No. 50: Notes on Horizontal Antenna Height

L. B. Cebik, W4RNL

In the last issue, we looked at the simple ¼ wavelength vertical monopole. In the course of our examination, we paid close attention to the effects of height on the pattern of the antenna. We discovered that if we chose a height too great, we could send our signals into space rather than into the skip layers that give us long-distance 10-meter communications. Although we have touched upon the subject in the past, we should give equally close attention to the height of horizontal antennas.

One advantage that all single layer horizontal antennas share is that their behavior is very regular as we change heights. By a "single-layer" antenna, I mean a single dipole, a single Yagi, etc. The situation does change a bit when we stack horizontal antennas. But most of us are limited to single layer antennas, and that is our proper starting point.

As we raise the height of a horizontal antenna, 2 things happen. First, the lowest elevation lobe gets lower and lower. That is an advantage, because on 10 meters, most of the DX skip occurs at angles from about 5° to 10° or so above the horizon. The second phenomenon is that when we raise the antenna, more and more elevation lobes develop. Fig. 1 shows the situation of a dipole at 0.5 wavelength intervals up through a 2 wavelength height.

For each half wavelength step, we find a new lobe. These lobes do not simply pop up, but develop. They first appear as a bulge straight up and then gradually split as we increase the height until they take their place in the set of lobes--with a new top bulge. Since we only have 90° of angular room for the lobes, every new lobe results in a lower angle for existing lobes, and each of them gets a bit thinner.

Note that each elevation plot is labeled in terms of the height measured as a fraction of a wavelength. A wavelength at 10 meters is about 35', at 15 meters 44', and at 20 meters 70'. So if you use a tri-bander at 70', then your 20-meter lobes will look like the 1 wavelength picture, on 15 like the 1.5 wavelength picture, and on 10 meters like the 2 wavelength picture. In fact, we should run the same plotting exercise with a 3-element Yagi just for comparison. See Fig. 2.

Note that in the forward direction, we find the same number of lobes at each height as for a dipole. At the lowest height, the Yagi main lobe is slightly lower than for the dipole as a function of the Yagi's added forward gain. But that advantage quickly fades as we raise the antenna higher. In addition, the rearward lobes of a Yagi tend to show some erratic properties, but so long as the front-to-back ratio is high--as it is in this design--the oddities create no problems. A 2 element Yagi, with lesser front-to-back ratio will show stronger rearward lobes with more standard differences in strength. The 3-element Yagi shown in the plot has a gain advantage over the dipole about 5.8 dB, although that advantage varies with height--assuming one of each type of antenna at the same height for comparison.

How high should I place my antenna? For maximum DX, place it as high as is feasible. I have heard arguments to the effect that a half wavelength is the optimal height, because we have only a single forward elevation lobe. Unfortunately, that argument fails to account for two facts. As we raise the antenna, the gain of the lowest lobe increases. The 2 wavelength dipole is about a half-dB stronger than the ½ wavelength dipole. The 2 wavelength Yagi is about 1.5-dB stronger than the ½ wavelength version. The second fact returns us to the primary skip angles on 10 meters, which are very low for the longest DX (most of the time). With a height of 1.5-2 wavelength, the Yagi main lobe angles fall well into the prime skip angles. Those same angles receive about half of the available power when the Yagi is only 0.5 wavelength up. If a half wavelength is all you can manage, you will still do well, especially in strong sunspot periods. But greater height will bring even better results.

These principles apply to what I have called the single-bay or single-layer horizontal antenna. The class of antennas includes single dipoles, Yagis, and flattop horizontal phased arrays, such as the ZL-Special or the 8JK. We can modify the lobe elevation structure by using horizontal antennas that are stacked vertically and phase fed. Many such arrays are possible, but here we can bring back an old favorite of mine just to illustrate how the elevation lobes change with stacking. The array is the expanded lazy-H. As shown at the top of Fig. 3, we run two extended double Zepp center-fed wires, each 44' or about 1.25 wavelength long. We place them one above the other, using a 5/8 wavelength spacing or 22'. Using parallel feeders, we run lines from each wire to a mid-point between them. At this junction, we run a parallel feeder back to the shack to a balanced antenna tuner. The antenna is usable on 40-10 meters with bi-directional narrow-beam high-gain patterns. The exact gain is a function of the overall antenna height, but with the base at 44' (about 1.25 wavelength) and the top is at 66', the gain is somewhat over 15 dBi--greater than the gain of the 3-element Yagi.

The lower portion of Fig. 3 answers 2 questions at the same time. 1. Where does all the gain come from? 2. Where did all the Yagi and dipole upper angle lobes go to? (These questions will also give some work to the preposition police.) When we place horizontal antennas at a vertical spacing of 1/2 wavelength, most of the upward energy cancels out and re-appears at lower angles. In the present configuration, the spacing is about 5/8 wavelength, which does not cancel all of the vertical energy, but does yield the highest gain in the lowest lobe. Note that compared to the dipole and the Yagi, even the extended lazy-H second elevation lobe is small, releasing more energy for use in the lowest lobe.

The actual mechanics of lobe formation is a matter of complex interactions of direct radiation and reflections from the ground at a distance from the antenna. My brief description above has a few terms that will not withstand mathematical treatment. However, we have a basic fact: an antenna will radiate all of the energy fed to it. If it does not go up at high angles, then it must go elsewhere. Since the linear nature of horizontal elements prevents the radiation going to the sides (in a well-designed array), then the energy must go into the main lobe or lobes. As a general rule, the higher the gain of an array, the narrower also that the beamwidth becomes. So if you plan to use a wire extended lazy-H, aim it well so that your gain goes in useful directions.

However, our main theme this time is not beamwidth in the horizontal plane, but the elevation lobes that form the pattern of any horizontal antenna. The higher the horizontal antenna, the lower will be the lowest and strongest lobe. Hence, DXers try for the highest antenna position feasible. The higher we place the horizontal antenna, the more elevation lobes that form. Stacking antennas by the right spacing can increase gain in the lowest lobes and reduce relatively useless very high angle radiation.

Finally, note that I always qualify my recommendation for height with the words "as feasible." There are questions of cost, maintenance, regulations, and simple family comfort that form part of our decision on how high to place an antenna. Higher is better, but only if you can handle it. My tower is only 35' tall, but my hill is nearly 100' above surrounding terrain. Mother nature has saved me a bundle of money and worry.

Updated 01-05-2006. © 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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