Practical Antenna Theory - Part 5

by AD5XJ ARRL Technical Specialist

One of the most often discussed topics in Ham Radio (and the most misunderstood) is SWR. In this series of articles fwe will learn some basics, some theory, and dispel a few myths that are generally perpetuated in the Ham Radio community.

The discussion last time involved some pretty heavy theory. We covered the most common type of simple antenna – the vertical mobile antenna. We also mentioned the close variation of the mobile – the base station vertical.

It is important that there is a thorough understanding of the material presented so far. Granted, the material can be overwhelming on first read. However, with patience and perseverance, you may find this knowledge to be invaluable to you in even the most simple of tasks where antennae of all type are concerned.

Our discussion this time involves a look at the influence that the ground and nearby objects have on the antenna and SWR in particular. This effect is not always what it appears to be. By that I mean many a CB’er has been fooled into thinking that the 1.01:1 SWR of his fiberglass whip mounted on the bed of his pickup 2 inches from the cab, is the best of all worlds. WRONG! This unsuspecting character has fallen prey to his own ignorance of the principals we are discussing in this series.

From the information and the illustrations provided last time, it should be obvious at this point why the CB antenna might look like it is functioning well. One need only review the discrete model antenna illustration to understand how this is total deception. The body of the pickup truck is the ground system for the antenna element. If the cab of the truck body is in close proximity to the vertical element of the antenna, there will be an increase in the capacitive component of the feedpoint impedance. Since this capacitance is a very large value (compared to the element being perpendicular to ground) radiation of RF from the antenna my indeed flow directly to ground instead of into the ether as expected. The low SWR reading is simply an indication that the ground of the truck body has absorbed most of the RF, the added capacitive component has been inappropriately offset by added length (making a non-resonant length), and there was little RF to be radiated.

This is but one of hundreds of similar false impressions that can result from ignorance of the material we are studying.

It does however, validate an old ham radio adage that has its roots at the beginning of the hobby – “Antennas should be erected as high as practically possible and free from surrounding objects”. For all antenna types except ground mounted vertical antennas, this is still the best approach for maximum utility. As hams, we know from experience that we don’t always have 40 acres of prairie land to raise our 100-foot tower. But we can use our knowledge of the effects of ground to our advantage when choosing the best location for that new base station vertical or raising that 50-foot tower for a beam.

It also explains the reasons for some unlikely objects proving to be a fairly decent antenna. Most of us have heard of the enterprising ham that connected a wire to his downspout and loaded up the metal gutter system on his house to work DX. It also illustrates why properly mounted and loaded mobile antennae rival fixed station performance from time to time. While these are not the best of applications, it indicates that no ham need do without a working antenna.

The characteristics we have discussed so far have been relative to the far-field radiation. The far-field term is used here to indicate the propagation of the RF wave into the atmosphere and reflected or refracted by the ionosphere then received some distance (more than 50 wavelengths) from the transmitting antenna. The angle of the transmitted wave and the conditions of the ionosphere influence how far the signal will travel and how strong it will be when arriving at a receiver. We have moved from the realm of antenna theory to the area of propagation effects. This far-field analysis is often referred to as sky-wave to designate the difference the effects of the ionosphere have on the transmitted signal versus the effects of objects near the antenna (e.g. the earth ground, buildings, car body, etc.).

Why do we care? The radiation pattern our antenna exhibits will almost directly influence how well we are heard in a targeted area. This is true for vertical and horizontal antennae. The physical methods for determining and adjusting for a given radiation pattern are different, but the factors that are influencing each are the same.

The important points to remember are that there are near-field factors and far-field factors that combine to influence the signal that is ultimately heard at a receiver. We are unable to control the atmospheric conditions that occur in far-field effects. We can just be aware of them and exploit them whenever possible. We can, however, control almost all near-field factors.

The first illustration depicts near-field characteristics of two types. The name of each is self-explanatory. Direct near-field space radiation describes RF that travels without interference to the receiving antenna through the air. Surface wave near-field radiation travels along the surface of the ground, sometimes even an inch or so beneath the surface (depending on frequency, soil type, distance, etc.).

Again, why do we care? Surface waves are stronger as we decrease frequency. At HF, their strength is very low except for 75 and 80 meters and below. In this low and medium frequency range they play a very important role in propagation. Surface waves are stronger in vertical antennae, whereas horizontal antennae exhibit a weaker but still present surface near-field wave, probably due to the reduced proximity to ground.

The specs for any antenna made commercially will publish the far-field radiation pattern for average terrain and average soil. The average is an arbitrary value taken from the difference between salt water and rocky, barely conductive soil of the desert or concrete city streets.

