ANTENNAS FROM THE GROUND UP
For a series with the title, Antennas from the Ground Up, we have said very little about the ground itself. That is close to heresy, and something that we shall begin to correct with this general note about grounds and grounding.
We are very careless with the ground. We tend to treat it as a single homogenous entity. In simplest terms, that means that we think about the ground in the same way, no matter in what radio context it appears. That habit can lead to trouble, since a satisfactory ground in one context may be no ground at all in another. Such thinking often leads to confusion-- including the use of confusing terms--replete with terms like "counterpoise," "ground plane," and others.
So let's go back to the beginning and begin sorting out some grounds and ground systems. First, a fundamental distinction. The very term "ground" has a built in confusion, since it is commonly used in the context where "common" or "common bus" should be used.
For example, we refer to a "chassis ground" (and this is accepted practice). However, the chassis ground is only the circuit common against which measurements are made. It also offers us a connection point for placing the chassis in common with the earth (often called an "earth ground"). As the figure shows, we use separate schematic symbols in the U.S. to differentiate chassis ground from earth ground.
Without some connection external to the chassis, the entire circuit might be--relative to the earth--floating or ungrounded.
Here are some rough and ready distinctions among grounds of relevance to HF radio work.
G1. The DC and static discharge ground: This is the ground of long ground rods by which we ensure that DC, small static charge build-ups, and power line AC are shunted to ground. Our station ground strap gets into the act here, because we make a common ground for all cases, keeping them at the same potential, hopefully ground potential if our distance from the rod into the earth is not too long.
For many purposes, the DC and static discharge ground rod is also an AC power safety ground. Hence, we often use the power company ground rod as a reference, connecting the station ground bus to it either directly or indirectly (via the "third" wire of the house wiring or a copper cold water pipe). For maximum safety relative to house power, a ground rod as near to the station equipment as feasible is always in order.
G2. Circuitry common bus (ground): There is a very important difference between a circuitry common and an earth ground. Sometimes we have learned this the hard way with shocks and tingles. As we initially noted, a circuitry common--or chassis ground--does not ensure an earth ground. In some kinds of cases, we may in fact need to keep some circuitry commons isolated from others. For example, many low-voltage power supplies intentionally leave the DC side of the circuit (relative to the power transformer) floating so that the supply, with suitable external contacts, can be used to provide either positive or negative voltages.
G3. Lightning ground: Lightning strokes and other sudden high voltage and high current impulses require more attention than G1-type grounds. The details of a satisfactory, equipment-protecting grounding system for lightning is more complex than that required for simply preventing the wind from building a static charge on our dipoles.
Truly safe operation during an electrical storm--or even safe conditions for inactive equipment--is itself a complex subject requiring extensive study and application of principles, techniques, and devices to each given station situation. In general, we can note two key ideas. First is the use of heavy, very low resistance interconnecting wiring to ensure that everything within the protective system remains at the same common potential as everything else, even in the face of sudden surges. Second is the use of devices to cut off or bypass spikes and surges, thus protecting our equipment. Essential to this process is bypassing the unwanted energy to the earth as efficiently as possible. This requires many regularly spaced long ground rods and heavy interconnecting cable or strap. The complex nature of lightning itself makes the design of an adequate lightning protection system a very specialized field of study.
G4. RF ground: Effective RF ground also requires attention to many details. Deep rods, while useful, may be less effective as RF grounds than we previously thought, and the U.S. Army developed a system of perimeter straps and a sequence of shorter rods to effect a satisfactory overall station RF ground, as sketched (with many missing elements) above. RF paths to and from ground via transmission lines, circuitry-to-case connections, common mode paths, and numerous other sources are receiving increased attention both by those who build equipment and by those who assemble operating stations.
There are many elements of a good RF ground system that are compatible with the elements of a good lightning-protection ground system. The use of long rods and a perimeter bus might serve both purposes. However, we should not assume without adequate planning that one system is doing the work of two.
We might extend this list--not to mention subdivide it. But let's turn to a couple of new categories created out of one old one. Both have to do with antennas.
We tend to think of the ground relative to an antenna as a single ground. Hence, we tend to lump together the ground from which signals reflect to contribute to antenna far field patterns and the ground directly under a monopole antenna. The "only" difference is their relative distance from the antenna itself. However, let's see where separating the two ideas leads.
G5. Far-field reflective ground: The ground quality is usually specified in terms of conductivity (given in Siemens or millisiemens per meter) and a dielectric constant (permittivity). ON4UN did some modeling with some vertical radiators that suggests the ground kicks in as a reflective medium somewhere around 2 and a half wavelengths from the antenna--possibly more for highly elevated antennas. Although the ground immediately under the antenna has some effect on antenna effectiveness relative to far field patterns, the effect is small (unless the ground is needed to complete the antenna). Dipoles, for example, exhibit only small gain changes as the quality of soil is ranged from very poor through very good.
Do not confuse ground quality with terrain considerations. The quality of ground at varying distances from an antenna is only one factor among many with which terrain evaluation is concerned. Slope and interfering objects are samples of other factors that go into terrain evaluation for determining the ultimate elevation pattern for a given antenna and site.
