Reflections: The Downside of Height

As my antennas get higher I run into novel difficulties. In one sense it's a nice problem to have considering that few hams have antennas that are large or high. Nevertheless it is a problem. In this article I'll review the affect of height on horizontally polarized antennas (primarily yagis) before discussing my own particular challenges and mitigation strategies.

Even for those without this problem the discussion may be of interest and educational. Some of the material is elementary. Antenna height has been covered numerous times in this blog (and countless times elsewhere) and I will reference earlier articles for details that are only touched on lightly in this one.

The antenna does not determine the path

Higher antennas are not always better. More precisely, for an antenna that is already reasonably high, higher is not always better. This is not due to diminishing returns so much as the potential mismatch between the antenna's pattern and what the ionosphere requires for communication.

Put another way, an effective antenna is one in which its most effective direction is the one that nature demands for the intended communication. Nature chooses the direction. Your job is to design and install an antenna that is effective for that direction. The antenna does not determine the path.

Direction is 3-dimensional so we must consider both azimuth and elevation. Good presentations of antenna patterns show both. Let's dispense with azimuth quickly by noting that the correct azimuth is the great circle route (short or long path); that is, except when it isn't! Skew path is not at all rare on the lowest HF bands and on the band closest to the MUF. Smart operators turn their antennas to find the optimum azimuth during difficult conditions.

With that out of the way let's turn to elevation angle. Good DX paths are more common at low elevation angles. There are exceptions; there are always exceptions. An effective vertically polarized antenna with its far field reflections from a high conductivity ground (e.g. seawater) can have an impressively low elevation angle for its main lobe. Most vertical antennas don't do nearly so well. 

Horizontal antennas usually do better over typical ground (medium to poor) when they are high enough. In context, high is with respect to wavelength. On 40 meters and down the height to put the maximum radiation at the required low elevation angle for most DX paths is difficult to impossible for most hams. Hence the prevalence of verticals on 80 and 160 meters. For DXing on these low bands, a moderately efficient vertical typically outperforms a horizontally polarized antenna with the same apex height.

When great height is possible there are dangers lurking. Higher isn't necessarily better. This is a lesson I am learning every day with my complement of low, high and higher antennas.

Modelling height

Software makes it easy to inspect the elevation patterns of antennas at various heights. Consider the following set of patterns for a 5-element yagi at heights from ½λ to 4λ over medium EZNEC ground. Although I am illustrating the effect of height with a 20 meter yagi the pattern is scalable to other bands. 

The elevation pattern scales with wavelength. If you get dizzy thinking about an 84 meter height for a 4λ on 20 meters, you can instead imagine a 2 meter yagi that is merely 8 meters high.

This is a busy plot that may be difficult to read. To help out I'll list a few key points:

• Starting at 1λ there is more than one forward lobe. Their quantity increases as height increases.
• There are deep nulls between those lobes, and that will cause difficulties on the air.
• Maximum gain increases with height due to the concentration of energy at lower angles.
• Low angle radiation increases quite a lot at greater heights. For example, at 5° the gain for the yagi up 2λ is ~6 db better than one up ½λ. Diminishing returns are rapid at greater height.
• By choosing heights that are an integral multiple of ½λ the radiation directly upward is cancelled. I did this deliberately for the plot and it is worth keeping in mind when planning your next tower.

As a general rule, the higher you go the greater the number of elevation lobes and nulls. These can be aggravating since when the signal comes in at an angle where there is a null the antenna will not serve you well. Unfortunately there will always be signals that strike those nulls. It is worth addressing for the contest enthusiast, but also for daily operating enjoyment and DXing.

Many VHF operators may be unaware of the problem since DX paths tend to be at low elevation angles. That is often not the case for sporadic E propagation, as one example. An elevation rotator for satellites and EME can deal with the problem at high elevation angles, where "high" means an elevation angle greater than the half-beam width of the main lobe in free space (see below). 

How those lobes and nulls form

The pattern of any antenna doesn't start at the horizon. All radiate downward. A free space elevation plot makes this evident. At right is one for the yagi discussed above.