The following comparative graphs illustrate the far-field effect of different soil types (very good vs. poor). Notice the take-off angle of each soil type for the same antenna (we will discuss take- off angle later – for now assume it to mean the angle at which the largest lobe of the radiation pattern occurs.). From the provided illustration below, you may notice there is a considerable difference. Depending on your antenna type, the purpose for which your analysis is performed, and your geographical location, a lower takeoff angle may perform better than a higher one.

For instance, in the far Northern Hemisphere, it is probably not as important to have a low radiation angle for East – West long distance propagation as it would be for the latitudes nearer the Equator. By the same logic, it is way more important on 80 and 160 meters to have a good ground system versus 10 meters.

Another consideration of near-field radiation also has to do with propagation. At frequencies of 75 meters and below, near-field ground-wave (space wave or surface wave) signals are usually much more intense than far-field sky-wave signals. Often, the far-field sky-wave signal will be strong enough to reach a receiver along with, but at a different time than, a near-field signal. The result is a signal that flutters, echoes, and fades rapidly (we often call it QSB and multi-path propagation). Our human ears can decode these fluctuations with comparative ease, whereas digital signals are difficult for computers to decode (the computer considers the flutter and echo to be “mistakes” (computer geeks call it data errors) and will not decode the information being transmitted).

Near-field effects also appear when considering antenna feedpoint impedance. Proximity to ground will affect antenna impedance directly. That is to say, the closer to ground, the lower the impedance of an antenna compared to free space. Just as the length of an antenna element varies with wavelength, the impedance effect of ground proximity varies with wavelength above ground.

It should be obvious from the graphs provided here, that the effect is more noticeable for heights below ½ wavelength. It is this proximity effect that makes ¼ wave vertical antennas exhibit much lower impedance than ½ wave horizontal dipoles more than ½ wave above ground.

 Whether we are talking about a completely flat horizontal mounted dipole, or an inverted “V” dipole, the closer any part of the antenna is to ground, the lower the characteristic impedance will be. By the same token, the more RF absorption will take place by the ground or surrounding objects as indicated by the radiation resistance curve on the supplied chart. Although the elevation is above ground, objects like trees, buildings, and mountains can be considered to be the same as ground.

Earlier we mentioned we have the ability to control some of the near-field conditions that affect our radiation pattern and strength. These conditions affect how well the RF from our vertical antenna is radiated (or not). This phenomenon is known as ground return loss. Signal strength (power) is often lost to the ground where poor ground quality is present. Imagine a huge resistor (high power) connected across the antenna terminals. That is what poor ground conductivity amounts to. On the ocean, this is hardly a problem, as this is as near to a perfect ground as we can practically come. But few hams live or work on the ocean. Most of us have to use the ground given to us where our ham shack is located. The conductivity of ground at your location will vary considerably from any place in town or across the state.

Ground conductivity can be improved by artificial means. An example on point would be the radial ground system of a vertical antenna. This series of wires or wire lattice is constructed to purposely improve the near-field surface conductivity and intensity. An increase in near-field intensity translates to a noticeable change in the intensity and takeoff angle of the radiated pattern. As a general rule of thumb, more and shorter radials are better than fewer and longer radials for a given frequency. Since the ground radials are a part of the antenna system, size DOES matter. Radials of no less than 4 in number but usually 12 to 16 of 1/8 to ¼ wavelength are preferred. When the vertical is elevated (as in a roof mounting), connecting the radials in a downward sloping angle will further lower the takeoff angle and help control the impedance of the antenna.

The possible exception to this phenomenon is the loop antenna and loop arrays. They are by no means immune to ground proximity effect, but they are much less affected by ground where feedpoint impedance is concerned. The far-field radiation pattern of loops and loop arrays are affected to the same degree as other above ground antenna types. It is, however, not necessary to construct a radial ground system for this or any other horizontal dipole or loop antenna including beams derived from these types.

Near-field radiation has considerations that go beyond communication to health and safety. The intensity of both the electric and magnetic fields in the near-field are several orders of magnitude higher than, say, 100 wavelengths away. In fact, at high intensity, RF waves can penetrate objects such as buildings, cables, and even human skin. At low power levels this is not a concern, but we often operate at very high RF power levels. High power levels mean more penetration of objects. High intensity RF radiation of the human skin can be lethal or at a minimum, damaging to human tissue for some frequencies. It is precisely for this reason the FCC has issued near-field operating power safety limits for each frequency band in which hams are licensed.

The current operating limits and safety guidelines are published in ARRL publications and on the web site:

http://www.arrl.org/news/rfsafety

and the FCC bulletin:

http://www.fcc.gov/oet/info/documents/bulletins/#65

Next time – grounding to control RF vs. grounding to minimize lighting damage and electrical hazard.