The 5/8-Wavelength Mystique
At VHF, both the 1/4 wavelength monopole and the 5/8 wavelength monopole are widely used, but in circumstances quite different from those applicable to an 80-meter monopole. Some 15 years ago, Don Reynolds, K7DBA, looked at "The 5/8-Wavelength Antenna Mystique" (in The ARRL Antenna Compendium, Vol. 1 [Newington: 1985], pp. 101-106). I knew my title had to have a source other than pure creativity. In any event, although the article must be read in the context of affiliations with AEA, which began back then marketing an effective telescoping 1/2 wavelength antenna for handhelds, the measurements used as the basis of the article will interest those who read further into this modeling study.
The VHF monopole is designed for elevated use, normally with 4 symmetrical 1/4 wavelength radials. We shall not here deal with monopoles of any length without radials, although radial-less dipoles will be noted. To understand the relative performance potential of these antennas, it was necessary to construct a number of models to account for the variety of configurations. Fig. 10 shows the span of models considered, where all models used 0.25" diameter aluminum for every element.
The test frequency was 146 MHz, where a wave is 80.84" long, making the selection of the length of the longer monopole obvious. The 1/4 wavelength monopoles were adjusted for resonance, with adjustment also to the sloping radial length to bring the radiator and radial lengths into rough alignment.
Each model was run over the standard soil varieties. Given that VHF antennas are used under varying circumstances, I approximated two major ones. For roof and tower mounting, I used a height of 25' (300") at the base of the monopoles and the center of the dipole. Further height increases would not have altered the results significantly in terms of operational trends. Vertical monopoles are also used in mobile and temporary installations. For that case, I used a base height of 5' (60").
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Table 7. 146 MHz 1/4 wavelength vertical monopole with 4 90° radials at 5' and 25' base height.
5' Base Height 25' Base Height
Soil Type Gain TO Angle Feed Z Gain TO Angle Feed Z
dBi degrees R+/-jX dBi degrees R+/-jX
Very Poor 2.76 12.2 26 - j 1 5.83 3.4 26 - j 0
Poor 2.32 34.3 26 - j 1 5.28 3.4 26 - j 0
Good 2.32 33.9 26 - j 1 5.28 3.4 26 - j 0
Very Good 2.90 33.5 26 - j 1 4.90 3.3 26 - j 0
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Table 7 summarizes the results for the 1/4 wavelength monopole with 4 radials at right angles to the radiator. At a height of 60", the radiated field strength of the antenna is best over very poor and very good soils. However, note that the TO angle for all but the worst soil is not the expected very low angle. The low angle corresponding to the strongest lobe over very poor soil has a reduced signal strength, although not down by more than about 1 dB or so from the higher-angle lobe. Fig. 11 shows the changing balance between the lower and higher lobes of the array as we change the soil quality beneath it at a base height of 5'.
At a height of 300", the expected low radiation angle for the major lobe is present. However, note the decreasing gain with increasing soil quality. Although the amount is not operationally significant, the trend is interesting.
A further factor to note is the lower-than-expected feedpoint impedance for the 1/4 wavelength monopoles. Received wisdom anticipates a feedpoint impedance of about 35-36 Ohms, based on ground mounted monopoles over perfect ground. An elevated monopole does not easily answer to this conception. The monopoles constructed for this model began with a free-space 1/2 wavelength dipole. One half of the dipole was replaced with 4 radials, with lengths adjusted until two conditions were achieved. First, the maximum current level is at the feedpoint, and the sum of the currents on the first radial segment of each of the four radials was close to the source segment current value. Second, the lengths of the radiator and the radials were adjusted to achieve resonance and to retain the current maximum position.
A useful technique for establishing a reasonable equality between the current level on the source segment of the monopole and the sum of the current levels on the first segment of the four radials is to use a maximum of segments consistent with maintaining a reasonable segment-length-to-radius ratio. As well, a close equality should be maintained between the segment lengths on the monopole and on the radials, since the source segment will be immediately adjacent to the first segments on the radials. For the 90° and sloping radial 1/4 wavelength monopoles, 31 segments per element provides a very usable figure while still maintaining a small enough model that does not require segment length-tapering to run on basic NEC-2 software. Small deviations from the goal of equal currents did not produce significant changes in the feedpoint impedance, which is about 38% lower than image-based conceptions of a monopole would dictate. Changing the number of radials does not materially affect the impedance or the vertical radiator length, if the radial lengths are adjusted to bring the assembly back to resonance.
The 1/4 wavelength monopole with sloping radials was constructed in the same manner. The length of the radiator and the lengths of the 45°-sloping radials were adjusted to place the maximum current at the feedpoint and to have a division of current among the radials so that sum of the currents in the 4 radials at their junction equaled the current level at the feedpoint, with the assembly as close to resonance as feasible. Consequently, both the radiator and the radial lengths are different from those used in the 90° radial model.
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Table 8. 146 MHz 1/4 wavelength vertical monopole with 4 45° radials at 5' and 25' base height.
