- Note: Some material here is by Author: R.J.Edwards G4FGQ © 6th January 2003 Getting experience on-the-air and developing basic operating skill is the first order of business for anyone, so use whatever antenna that you have on hand to begin. Once you're ready to contemplate station improvements, before scrapping your old rig and mortgaging the house to buy a new DX4000Pro IV transceiver so that you can "work more DX", be advised that the most effective investment that you can make is in the antenna that you use. Indeed, aside from P.E.P.S.I., the next three most important things for DXing are (i) the antenna, (ii) the antenna, and (iii) the antenna! Why is this? Well, on receive, if the antenna cannot produce enough signal strength, then the most sophisticated and expensive receiver can do little more than light up and look pretty; while on transmit, a KW amplifier will do little good if all of the radiated power is going straight up to warm the clouds or straight down to warm the worms. Once you reach the point that you're ready to improve your station, then it's time to learn more - and as much as you can - about the fascinating field of antenna theory, design and, most importantly, practice. As in almost all of the other subjects having to do with amateur radio, antennas can be as complex as one has time to invest in leaning about them, or as simple as taking someone else's advice and hoping that they're right. What follows is a highly over-simplified attempt to distill and summarize some of the common questions and points of interest to DXers. Introduction to Antennas & feedlinesNow that you've got a feel for the basic principles of RF propagation, let's review a few things about antennas - the part of the station that actually puts the signal into the air! First, there are two parts to any antenna system: the antenna elements that radiate the signal, and the antenna feedline that carries the signal from the transmitter to the radiator. Most of the time (as in the following), the main focus of interest is the radiator portion, so that is what we generally mean when we speak of "antennas". The reason is that feedlines are relatively simple in comparison to the radiators. However, would be a mistake to neglect feedlines completely because a poor feedline can easily negate the performance of the best of the radiators. While most of the discussion below will be on the radiators, some key concerns about feedlines will be stressed as well. First, the bench-mark (or "standard") antenna is considered to be the half-wave dipole antenna, shown in the figure below as it is most commonly used today. The dipole is the reference antenna to which all others are compared. It is also serves as a nice example of the relationship between the radiator and the feedline. What defines a "dipole" is the balanced symmetry of the center-fed half-wavelength configuration: both sides of the feed-point are identical. In the early days before WWII (and therefore before coax), dipole feedlines were typically two parallel wires separated every few feet by insulated spacers. Because of its appearance, it was called "ladder-line", or later, "twinlead". Now picture this in your mind: the antenna then consisted of two wires leaving the transmitter and running in parallel out to the tie-point of center of the dipole, then "splitting up" at right-angles to each other to finally extend horizontally outward for one-quarter wavelength. At this point, you may (should!) well ask: "When the signal leaves the transmitter and enters the feedline, how does it know where antenna begins and the feedline ends?" An excellent question! The answer is that since the parallel wires of the feedline are symmetrical, the currents flowing in each are, at all times, equal in magnitude but opposite in direction. Any radiation from one side of the balanced feedline would then cancel that from the other. Not only is this "balanced feedline" answer simple to understand, it also serves to underscore an important issue about coaxial feedlines used today - they are not symmetrical, and are therefore "unbalanced". What this means is that unbalanced current flows on the exterior surface of the coaxial cable shield may produce radiation from the feedline. This becomes a concern when one has erected a directional antenna that is supposed to transmit and receive signals in a specific direction - radiation from the feedline can significantly disrupt the radiation pattern. There are two ways to address this: some means is needed to provide a balanced-to-unbalanced matching of signal flow, or some form of RF choke is needed to supress current flow on the exterior of the coax shield. One solution is to use a balanced-to-unbalanced transformer device called a "balun" that can be constructed or purchased for the purpose. Not only will a balun suppress unwanted current flow, but they can also be designed to provide impedance matching when needed. However, if only current suppression is required, then a simple but effective RF current choke (sometimes called a "choke balun") can be fashioned using the coax feedline itself by winding part of it into a tight coil near the antenna feedpoint. The length of coax in the coil should be approximately 10-15% of the resonant frequency wavelength (e.g., about 12ft for 20m) and wound into an 6-inch diameter coil of 6-8 turns. For more exact construction details, see the ARRL Handbook section on "Transmission Lines". For simplicity, baluns are not shown in the folloing antenna illustrations, but should always be used whenever unbalanaced feedline (coax) is used with a balanced antenna. How do we compare antennas?Basically, there are four major attributes of an antenna that are of concern for DXing:
These attributes are not necessarily independent of one-another, and trade-offs among them are usually required. What follows is a highly simplified categorization of the most common antenna types and listing of the most familiar configurations in order of increasing complexity and cost. - Radiation Angle Almost any conductor connected to the output of a transmitter will radiate some RF energy, so what we must be concerned with is the efficiency of the radiator for its intended purpose. For example, the need for reliable close-range communication would warrant the use of an antenna that concentrated most of the radiated energy locally, that is, an antenna that produced a spherically symmetrical radiation pattern (figure below) so that most of the RF energy went straight up, some of which will be reflected back down in the immediate region, as in the figure.
