The 5/8-Wavelength Mystique
From various sources, the idea persists that it is possible to create a "gain" vertical monopole for the upper HF bands by using a 5/8 wl radiator instead of the more usual 1/4 wl radiator. From various books, the idea tends to migrate on occasion into magazine articles, where new generations of amateurs accept the premise as gospel. The purpose of this final section of our investigation is to see what truth, if any, lies in this persistent notion.
To accomplish the investigation, I modeled monopoles with 4 1/4-wl radials at 20 and 10 meters--specifically, at 14.15 MHz and 28.5 MHz. The 20-meter monopoles used 1" diameter radiators, while the 10-meter models used 0.5" vertical elements. All radials were #12 AWG wire.
The 20-meter radials were 199.4" long, with a 211" 1/4-wl vertical and, alternatively, a 41.5' 5/8-wl vertical. For the 1/4-wl monopoles, I used the same standard as with the VHF monopoles: the sum of the currents on the first segments of each radial added up to the value of the source current on the main radiator. The one exception to these dimensions is the last section of Table 13, where I revised the dimensions of the 1/4-wl monopole with radials that sloped at a 45-degree angle to bring the system to resonance and to effect the "equal current" standard. In this revised monopole, the radiator is 200" long, with 191" radials. No such revision was made to the non-resonant 5/8-wl radiator.
Similarly, at 10 meters, I used a 104.75" radiator with 99" radials for the 1/4-wl monopole system. Since there was no significant change in any parameter except the feedpoint impedance when revising the 20-meter sloping-radial model, I did not perform the same operation on the 10-meter antenna. The 5/8-wl monopole used the same radials, but with a 255" vertical radiator.
Fig. 18 shows the 4 types of models for each band. Both the 1/4-wl and 5/8-wl monopoles were modeled with radials at 90 degrees to the radiator and with radials sloping 45 degrees downward. Each 90-degree monopole was modeled at heights of 1, 5, and 25 feet to simulate ground, low, and roof-top positioning of the antenna base. Only the 25' high monopole was adapted to sloping radials. Since the fixed physical heights for test modeling represent different heights in terms of wavelengths for each of the two bands, I modeled each band separately.
As with all other antennas--and subject to some speculative discussion at the end of this section--the antennas were modeled over the standard spectrum of soil quality samples: very poor, poor, good, and very good. See part 1 of these notes for full details on the conductivity and permittivity of each soil grade.
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Table 13. 20-Meter 1/4-wl monopoles over various soils.
Soil Type Gain TO Angle Lobe Feedpoint Impedance
dBi degrees No. R +/- jX Ohms
1/4-wl Monopole, 90-degree radials, 1' above ground
Very Poor -1.03 28 1 35.8 + j 6.7
Poor 0.00 26 1 38.5 + j 10.2
Good -0.24 25 1 37.6 + j 9.9
Very Good 0.76 22 1 42.8 + j 12.1
1/4-wl Monopole, 90-degree radials, 5' above ground
Very Poor -0.39 25 1 29.2 + j 0.4
Poor 0.43 22 1 31.3 + j 0.7
Good 0.16 22 1 31.3 + j 0.2
Very Good 1.05 19 1 33.6 + j 0.3
1/4-wl Monopole, 90-degree radials, 25' above ground
Very Poor 1.37 17 1 23.2 + j 4.2
Poor 1.11 14 1 22.8 + j 4.1
Good 0.55 14 1 22.8 + j 4.2
Very Good 1.00 33 2 22.4 + j 4.3
1/4-wl Monopole, 45-degree radials, 25' above ground
Very Poor 1.43 18 1 55.3 + j 35.8
Poor 1.54 15 1 54.9 + j 34.7
Good 1.05 15 1 54.6 + j 34.7
Very Good 1.17 12 1 53.8 + j 33.6
1/4-wl Monopole, 45-degree radials, 25' above ground (revised dimensions)
Very Poor 1.40 18 1 47.5 + j 0.8
Poor 1.54 15 1 47.2 - j 0.2
Good 1.05 15 1 46.9 - j 0.3
Very Good 1.19 12 1 46.3 - j 1.2
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Table 14. 20-Meter 5/8-wl monopoles over various soils.