Self-contained vertically polarized antennas, running from the vertical dipole to complex arrays like the bobtail curtain, exhibit far field radiation properties with respect to ground similar to those of dipoles and other horizontally polarized antennas at HF. The surface wave of an HF antenna is small, relative to the sky wave. Losses may be slightly higher than with a high horizontal antenna, but generally are not significant.
G6. Antenna-completion ground: Monopole antennas are generally analyzed as having their missing pole (relative to a dipole) within the ground. This is often pictorially presented as an "image" antenna sticking straight into the earth. While this portrayal allows the solution to certain basic equations, it is actually a very poor picture of what is going on.
It is the surface volume of the ground that provides the completion of the antenna. Signals penetrate the ground to depths that vary directly with wavelength and inversely with frequency. However, even with a monopole, the penetration does not act like a spear into the ground.
A more correct picture is a surface area (with some depth) around the monopole. The conventional radius of this surface is about 1/4 wl. Since even the best soil is lossy compared to conductors, we lay screens and radials under the soil to improve its conductivity. Somewhere between 60 and 120 radials approaches the conductor saturation point of most soils. This is the most traditional ground plane.
Of course, we can also elevate the ground plane. A number of investigators (nicely referenced by ON4UN) have discovered that even a few feet of elevation can improve the performance of a monopole over that obtainable with buried radials.
Another interesting phenomenon is that rooftop monopole vertical antennas do not seem to benefit from littering the roof with 60 to 120 radials. 4 to 8 seems to be enough to achieve all the performance of which the antenna design is capable. Experimenters have also discovered that the best length for radials is slightly less than 1/4 wl. Of course, these were elevated radials. Moreover, if we model a monopole with radials in free space, it does not care what its orientation is.
What Kind of Ground are We Talking About? This is a key question to ask whenever you encounter talk of ground. Very often, terms are confused, and the speaker may misinterpret a visible structure for a function. A perimeter RF ground system might look from the surface like a lightning protection ground but be wholly inadequate to the task.
Let's take a closer look at another kind of example. In the past two episodes, we examined 1/2 WL and 3/8 WL inverted-L antennas. The base-fed design, with the associated L-network tuner, is most interesting to us here.
No Ground Plane 10-Radial Ground Plane Freq. T-O Ang Gain Feed Z T-O Ang Gain Feed Z MHz degrees dBi R+/-jX degrees dBi R+/-jX 3.7 31 -0.15 125+480 31 0.52 100+480 7.15 40 5.99 70-220 40 3.33 150-260 10.1 27 5.27 1440+625 27 5.19 1400+660 14.15 24 4.28 235+350 25 3.87 325+420 18.1 16 6.38 145-35 16 5.80 160-40 21.2 44 6.16 785-480 43 5.96 880-570 24.9 41 7.17 450+405 41 7.10 480+395 28.5 10 6.88 165-5 10 6.02 205-5 3/8 WL Base-Fed Inverted-L With and Without a Ground Plane: Modeled Values
The table lists modeled values for the antenna system over all of the HF ham bands. In one case, the antenna uses no ground plane, but the ground beneath it is average (conductivity = 0.005 S/n; dielectric constant = 13). In the other case, a modest ground plane of 10 radials has been added, with the same soil quality. The ground plane chosen is for many hams larger than one that might be installed, since the average is 4-6 radials. Still, it is far smaller than many vertical antenna experts recommend (60-120 radials).
Except for the lowest band, the ground-plane system, consisting of 1/4 WL radials, actual decreases performance on almost all of the bands. At the same time, for antennas that are equally self-complete, one often hears claims to the effect that the antenna operated poorly until some radials were added. Such claims are often accompanied by further reports that performance did not improve with the addition of further radials. Moreover, the addition of the radial system did not affect antenna resonance. Such claims are not consistent with either good theory or good practice associated with ground plane systems used with 1/4 WL monopole systems.
What's going on here? The answer is quite straightforward. What the L-network for this inverted-L requires--as do many other types of self-complete antennas mounted near the ground and base-fed--is a good RF ground relative to the source. A single ground rod may or may not provide the necessary RF ground. However, a few radials might do the job in some instances. In such cases, their length and position is likely to be highly non-critical.
Our bad habits lead us to refer to these radials as a ground plane, when in fact they provide no antenna-completion function at all. Instead, they provide the rudimentary conditions of an adequate RF ground to ensure antenna operation, especially when receiving.
The table shows representative figures for the subject antenna with just a good ground and with a radial system. The good RF ground is sufficient for good antenna performance. Without the good RF ground system, voltages across the antenna side of the L-network tuner may float and not vary with reference to a standard value, namely, earth ground. Voltage swings, upon which reception is based at the other end of the coaxial cable, will likewise not be as large and signals will be weak.
There are many other important reasons for having a good RF ground systems for an amateur station--too many to list here. As well, it is important that all of the other types of grounds be studied and improved to the highest degree possible. The listing has only scratched the surface--and not very deeply--of this fundamental subject.
However, in the process of evaluating our ground systems, it is equally important that we distinguish among the grounds that we are investigating. The history of radio (including amateur radio) is full of reports of techniques that seemed to fulfill a purpose, with the later discovery that how it did the job was not fully understood at the time. The result has been the adoption of terms that later proved inapt but continued in common usage.
The lesson then is not to get lost in short-cut talk and inexact terms. Think through every aspect of your ground systems and be certain each is the best that you can implement.
Updated 11-15-2001. © 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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