The elevation patterns above and below the horizontal axis (0° elevation) are mirror images. When placed over ground the downward radiation is absorbed (dissipated) or reflected upward. 

The half of the radiation that reflects from the ground adds to the direct (skyward) radiation to form an interference pattern by superposition. That interference pattern is the series of lobes (reinforcement) and nulls (cancellation). 

Perfect addition and subtraction requires the amplitude of the ground reflection to be equal to the direct radiation. This is approximately true for horizontal polarization even for poor ground quality. 

When the fields add the radiation in the lobe is boosted by 6 db. Where the phase difference is 180° the null is exactly zero. Of course it's never exactly zero, and over real ground will rarely be deeper than -20 db below the peaks of the adjacent lobes.

At right is the broadside elevation pattern of a simple dipole 4λ above ground. Perfect and poor ground are compared. The effects of ground are easier to see with a dipole than with a yagi.

The first thing to notice is that over poor ground the nulls are not so deep and the lobes not so large. Poor ground absorbs more at high incidence angles so the reflections are weaker. In addition to that, the phase shift will not be 180°. Perfect ground does not have these deficits. The effect of poor ground is modest with respect to filling the nulls, and is negligible at low angles that are of interest to DXers. At low angles even poor ground reflects well.

The second is that the angle between adjacent lobes and nulls is smaller at low angles than at high angles. This is due to the more rapid change in the path length of reflections as the elevation angle decreases. This puts more of those deep nulls at low elevation angles where we don't want them. The higher the antenna the worse the problem.

This review should have been elementary to most hams. It is nevertheless worth recapping the basics before going further.

Terrain

The previous discussion assumes flat terrain, and that is not the case for many. Terrain plays an important role in determining the elevation angles of lobes and nulls, and can be the dominant factor where there are major slopes, hills and other large geographic features.

To solve the terrain problem the usual antenna modelling systems are not helpful. Modelling terrain requires real topographic data and a ray tracing tool like HFTA. I have never used HFTA for my station since the land here is quite flat, with gentle slopes and undulations for many kilometers in all directions. Urban hams also have little to gain from HFTA since although the land may be flat all those buildings and metal infrastructure are near impossible to model.

For those with topography that is not flat and open it is advisable to use HFTA to investigate candidate antenna heights and to calculate the elevation angles of the lobes and nulls. In some cases a low tower will be sufficient to achieve both a low elevation angle and few nulls. For others no tower is high enough. 

One recent correspondent who is working on a 40 meter wire yagi put up the first inverted vee element. It is outperforming his vertical on his most important DX path. The reason is that the land slopes downward in that direction. His experience is typical. It is possible to get an idea of how an antenna in his location will perform in NEC2 by tilting the antenna upward by the same angle as the downward slope (or vice versa for an upward slope) and subtracting that angle from the elevation plot.

Beyond this rudimentary advice I have little to say to those with complex terrain. Use HFTA to find what will work best in your unique circumstance.

Mitigation

You cannot fill an elevation pattern null by aiming above the horizon on any HF band. The ground reflection remains and dominates the far field pattern. For high gain stacked arrays for 2 meters and above tilting can work very well. For these antennas the elevation beam width is narrow and the ground-directed radiation falls off rapidly when the antenna is tilted up. In essence, the ground disappears and the antenna performs as if in free space.

An elevation rotator is routinely used for satellite and EME communication. For low elevation angle terrestrial paths or for low elevation EME and satellite work the ground is as much a factor as it is at HF. At the lowest elevation angles the ground reflections will dominate and cause elevation pattern nulls. Too often those nulls are at inconvenient angles. The only good options are to switch to vertical or circular polarization or to wait a minute or two for the moon or satellite to move.

To fill elevation pattern nulls there are a few common strategies:

• More than one antenna: Having one horizontal and one vertical antenna is perhaps the easiest way to deal with nulls. Switch between antennas and see which is better. The comparison must be longer than a few seconds because of signal fading and Faraday rotation that continuously changes signal polarization.
  
• Stacking: Yagis at different heights are fed in phase, out of phase (BOP) or separately to select the one that works best.
  