5' Base Height 25' Base Height
Soil Type Gain TO Angle Feed Z Gain TO Angle Feed Z
dBi degrees R+/-jX dBi degrees R+/-jX
Very Poor 3.24 13.0 51 - j 0 6.74 3.5 50 + j 0
Poor 2.47 37.5 51 - j 0 6.18 3.4 51 + j 0
Good 2.48 37.5 51 - j 0 6.18 3.5 51 + j 0
Very Good 3.03 36.7 51 - j 0 5.79 3.4 51 + j 0
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Table 8 collects the results of running the model at both high and low levels. At 5', the sloping-radial monopole shows the same pattern of high-angle main lobes except over very poor soil. At 25', the pattern of decreasing gain with increasing soil quality also reappears. The chief difference in performance is a noticeable increase in gain at all levels and soils relative to the 90° radial monopole. At 25', the difference amounts to nearly 1 dB.
The reason for the gain increase is simple: the sloping radials have both a horizontal and a vertical component to their radiated fields. The horizontal components cancels (if the radials are identical in length and symmetrically distributed). However, the vertical component--non-existent in the 90° monopole--contributes to the overall radiation of the antenna, yielding a small gain increase.
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Table 9. 146 MHz 1/2 wavelength vertical dipole at 5' and 25' (feedpoint height).
5' Base Height 25' Base Height
Soil Type Gain TO Angle Feed Z Gain TO Angle Feed Z
dBi degrees R+/-jX dBi degrees R+/-jX
Very Poor 3.08 13.2 72 - j 0 6.79 3.6 72 - j 0
Poor 2.19 11.0 73 - j 0 6.22 3.5 72 - j 0
Good 2.14 11.1 73 - j 0 6.22 3.5 72 - j 0
Very Good 2.42 38.0 73 - j 0 5.83 3.4 72 - j 0
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If the sloping radials increase gain, then a true vertical dipole should show additional increases in antenna gain. Therefore, I modeled a reference dipole for the test frequency--with the dipole center at the test heights so that the maximum current position would be consistent among the models. Table 9 reports the modeling results. At 25'. the dipole shows a minor (and operationally insignificant) increase in gain over all soils relative to the 45°-radial monopole. At the lower height, the dipole shows a reduction in gain--largely as a function of the closer proximity of the lower antenna end to the ground. However, except over very good soil, the TO angle of the strongest lobe is now at the expected low angle. Hence, for poor and good soils, the gain comparison is not especially valid, since the radiation is headed in different directions. Nonetheless, the dipole can be considered to have a slightly weaker signal, but one more likely to be directed at a favorable elevation angle. Fig. 12 compares elevation patterns of the three antennas so far considered at a height of 25' above good ground. In practical terms, the low- angle lobes of the dipole and the 45° sloping-radial monopole overlap, with the 90°-radial monopole slightly weaker.
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Table 10. 146 MHz 5/8 wavelength vertical monopole with 4 90° radials at 5' and 25' base
height.
5' Base Height 25' Base Height
Soil Type Gain TO Angle Feed Z Gain TO Angle Feed Z
dBi degrees R+/-jX dBi degrees R+/-jX
Very Poor 3.74 9.9 65 - j 227 5.78 3.2 65 - j 227
Poor 2.87 25.2 65 - j 227 5.28 3.2 65 - j 227
Good 2.87 25.2 65 - j 227 5.28 3.2 65 - j 227
Very Good 3.36 24.5 65 - j 227 4.94 3.2 65 - j 227
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The key question is whether a 5/8 wavelength radiator will have any advantage over the array of monopoles and dipoles we have so far explored. Table 10 tells the story for a 50.5" radiator over quarter wavelength radials set at 90° to the radiator. At a base height of 5' above ground, the 5/8 wavelength monopole shows the same progression of gain values and the same relatively high-angle main lobes as the 45° sloping-radial monopole, the better of the two 1/4 wavelength monopole arrays. Fig. 13 reveals, as do the tabular numbers, that the 5/8 wavelength monopole has a slightly lower-angle lowest lobe and slightly greater strength, with the elevation pattern taken over good ground.
Notable in the comparison of patterns is the fact that the elevation pattern of the longer monopole has one more lobe than the pattern for the 1/4 wavelength antenna. Hence, while the lowest lobe is stronger than that of the shorter antenna, the longer monopole also has stronger radiation at considerably higher angles than the 1/4 wavelength monopole. The consequence of the third lobe is to increase the complexity of the lobe structure variations as we change the soil type over which we operate the antenna at a height of 5'. Fig. 14 shows the elevation patterns for the 5/8 wavelength monopole for very poor, good, and very good soils in order to demonstrate the changing balance among lobes. (Poor soil is omitted since its pattern overlaps the pattern for good soil.)
Although the 5/8 wavelength monopole shows a very slight advantage over the 1/4- wavelength monopole with sloping radials, the advantage disappears when we raise the longer antenna to 25' at its base. Comparing the numbers in Table 8 and Table 10 sets the stage for examining Fig. 15. With both antennas over good soil, the 1/4 wavelength monopole with sloping radials shows nearly a full dB additional gain over the 5/8 wavelength model.