Soil Type Gain TO Angle Lobe Feedpoint Impedance
dBi degrees No. R +/- jX Ohms
5/8-wl Monopole, 90-degree radials, 1' above ground
Very Poor 1.70 19 1 107 - j 366
Poor 1.74 16 1 107 - j 366
Good 1.29 16 1 107 - j 366
Very Good 1.26 14 1 107 - j 366
5/8-wl Monopole, 90-degree radials, 5' above ground
Very Poor 1.98 18 1 106 - j 370
Poor 1.81 15 1 106 - j 369
Good 1.33 15 1 106 - j 369
Very Good 0.96 12 1 105 - j 369
5/8-wl Monopole, 90-degree radials, 25' above ground
Very Poor 3.00 14 1 108 - j 369
Poor 2.67 38 2 109 - j 369
Good 2.87 37 2 109 - j 369
Very Good 3.95 37 2 109 - j 369
5/8-wl Monopole, 45-degree radials, 25' above ground
Very Poor 3.29 36 2 104 - j 351
Poor 4.18 37 2 105 - j 351
Good 4.31 36 2 106 - j 352
Very Good 5.27 35 2 107 - j 352
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According to the modeling analysis, at 1' above ground, the 5/8-wl antenna shows a 1.37 dB average gain advantage over the 1/4-wl monopole. As we raise the base height to 5', the advantage on average remains about 1.31 dB--very consistent with the advantage at the lower height. However, when we place that antenna atop a 25' roof, the 5/8-wl vertical outstrips the 1/4-wl assembly by 2.11 dB with 90-degree radials and by 2.96 dB with sloping radials. From this quick readout, it would appear that the 5/8-wl vertical has finally achieved the 3 dB advantage so many claim for it relative to the shorter vertical.
However, for the elevated 5/8-wl monopole, we must pay special attention to the "Lobe Number" column in the tables. For the 1/4-wl antenna, as partially illustrated by Fig. 19, the elevation patterns show their strongest radiation from the lowest lobe--if there is more than one. The 1' high antenna (1-90 in the figure) and the 5' high antenna (5-90 in the figure) have only a single lobe in their elevation patterns due to the very low mounting heights. The higher versions with 90-degree or 45-degree radials (25-90 and 25-45 in the figure) show the emergence of a second lobe at a higher angle. Only for the 90-degree radial 25' high monopole over very good ground does the second lobe grow stronger than the lower lobe. In all other cases, the lower lobe dominates the pattern. Hence, the recorded signal strength of maximum gain is almost always for the lowest lobe (#1) for the 1/4-wl monopole at all heights.
When we turn to the 5/8-wl monopole, partially illustrated in Fig. 20, we discover that the situation is not at all the same. For almost all cases where the antenna is mounted at 25', the strongest lobe is not the lowest one, but instead is the higher-angle second lobe. The only exception among angles tested is the 90-degree-radial 5/8-wl monopole when placed over very poor soil. This particular case is the basis for some speculations on ground quality at the end of this study.
As Fig. 20 reveals, there is something curious about the patterns for 5/8-wl monopole. The lowest lobe remains at a relatively constant angle and strength. Lowest-lobe signal strength varies between 1.29 and 1.51 dBi over good soil as we change the base height, and the lobe TO angle is between 12 and 16 degrees above the horizon. The higher maximum gains reported by the models when the antenna has a base height of 25' are wholly a function of the upper lobe, the TO angle of which ranges above 35 degrees. If we use the lower lobe of the 5/8-wl pattern as a guide (instead of the stronger upper lobe), then the 5/8-wl monopole advantage over the 1/4-wl monopole at the same base height drops to the 0.5 to 1.0 dB range--far below the desired 3-dB advantage.
Fig. 21 shows what the gain advantage amounts to, using the 90-degree radial versions of the two monopoles, each with a base height of 25' above good ground. In operation, there would be little to choose between the two antennas. The very slight gain advantage of the longer monopole is offset by its greater sensitivity to high-angle QRN. Whether a 41.5' rooftop radiator is sufficiently easy to maintain relative to the standard 17.6' 1/4-wl radiator to go for the extra partial dB is a user judgment.
