by Edward Farmer
On Dec. 12, 1901, Guglielmo Marconi transmitted the first radio signal across the Atlantic � from Poldu in Cornwall, England, to St. John�s, Newfoundland. This feat of scientific achievement succeeded by blind luck � with no knowledge of radio-propagation science, Marconi made a fortunate choice of frequency, and he happened to pick antenna locations and an overwater path that made the most of his kite-lifted vertical antenna.
Radio science has moved a long way since then. The excitement of discovery may be diminished, but we now can plan radio communications with a confidence beyond Marconi�s dreams.
There are always two essential factors in high-frequency radio communications: frequency selection and antenna design. Essentially, the frequency used must support propagation over the required distance, and the antenna must radiate enough power at the angle required to make the path.
Nothing absolves the communications planner of addressing these two issues. Automatic link establishment merely automates trying each frequency in a user-selected suite of frequencies, using whatever antenna system is connected. ALE can make the best of a bad situation, but it can also waste a lot of time trying frequencies that can�t possibly work. Optimizing link time depends on the user providing a suite of viable frequencies.
Near-vertical-incidence skywave communication has been thoroughly discussed in Army Communicator and other technical literature. NVIS is a �one-hop� system (earth-ionosphere-earth). Its effective use requires antennas that predominantly radiate at very high (nearly 90 degrees) angles, along with frequencies low enough to refract from the ionosphere when the angle of their contact with it is nearly 90 degrees. (See my article titled �NVIS propagation at low solar flux indices� in Army Communicator�s Spring 1994 edition for a more complete discussion of critical angle and frequency selection.) It�s hard to improve on a dipole-like antenna mounted about 0.15 to 0.3 wavelengths above the ground that uses frequencies between two and 12 megahertz � depending on the time of day, month and position in the 11-year solar cycle.
Long-range communication involves a much different set of requirements. Making a circuit requires several �hops� (reflections involving earth-ionosphere-earth). Having as few hops as possible enhances circuit quality. Further, it�s often attractive to select a path in which the �earth� portion of the hop takes place over seawater, even if it isn�t the �shortest distance between two points.� Thus we�re concerned with both the �short path� and the �long path� between circuit ends.
In either case, the geometry of these paths requires radiation at lower angles, the exact angle depending largely on the path length. Since radiation at these low angles encounters the ionosphere at shallow angles, much higher frequencies will refract than is the case with NVIS paths. This effect is similar to firing a bullet at a steel plate. If the bullet encounters the plate squarely, it may penetrate, while if the bullet encounters the plate at a grazing angle, most likely it will deflect.
Since we have no control over the ionosphere, there�s no choice but to select frequencies based on what it will do for us in any given situation. Long-range communications planning, however, requires circuits that are available a high percentage of the time. This means they need to work during all hours of the day, all months of the year and over the entire range of sunspot numbers. These frequencies are found by running a propagation-prediction program for every possible case.
While there are quite a few propagation programs available, my approach uses ICEPAC. ICEPAC was developed by Voice of America and is available, without cost, from the National Bureau of Standards. Due to development work done by Dr. Greg Hand and his associates, ICEPAC is among the best point-to-point HF-propagation-prediction software.
To consider each hour of the day, each month of the year and a representative number of sunspot numbers (for example, 0 to 200 at increments of 10), one would have to run ICEPAC thousands of times, then compile and analyze the results. Depending on your computing horsepower and bookkeeping skills, this could take weeks. However, I developed software to automate the process. The following figure shows the results of an analysis of the path between Sacramento, Calif., and Fort Meade, Md. I use this path as an example because it�s a typical one with typical results. For best confidence, each situation should be specifically analyzed.
|MUF for a path between Sacramento, Calif., and Fort Meade, Md., under all normally occurring conditions. The graph shows the percentage of cases tested at which communication takes place at each frequency in the HF range. If the path were shorter (for example, to Colorado), the frequency suite would shift to the left.|
Propagation programs report the maximum useable frequency because there�s a scientific basis for determining it. The �best frequency� is called the frequency of optimum traffic. The FOT is usually a bit below the MUF; some literature suggests 80 percent of the MUF is a good estimate. In any case, the figure above provides a good indication of the range of frequencies that will be useful in making this circuit.