Such an antenna, usually called a "cloud warmer", is very effective for local HF communications within couple of hundred miles or so. A cloud warmer is an antenna with most of its radiation going up at very high angles, rather than out towards the horizon. This brings up an important fact about antennas: the radiated waves interact with the ground surface (and possibly other nearby structures). For example, the figure above is a the radiation pattern of an 80m dipole at the height of 10m (33.3 ft) above ground. Many hams expect that all dipoles radiate energy in a doughnut pattern, broadside to the wire, which, if sliced in a plane parallel to the ground, would look like the "dumbbell" or "figure-8" azimuthal shape seen below for the 20m dipole. While this may be true for an antenna far enough removed from the ground, that is NOT the case for low antennas, due to the significant interaction (absorption and reflection) of the waves with the ground surface. A rule-of-thumb is that unless an antenna is at least a wavelength above the surface, one may expect significant ground effects. Keep this in mind! OK, fine, but cloud warmers are NOT what we want for DXing! We want an antenna that will radiate a significant portion of its energy at low angles towards the horizon so that the signal will reach out as far as possible, because we learned in the section on Propagation that radio waves can "ricochet" from the ionosphere and back to Earth. The figure below is a refresher of that seen in "propagation". In (A) the skip phenomena is shown, and (B) illustrates the rule that the smaller the angle of radiation, the greater will be the skip distance for each "hop" that the signal will make.
Now, you ask, "What exactly is the 'radiation angle'? If the radiation leaves the antenna as an ever-increasing spherical blob, how can we speak of "an angle of radiation" ... isn't radiation emitted in all directions? The answers is "YES", but we are really interested in getting some significant portion in a favored direction (or angle), knowing that the rest will go elsewhere. In the figure below, we see a 2-D elevation view of a slice through the radiation pattern for a 40m vertical antenna, where the antenna is seen as the vertical line among the numbers. The actual 3-D figure would be akin to a doughnut on its side. The dotted arrows indicate the angle at which one would measure the most energy flowing through a unit area of the pattern. Note that a significant portion of the energy is leaving at a relatively low "radiation angle", just as we would want to happen.
The angle of RF radiation from an antenna is dependent upon a number of factors: the antenna design; the height above ground; the ground conductivity in the region around the antenna; and other objects (especially conductors) in the vicinity of the antenna. Since it is often difficult to do anything about the last without re-locating, DXers usually concentrate their efforts on the first two - antenna design and height above ground - while either accepting whatever the local soil conductivity may be or perhaps making some attempts to improve it. What to do about height above ground is self-evident, but a general rule-of-thumb is that for horizontal antennas to have reasonably low radiation angles, the minimum height above ground should be one-quarter wave-length at the operating frequency. As for ground conductivity, The usual way to try to improve it is to put down as much conductive material as possible below the antenna. Typically, this is in the form of wire mesh or multiple wires radiating from the base of supports, either laid on the ground, or just below the surface. This is usually called a "radial" system (more below). - Forward Gain & Front-to-Back RatioIn evaluating performance, one must have some measurable parameters to use in making comparisons among different antennas. One of these is the ability to focus the antenna in a desired direction in order to "amplify" the signals. When strength of a signal from a directional antenna is compared to that of an reference antenna, the ratio is called the gain of the directional antenna over the reference antenna. The reference standard for defining antenna gain is the half-wave dipole operating at the same frequency and under identical environmental conditions as the antenna under evaluation. Directional antennas are constructed by combining two or more radiators ("antenna elements") in order to make use of the property that waves have for interfering with themselves. Proper layout of these elements can cancel signal radiation in some directions destructive interference while enhancing radiation constructive interference in the desired direction. Given a directional antenna, the ratio of the signal strength transmitted from the intended maximal radiation direction ("front" of the antenna) to that transmitted from the intended minimal radiation direction ("back" of the antenna) is called the front-to-back ratio. The gain of a directional antenna is dependently coupled to the front-to-back ratio - as one goes up, the other may go down - so that proper design is important in order to optimize the figure of interest. Although one frequently hears only comments about antenna gain, experienced DXers know that the front-to-back ratio is also of great interest, since one of the benefits of using a directional antenna is to be able to attenuate strong signals from areas in the opposite direction of the incoming signal, making it easier to select weak signals in a pileup of strong signals. Remember that since an antenna's effective gain (what you're interested in when you're working DX!) will also be affected by the angle of radiation, we can expect that height-above-ground will be a contributing factor. Antenna design, modification, and optimization would be a tedious process
if one had to continually test new models or changes to see if they performed
as expected. Since it is difficult (more to the point: IMPOSSIBLE!) to control
the environment around antennas when making comparisons, a theoretical construct
called a free space environment is used for comparing antennas during
the design process, in which the antenna is imagined to be in a region of
space far removed from Earth without the presence of any interfering structures
(including ground). This is very useful for purposes of theoretical design,
but it is of less use (and often confusing) to everyday users such as hams.
