna texts fold vertical monopole concepts into more general considerations, although many antenna texts through at least 1970 present the theoretical relationship of a 1/4 wavelength radiator to a 5/8 wavelength radiator in the classic terms of Fig. 1. This idea persists in amateur radio literature. For examples, see Orr and Cowan, Vertical Antennas (RAC, 1986), p. 162, and by the same authors, Simple and Low-Cost Wire Antennas (RAC, 1990), p. 115.
A second factor contributing to the reputation of the longer monopole for higher gain is the current distribution along the element. Fig. 2 shows the distribution for both the long and short monopoles, with the ground plane elements omitted for clarity. The 1/4 wavelength antenna presents its "half-dipole" current distribution curve, while the 5/8 wavelength antenna provides a "half-EDZ" distribution curve. The peak current at a position well above the top of the short antenna is said to give the longer monopole a lower-angle of radiation and additional gain.
Although the claim of "3 dB more gain" for a 5/8 wavelength vertical radiator has considerably quieted in recent years, little by way of systematic exploration of the claim has appeared in amateur literature. Therefore, a little study seemed in order. However, the examination is complicated by the fact that the 1/4-vs.-5/8 question arises in two ot three different contexts. In the lower HF region of the spectrum, vertical monopoles are implemented with their bases normally at ground level, with a buried set of ground-plane radials. In the VHF region, vertical monopoles and their radials are elevated at least 1 wavelength above ground and often up to several wavelengths. The third main but lesser used region for monopoles are the upper HF bands from 20 through 10 meters, where they are sometimes employed by space-restricted hams. Consequently, our look into the 5/8 wavelength monopole question will require three related but relatively independent investigations, one for each frequency region and antenna assembly.
The three-part investigation appears in the following items:
The exploration proceeded using NEC-4 as a suitable modeling vehicle for examining the question of relative gain. The VHF and upper HF portions of the study might well have been undertaken with NEC-2 in any of its basic commercial implementations, since the number of elements are few and the antenna geometry does not challenge any of the software limitations. However, NEC-2 cannot directly model a buried radial system, and many basic NEC software packages are limited in the total number of segments a modeler can use. Therefore, professional software using NEC-4.1 becomes the necessary tool for buried ground-plane systems involving up to 128 radials.
Updated 04-17-2001. © 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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