Did you notice that the two bullets are related? A stack has more than one antenna, and it is important that they can be individually selected. This is typical for HF stacks since it is so useful for optimizing elevation angle to the path and for avoiding nulls. Yagis in a stack for 2 meters and up are rarely configured to allow selection of one or a subset of them since it isn't as beneficial as at HF.

By feeding the yagis out of phase the nulls and lobes largely reverse. The elevation plot compares BIP and BOP for my stack of 5-element 20 meter yagis. The reversal isn't perfect but it is close. Gain in the BOP lobes provided little if any advantage over selecting the lower yagi alone. BOP was enough additional work for my home brew stack switches that I decided it wasn't worth the bother. Many commercial stack switches have the BOP feature, at a price.

For maximum versatility of elevation angle it is necessary to have at least two antennas, either in a stack or at different heights. One antenna, be it high or low, is a competitive disadvantage for contests and for DXing. I am sure that readers struggling to raise just one antenna per band, or even just one multi-band antenna, are feeling less than sympathetic about my plight! Nevertheless this is what I must deal with to achieve my operating objectives.

By the end of this year I'll have stacks or multiple antennas for 40, 20, 15 and 10 meters. For 80 meters I may reinstall my trusty inverted vee to have a high angle antenna to complement the vertical yagi. It would be useful on some paths, especially when I'm not running low power or QRP. With a kilowatt I can almost always work the nearby stations with the less effective low elevation angle of the vertical array.

On 6 meters I have a problem with just the one yagi. It is up 4λ and has the elevation pattern shown in the plot at the top of this article. Sporadic E and aurora often have optimum paths well above the horizon. That said, the antenna works well for the longest DX paths with that very low main lobe. There are times when friends nearby with lower yagis do better, which is strong evidence that the elevation pattern nulls are putting me at a disadvantage some of the time.

I would like to have at least 2 yagis in a stack for 6 meters. Unfortunately that's not a project for this year. It isn't even obvious where I could put it; the towers are rapidly filling with HF yagis. I need the gain and I need to fill those nulls to aggressively increase my country count on the magic band. I'll come up with a plan next winter in the hope of building a stack in 2022.

Now let's talk about verticals. These include vertical dipoles, monopoles with a radial system and vertically fed loops. When ground mounted or close to ground they have no nulls between 0° and at least 45°. Like horizontal antennas, the main lobe will split and form a null at greater heights. Although there is no critically located null for most verticals there are other difficulties.

First, verticals for the high HF bands are short, and in almost all locations will have to radiate through buildings, utilities, foliage and other common obstructions. This impairs both efficiency and effectiveness. Even with a good radial system a hex beam mounted on the roof of a house will outperform a vertical monopole or dipole, though usually not a full wave loop in its favoured directions. Ground reflections are not as reliably strong as they are for horizontal polarization, and that can cost a few decibels.

The elevation plot at right compares a ¼λ vertical with 8 full-size radials over medium ground versus a hex beam up ½λ. This is an estimate of what to expect from a ¼λ vertical in many urban and suburban situations. I believe it is fair to compare the vertical to a small yagi with gain (directivity) since the installation difficulties are of similar order. Only half the forward lobe difference is due to the yagi's gain. The rest is due to near field and far field ground loss.

On the low bands it is rare for a horizontal antenna to be very high, and that is why verticals are popular for 40, 80 and 160 meters. Unfortunately the vertical is not a good way to fill nulls on those bands since the horizontal antennas are so low they typically have none. Go back and look at the first plot in this article for yagis up ½λ and 1λ. Verticals are used on the low bands since, as inefficient as they often are, they are superior for low elevation angles over horizontal antenna at practical heights.

No magic

If you were expecting me to propose one highly effective antenna with the magical property of having few or no elevation nulls, I am sorry to disappoint. There are no easy solutions. It is no surprise that big gun stations have lots of antennas on each band, since that is the only reliable way to deal with the vagaries of propagation.

The smart operator at a large station will periodically try different antennas and stack combinations. Propagation changes throughout the day and night, and what worked best an hour ago may not be what works best now.