In the end, it is dubious whether a 5/8 wavelength monopole has any significant operating benefit over a 1/4 wavelength monopole, each with 4 radials. Perhaps the higher top height of the longer antenna will yield some benefit when its base is very close to the ground. However, the 5/8 wavelength monopole always requires some form of matching system for use with a 50- Ohm coaxial cable, and matching systems at VHF are not without loss. At rooftop and higher levels, the sloping radial monopole with a 1/4 wavelength radiator or a half wavelength dipole will do as well--or better.
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Table 11. 146 MHz 5/8 wavelength vertical monopole with 4 45° radials at 5' and 25' base
height.
5' Base Height 25' Base Height
Soil Type Gain TO Angle Feed Z Gain TO Angle Feed Z
dBi degrees R+/-jX dBi degrees R+/-jX
Very Poor 3.02 50.2 65 - j 191 3.95 36.2 65 - j 191
Poor 3.83 51.1 65 - j 191 4.90 36.1 65 - j 191
Good 3.84 51.0 65 - j 191 4.90 36.1 65 - j 191
Very Good 4.21 51.1 65 - j 191 5.30 36.1 65 - j 191
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One tendency of those who home brew VHF monopoles is to attempt to replicate the advantages of the sloping radial system that adds to the performance of a 1/4 wavelength monopole. Hence, they slope the radials of a 5/8 wavelength monopole. In principle, the radiation resistance goes down, requiring only a series inductance as a means of compensating for the capacitive reactance at the feedpoint. Unfortunately, the maneuver results in a major change in the elevation pattern of the antenna over all qualities of soil, as revealed in Table 11. Radiation from the lowest lobe is over 2 dB down from the flat-radial system. Fig. 16 shows the situation graphically over good soil at a height of 25'. The number of lobes in the pattern does not change with the change in radial angle, but the power distribution changes very significantly. In general, a 5/8 wavelength monopole with sloping radials cannot be recommended for general communications.
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Table 12. 146 MHz 1.25 wavelength vertical EDZ at 5' and 25' (feedpoint height).
5' Base Height 25' Base Height
Soil Type Gain TO Angle Feed Z Gain TO Angle Feed Z
dBi degrees R+/-jX dBi degrees R+/-jX
Very Poor 4.31 10.5 115 - j 389 9.49 3.5 108 - j 385
Poor 3.70 9.3 118 - j 390 8.93 3.4 108 - j 385
Good 3.65 9.4 118 - j 390 8.93 3.5 108 - j 385
Very Good 3.22 8.7 120 - j 391 8.54 3.4 108 - j 385
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There is one more antenna that deserves passing recognition before we summarize the results of the VHF use of monopole. Although completely impractical at 80 meters, a vertical EDZ can be effectively used at VHF in many circumstances. Table 12 summarizes the results of modeling a 111" center-fed EDZ at the center heights used for the dipole: 60" and 300". At the lower height, the antenna is only 10" above the ground, but it outperforms any of the monopoles and the dipole. It shows a consistent low angle for the main lobe, a fact illustrated in the 5' outline of the elevation pattern in Fig. 17. As well, the antenna shows the decreasing gain with improvements in soil quality so that with very good soil, the gain drops just below the level achieved by the 5/8 wavelength monopole with its base at 60" above ground.
With the antenna center at 25' above ground, the EDZ provides just under 3 dB additional gain relative to the best of the other antennas in the collection examined here. The elevation pattern is also shown in Fig. 17. The feedpoint impedance of the vertical EDZ will be a challenge to transform to what a 50-Ohm cable requires, but there are both network and linear transmission line methods of achieving the match. Even allowing for the losses in such systems, the vertical EDZ may be worth considering for some applications. See, for example, Rick Littlefield, "The 2-Meter PVC-EDZ Antenna, Communications Quarterly (Summer, 1997), pp. 104-106. As well, see "Feeding the EDZ" at this site for additional notes on this general technique, sometimes called using "delay lines."
Whatever the advantages of using a straight vertical radiator, the question that formed the basis for this VHF study was whether there is any profit in using a 5/8 wavelength monopole in preference to a 1/4 wavelength monopole when each is equipped with 4 radials. The results of our investigation might be summarized as follows.
The modeling application to house-top mounted monopoles is--except for nearby metallic structures--applicable to reality without much adjustment. However, the results for lower mounting heights would need adjustment for the effects of the antenna surroundings. Hand-held antennas are almost impossible to predict relative to surrounding influences on radiated signals. However, Dan Richardson, K6MHE, is performing some studies of typical monopoles and other antennas mounted on various types of vehicles, as simulated by a series of detailed wire-grid structures. I shall await the results of his work to see if the greater height of the current maximum for a 5/8 wavelength monopole mounted on the trunk of a car makes a significant difference relative to the lower current maximum of a 1/4 wavelength monopole mounted in the same position. In short, a final decision on which type of antenna may be best (including both vertical dipoles and EDZs) must rest on a full analysis of the operating circumstances. At best, these general notes are indicators, but they are not final or universal judgments.
Before we can complete the overall picture of 5/8 wavelength monopole performance relative to the more familiar 1/4 wavelength monopole, we must look at one more frequency region: the upper HF bands.
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.
Go to Part 3: Upper HF Monopoles
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