In this case, frequencies below eight mhz are very unlikely to be useable � consequently, asking an ALE system to constantly scan and try them is a waste of time that could be spent actually communicating. Sometimes there�s no frequency that can get the job done. Propagation anomalies such as solar flares can eliminate radio propagation for periods of time. Nothing in our present science can overcome these problems.
The �best� frequency under any given set of conditions must be determined by evaluating each specific case. System design, however, needs to define the limits that bound the problem.
Once we know the frequencies, the next task is to determine what take-off angles from our antennas are required to make this circuit. The same computer program that produced the above figure was also used to produce the following figure.
|Take-off angles required to complete the circuit over the entire range of possible conditions. Note that for this path, the most important angles are in the vicinity of 4 degrees and 12 degrees. An antenna with a vertical radiation pattern that covers the range of 1 to 28 degrees would be ideal. A shorter path will generally involve greater angles.|
An antenna that concentrates its radiation at low angles (1 to 28 degrees, in this case) and in the specific direction required to reach the circuit�s other end would be ideal for this application. This is clearly not your NVIS dipole. Since these requirements are as old as radio, there�s quite of bit of science as well as practical experience available.
Probably the �gold standard� for long-range HF is the terminated rhombic antenna. It focuses its radiation in a narrow, low-angle beam. Its geometry can be adjusted to control its take-off angle. Unfortunately, it�s very large � several wavelengths long. The size doesn�t permit easy reorientation, so versatility is low. Changing frequency can mean changing its size. While it�s truly a remarkable antenna, it isn�t a practical choice for most missions. A cousin of this antenna, the V-beam, is a practical field-expedient wire antenna that provides good performance for military missions.
Another common antenna is the log periodic dipole array. LPDA is available in �rotateable� versions and in sizes that cover the frequency range most often used for long-range HF communication. These antennas provide good directivity in azimuth and acceptable patterns in elevation. They also have the decided virtue of operating over a wide range of frequencies without any reconfiguration or adjustment.
The following figure shows a typical azimuthal pattern for an LPDA. Note the radiation pattern is focused along the antenna�s axis.
|Azimuthal pattern of an LPDA. The gain of an LPDA of any particular design varies with frequency. This particular one is 67 feet long and incorporates 29 elements. Maximum gain is 10.8 dBi or about 8.2 dBd. That means its performance is about 6.6 times better than a dipole at the same height.|
The main concern is the elevation pattern and the antenna�s ability to radiate at low angles. The following figure shows this antenna�s performance. The blue-shaded area defines the angles important in making the circuit of interest.
|LPDA�s elevation pattern with the angles needed for the Sacramento-to-Fort-Meade path shaded. Note that even an antenna as large and well-designed as this one is down about 20 decibels at the lowest angle of interest. A 20-dB reduction in antenna gain is analogous to lowering transmitter power from 100 watts to one watt. This illustrates how little transmitter power is really needed for many paths, but also how difficult it is to get the transmitter�s power radiated at the angles we need. A shorter path would involve a similar blue wedge covering higher angles.|
The actual pattern of a specific antenna depends heavily on its height above ground and on the electrical properties of the ground under the antenna. For most horizontally polarized antennas, the pattern favors high angles at the expense of low angles as the antenna is lowered toward the ground. A minimum height for good low-angle radiation is one-half wavelength.
There are other antennas suitable for long-range communication; it would be easy to spend an article much longer than this one discussing any one of them. It is, however, instructive to evaluate the simple horizontal dipole and a quarter-wave vertical.
A dipole, such as we use for NVIS effect, loses its overhead radiation and gains lower-angle radiation as it�s raised farther above ground. The following figure shows the elevation pattern of a dipole mounted at a half-wavelength and at one wavelength above average ground.
|Dipole elevation pattern at one-half and one wavelength over average ground. Note that even though a high-angle lobe appears when the dipole is raised to one-wavelength, the radiation at most of the critical low angles still shows an increase. The �best� gain of this antenna is 7.81 dBi, considerably less than the LPDA.|
Vertical antennas have a reputation for excellent low-angle radiation. This is true only if the antenna installation includes a great many radials or if it�s mounted over very conductive ground such as seawater. The following figure shows the radiation pattern of a quarter-wave vertical over perfect ground compared with the same vertical over average ground with 12 in-ground radials. A dipole mounted a half-wavelength above average ground is included for reference. (Whenever my discussion is in wavelengths, it�s implied that the antenna is designed for a specific frequency or narrow range of frequencies. The antenna will work on other frequencies but may require a tuner. In most cases, performance changes when operation moves from the design frequency.)