As shown in Fig. 22, a set of elevation plots for the 5/8-wl monopole with 90-degree radials at a base height of 25', the vertical antenna exhibits some of the same properties as the VHF verticals. As we improve the soil quality both beneath the antenna and in the ground-reflection region, the lower lobe shrinks and the upper lobe increases in strength. Contrary to the expectations of many, the strongest lowest lobe occurs over very poor soil.
The 20-meter monopoles at a 25' base height are only about 1/3 wavelength above ground. That same physical base height is above 2/3 wavelengths on 10 meters. Therefore, the 10-meter versions of our monopole sets deserves independent attention.
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Table 15. 10-Meter 1/4-wl monopoles over various soils.
Soil Type Gain TO Angle Lobe Feedpoint Impedance
dBi degrees No. R +/- jX Ohms
1/4-wl Monopole, 90-degree radials, 1' above ground
Very Poor -0.66 27 1 33.4 + j 4.2
Poor 0.23 25 1 36.1 + j 6.1
Good 0.07 25 1 36.4 + j 5.8
Very Good 0.43 23 1 38.8 + j 6.2
1/4-wl Monopole, 90-degree radials, 5' above ground
Very Poor 0.43 22 1 25.2 + j 1.3
Poor 0.95 19 1 26.1 + j 0.5
Good 0.75 20 1 26.0 + j 0.3
Very Good 0.81 17 1 26.5 - j 0.5
1/4-wl Monopole, 90-degree radials, 25' above ground
Very Poor 2.99 13 1 24.1 + j 4.0
Poor 2.67 35 2 24.2 + j 3.9
Good 2.73 34 2 24.2 + j 3.9
Very Good 3.78 33 2 24.1 + j 3.8
1/4-wl Monopole, 45-degree radials, 25' above ground
Very Poor 3.11 13 1 59.3 + j 38.3
Poor 2.42 38 2 59.6 + j 38.2
Good 2.51 37 2 59.6 + j 38.1
Very Good 3.52 36 2 59.8 + j 38.0
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Table 16. 10-Meter 5/8-wl monopoles over various soils.
Soil Type Gain TO Angle Lobe Feedpoint Impedance
dBi degrees No. R +/- jX Ohms
5/8-wl Monopole, 90-degree radials, 1' above ground
Very Poor 2.13 19 1 82 - j 307
Poor 1.97 16 1 82 - j 307
Good 1.72 16 1 82 - j 307
Very Good 1.05 13 1 82 - j 308
5/8-wl Monopole, 90-degree radials, 5' above ground
Very Poor 2.43 17 1 83 - j 307
Poor 1.88 14 1 83 - j 307
Good 1.63 14 1 83 - j 307
Very Good 2.21 46 2 83 - j 307
5/8-wl Monopole, 90-degree radials, 25' above ground
Very Poor 4.13 10 1 84 - j 308
Poor 3.16 26 2 84 - j 308
Good 3.19 25 2 84 - j 308
Very Good 4.19 24 2 84 - j 308
5/8-wl Monopole, 45-degree radials, 25' above ground
Very Poor 3.02 10 1 83 - j 283
Poor 3.57 25 2 84 - j 283
Good 3.54 25 2 84 - j 283
Very Good 4.26 23 2 84 - j 282
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If we again begin with a casual reading of the tabular data, we find that the 5/8-wl monopole has an advantage over the 1/4-wl antenna at every height. At the 25' level, the advantage is about 0.6 to 0.7 dB. The reason for this low figure, compared to what we initially encountered on 20 meters at the same height, is that both the 1/4-wl and the 5/8-wl antennas show maximum strength in the second, higher lobe. The common exception is once again when the antenna is over very poor soil, where the lower lobe dominates.
On 10 meters, even the 1/4-wl monopole shows a strong development of the second elevation lobe when the base of the antenna is 25' up. As shown in Fig. 23, over good ground, the differences between the flat-radial and sloping radial systems are minimal. Similarly, the there are no highly significant differences in the patterns for 90-degree and sloping 45-degree radial systems when we turn to the 5/8-wl monopole. With respect to the two lower lobes, as shown in Fig. 24, the structures are similar. However, note that the 5/8-wl monopole has developed a third elevation lobe near the 50-degree elevation angle. With sloping radials, this lobe is stronger than the lowest lobe. With the 90-degree radial set, the situation is reversed, even though the second lobe remains the strongest in both cases.