In fact, it can be very misleading when applied to real-life applications
(thereby making it a good marketing tool. For an example of how the ground affects antennas (and our perspective of real-world vs. theoretical), let's use a theoretical modeling program to look at the simple dipole antenna. In the figure below are some cross-sectional views of a dipole azimuthal radiation pattern at an elevation of 10 degrees from the horizontal plane of the antenna in three environments: Free Space, 10m (33ft) above Earth ground; and 3m (10ft) above Earth ground. The dipole orientation in the figure is from top-to-bottom and in the plane of the paper.
You may recall that the popular conception of a dipole is usually thought to be of a "dumbbell" shape, with total attenuation of radiation from the ends of the antenna and all energy being radiated broadside to the antenna. In fact, this is the theoretical "free space" pattern that is seen in teh figure as the inner=most pattern labeled "A". However, when erected above a moderately reflecting ground, this changes as a function of height as seen in patterns (B) and (C). Note that at a height of approximately one-sixth wavelength (3 meters), the radiation pattern is almost omni-directional and certainly no longer the classic "dumbell" shape. So it is with almost all antennas - the height above ground is a key factor in their performance characteristics. As a rule-of-thumb, antennas that are mounted at a height of one-quarter wavelength or more at the intended operating frequency will begin to approach the theoretical predictions of Free-space calculations, while those at a lessor height cannot be expected to do so. For example, scaling the dimensions from the above figure for an 80m dipole (multiply all factors by 4: 4 x 20m = 80m and 4 x 3m =12m, or ~40ft), one can easily see why there is little directivity to be expected for dipole antennas on the low bands. Moreover, depending upon variations in ground conductivity and the presence of surrounding structures, performance of otherwise identical antennas can vary widely with location, a fact that contributes to frequent consternation and debate among DXers! So it is that we should all remember that when you see data or hear comments about "high gain" or "great front-to-back ratio", you should always keep in mind that these numbers are really only approximations at best! - Noise Susceptibility Radio waves have electric and magnetic field components that "leap-frog" through space once the wave is radiated from its source. By convention, the orientation or " polarization" of the wave is described in terms of the orientation of the electric field component (probably because radio waves originate from the presence of a time-varying electric field that accelerates electrons into motion). For this reason, horizontal antennas, such as a dipole, will produce "horizontally polarized" radio waves, while vertical antennas will produce "vertically polarized" waves. It follows that if the antenna orientation is the same as that of an incoming wave polarization, then it will be somewhat more sensitive to signal induction by the wave. Wave polarization may be altered by several factors, the most common being reflection. Locally generated natural and man-made noise happens to be predominantly vertically polarized, whereas radio waves from distant stations, having undergone atmosphere-ground reflections, can have a variety of polarization angles. This means that a vertical antenna will be somewhat more sensitive to receiving locally generated noise. For this reason, one will learn that vertically oriented antennas, while they can be very effective transmit antennas, have a reputation for being "noisy" receiving antennas. Antenna Types and DesignsThere are as many different antenna designs as there are ways to draw lines on paper, but here are a very few of the more popular ones. These are presented in approximate order of increasing effectiveness for DXing, but this is based upon many assumptions, such as the fact that there are usually limits on the money, real estate, and height (in that order) that one has available! - Random-length or "Long-wire" antennas: These are by far the simplest of all (figure),
yet they can be very effective. They consist of a length of wire, mounted as long and as high as possible, that runs directly from the radio room, out to the first support, then extending out to the extent of its length. Although the best configuration is a straight run of the horizontal portion, it is not essential. The wire can be laced through trees and supports, up, down, and around, as needed. Use of small-gauge wire also makes this a good "stealth" antenna when the situation demands it. Since they are also of random impedance, an impedance matching unit ATU is required to match the feedpoint impedance of the antenna to the transceiver output impedance. One can use any conduction material of any length, orientation, or layout that is conveniently available (wire, aluminum foil strip taped to the wall or window, window frames, copper gutters insulated from ground, or anything that your imagination can fathom) being careful that the conductor is reasonably protected from curious and wandering hands!!! Random wire antennas are typically omnidirectional, except that when they are over a wavelength in length, they begin to exhibit some directional lobes. For the higher bands, these lengths are more than feasible. If you have access to enough real estate to put up a random wire of 150 feet or more in length, you will not only have an all-band antenna, but also an excellent chance of working some good DX on the higher frequency bands (14 - 28 MHz), as it may have some very good low-angle lobes, as can be seen in the figure below.
Also, if the random wire antenna has an appreciable portion that is of vertical wire run (e.g., one-eighth wavelength) as well as the horizontal portion, it may function well as a vertical radiator, providing additional low angle components at lower frequencies (see the inverted-L next page). Reference Websites
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is because ground reflections, ground absorption, and absorption with re-radiation
by nearby structures will drastically affect radiation patterns. )
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