|Comparison of vertical over perfect ground, vertical over average ground with radials and dipole at �-wavelength over average ground. For low-angle radiation, the vertical over perfect ground is hard to beat � unless you�re on the ocean! Note that the dipole actually outperforms the vertical-over-average-ground at most angles, even though most of its pattern is at higher angles. This illustrates the often-overlooked fact that while the shape of the pattern matters, it�s the amount of gain in the desired direction that�s the most important feature.|
A vertical antenna over saltwater is a very good low-angle radiator � probably the best there is. Unfortunately, the farther from seawater, the poorer the performance. There are many reasons for this, but the important information here is the somewhat surprising conclusion that a dipole mounted at a half-wavelength will usually outperform a practical vertical at the low angles required for these paths.
As is apparent, selecting the best antenna for the job requires some analysis. I analyzed these antennas using NEC-4, which is freely available to Defense Department entities. The user interface to the NEC computing engine, provided by EZNEC Pro � a commercial product � makes using NEC-4 much simpler and more pleasant. (All the comparison plots in this article are from EZNEC Pro.) Since actually testing HF antennas in a relevant way is pretty difficult, an analysis using NEC-4 generally is a more reliable indicator of antenna performance than can be discerned on an antenna range. In any case, if a vendor�s claims for an antenna are not supported by a NEC-4 analysis, the vendor is probably wrong.
Clearly, long-range HF communication requires that we address the two basic issues in all HF circuits: frequency selection and antenna design. Analyzing frequency selection and antenna requirements is greatly facilitated by the automated use of a propagation-analysis program such as ICEPAC. While radiation patterns for many common antennas are readily available in manuals and technical literature, a computer analysis can provide a more precise look at a specific design�s performance in a particular situation.
Marconi showed us what was possible. We now have the tools to expediently design and implement the long-range systems that were certainly in Marconi�s dreams.
Mr. Farmer, a lieutenant colonel in California�s state military reserve, is a professional engineer and president of EFA Technologies, Inc. The former Signal soldier has a bachelor�s degree in electrical engineering and a master�s in physics, both from California State University. He has published two books and more than 40 articles, and he holds two U.S. patents.
ICEPAC information is available from Dr. Greg Hand�s website at elbert.its.bldrdoc.gov/hf.html. The entire propagation-prediction suite can be downloaded without charge, as can extensive documentation. This is the best place for anyone interested in propagation prediction to start.
Another useful tool is the program I developed that works with ICEPAC to evaluate paths in determining useful frequency ranges and antenna take-off angle requirements. My program requires a high-speed Pentium-class personal computer and Windows 2000 or better. A free copy on CD-ROM will be provided to any Defense Department entity that requests it on letterhead. Address your request to: MG Paul Monroe, the adjutant general for the California National Guard, 9800 Goethe Way, P.O. Box 269101, Sacramento, Calif. 95826-9101.
An outstanding reference on antennas � especially the V-beam and other interesting long-range antennas � is The ARRL Antenna Book (Dean Straw, editor), most notably Chapter 13. The ARRL Antenna Book is available in bookstores and from The American Radio Relay League, 225 Main Street, Newington, Conn. 06111-1494. Also, try ARRL�s website at www.arrl.org. Anyone seriously interested in radio communications should read The ARRL Antenna Book from cover to cover as many times as it takes.
The NEC code can be obtained from Lawrence Livermore National Laboratory, Attn.: Gerald Burke L-156, P.O. Box 5504, Livermore, Calif. 94550.
EZNEC Pro is a commercial product developed and marketed by Roy Lewallen, W7EL, P.O. Box 6658, Beaverton, Ore. 97007. Under some circumstances, EZNEC Pro can be provided with the NEC-4 computing engine fully integrated, thus saving the trouble of acquiring it in raw form from Lawrence Livermore National Laboratory. You may also obtain the NEC-2 version at lower cost. Try the webpage at www.eznec.com.
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Army Communicator is part of Regimental Division, a division of Office Chief of Signal.