At a 25' height, the lowest lobe for either version of the 5/8-wl monopole is between 1.7 and 1.8 dBi. The lowest lobe of the 1/4-wl vertical with the same base height is about 1.5 dB whether the radials are straight or sloping. Hence, at the most desired angles of radiation, the 5/8-wl monopole advantage drops to a small fraction of a dB--hardly worth the extra 12' feet of vertical radiator in most cases.
We have tended to focus on the performance of the antennas at roof-top height. However, the two types of monopoles are also worth comparing at lower base heights. Here the 5/8 wl gain advantage is in the 1.3 to 1.7 dB range on 10 meters. Perhaps of equal importance is the fact that elevation angle of maximum radiation is 6 to 8 degrees lower than for the 1/4-wl monopole--indeed, it sits squarely in the most desire elevation-angle region for effective DX operation. Note as well that the 1/4-wl monopole shows its highest gain over poor soil, while the 5/8-wl version has its highest gain over very poor soil. That phenomenon accompanies another: the 5/8-wl monopole at a 5' base height already has sufficient second lobe development over very good ground to place the maximum gain at a high angle.
1. At roof-top heights, the advantage of the of an upper HF 5/8-wl monopole over a 1/4-wl version is well under 1 dB, while the added radiator length above the roof-top may create significant mechanical challenges to offset the small gain in signal strength.
2. At base heights near the ground, the 5/8-wl monopole advantage may sometimes exceed 1 dB at lower angles of radiation, but does not reach 2 dB, even on 10 meters. The lower TO angle of the 5/8-wl monopole may prove to be of some advantage in such situations. Although each advantage of the longer monopole may be individually somewhat marginal, together, the advantages may add up to a noticeable improvement in performance over a 1/4-wl vertical.
Although I have used good ground for most of the illustrative patterns in this study, we should take note of the performance of all of the monopoles over very poor soil. On 10 meters, we can show the effects by comparing 25' high 5/8-wl monopoles with 90-degree and 45-degree radial sets. See Fig. 25.
The 10-meter patterns show once more that the use of sloping radials is marginal or detrimental, just as it was in the VHF region when the antenna was several wavelengths above ground. The flat-radial monopole has the greater signal strength at the lowest elevation angles. However, what may be more important is the fact that for either case, the poorer the ground, the better the antenna performance, both in terms of gain and in terms of pattern shape.
Now, if we only knew how good or bad the ground was beneath and around our antenna. . .
This section of the investigation is somewhat speculative and based on inadequate evidence. However, it may have some use in fostering further investigations that might better resolve the questions we shall raise.
In some 7 MHz investigations of vertical antenna performance in concert with Dave Bowker, W1FK, he measured the soil conductivity of his Maine antenna site at 0.0002 to 0.00025 S/m. The readings were not only repeatable with good equipment, but as well showed the proper trends for rainy and dry spells. Using these values and some careful measurements of a 7-MHz vertical with a number of different elevated radial fields, we developed a permittivity value of about 7 for the area. First, this value of dielectric constant turned out to be assigned to shale, which is not unlike the constitution of the antenna site sub-soil. Second, using the value permitted careful models of the antenna and radial system to track all variations such that the modeled resonant impedance and the measured resonant impedance were always within 1 Ohm of each other--or less.
This anecdote--and that is all it can be at this stage--is coincident with reports that soil conductivity decreases with increases in frequency. (This claim is not the same as saying that for a given conductivity, ground losses increase with increasing frequencies. This other principle is already accounted for in the Sommerfeld-Norton ground calculation algorithms used by NEC. The new claim is not, since the user enters values--presumably measured or accurately estimated--of conductivity and permittivity (relative dielectric constant) into the S-N ground calculation system.) The measured value of the Maine antenna site is well below any value listed in standard soil quality charts--most of which were generated for AM broadcast antenna use. Indeed, a conductivity value of less than 1 mS/m is no where in such charts associated with a dielectric constant as high as 7.
The sum of these considerations is a question: just how poor is the soil beneath our upper HF vertical antennas? Although I cannot answer that question directly, I can look at some models using very low values of conductivity and permittivity. In fact, we can look together at the results of some speculative modeling. I used an initial value of 0.0002 S/m for conductivity and reduced the value to 0.0001 S/m. For a constant dielectric constant of 7, there was no significant change in the modeled performance of the subject antennas for the 2:1 change in soil conductivity. However, at low values of conductivity, changes in permittivity have a stronger influence on performance. See Part 2 of Some Facts of Like About Modeling 160-Meter Vertical Arrays: Part 2: Appreciating Conductivity and Permittivity. Therefore, using the 0.0001 S/m value of conductivity, I reduced the permittivity in steps from 7 to 1.
The modeled antennas were the 20-meter and 10-meter 1/4-wl and 5/8 wl monopoles at 25' above ground using 90-degree radial sets. The results of the modeling tests appear in Table 17.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 17. 20-Meter and 10-Meter 1/4-wl and 5/8-wl monopoles over low-quality soils. Soil Cond. Gain TO Angle Lobe Feedpoint Impedance & diel. const. dBi degrees No. R +/- jX Ohms 1/4-wl 20-Meter Monopole, 90-degree radials, 25' above ground .0002/7 1.54 16 1 23.1 + j 4.1 .0001/7 1.57 16 1 23.1 + j 4.1 .0001/5 1.72 17 1 23.2 + j 4.0 .0001/3 1.98 18 1 23.5 + j 4.0 .0001/1 3.01 14 1 24.0 + j 3.8 5/8-wl 20-Meter Monopole, 90-degree radials, 25' above ground .0002/7 2.86 13 1 109 - j 369 .0001/7 2.88 13 1 108 - j 369 .0001/5 3.19 14 1 108 - j 369 .0001/3 3.61 15 1 108 - j 369 .0001/1 4.30 11 1 108 - j 370 1/4-wl 10-Meter Monopole, 90-degree radials, 25' above ground .0002/7 2.71 12 1 24.2 + j 4.0 .0001/7 2.72 12 1 24.2 + j 4.0 .0001/5 3.06 12 1 24.2 + j 4.0 .0001/3 3.52 13 1 24.2 + j 4.1 .0001/1 3.72 9 1 24.2 + j 4.3 5/8-wl 10-Meter Monopole, 90-degree radials, 25' above ground .0002/7 3.86 10 1 84 - j 308 .0001/7 3.87 10 1 84 - j 308 .0001/5 4.18 10 1 84 - j 308 .0001/3 4.56 11 1 84 - j 308 .0001/1 4.50 8 1 84 - j 308 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
For all of the models, there is a significant increase in gain as we decrease the dielectric constant, using a very low value of conductivity. To gain a sense of what the table shows, we can turn to some illustrations.
Fig. 26 shows the tabular extremes for the 20-meter antennas. Note that the lower lobe of the best 1/4-wl monopole performance coincides closely with the lowest lobe of the 5/8-wl monopole using the starting values of the progression. The improvement on 20 as we decrease the dielectric constant is about 1.5 dB total.
In Fig. 27, we see the same data in pattern form for 10 meters. The increment on this band is only about 1 dB, possibly due to the more complex lobe structure that results from the antenna being higher in terms of wavelengths than the 20-meter models.
The upshot of the speculative modeling is simply this: the worse the soil, the better that most verticals perform. This statement has some assumptions built into it. The most important is that we are not located near a large body of salt water and that the soils with which we are making comparisons are not any better than that which we classified as very good. However, the pattern continues to repeat itself: lowest lobes dominate over the poorest soils, while with better soils (poor to very good), higher lobes often take over, to the detriment of low-angle radiation.
It would be glib to jump to conclusions. However, I do not know the answer to the question of how well our upper HF vertical perform, since I do not have decent data at hand for the conductivity and permittivity of the usual soils at the upper HF frequency range. All that I can do is draw from these notes a conditional conclusion: if any of this speculative material has relevance to real antenna performance, then we may one day have to re-evaluate our judgments of the performance of vertical antennas in the upper HF range.
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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