by Ed Farmer and Dave Fiedler
Over the past several years, the Signal Regiment has become challenged by the Army�s increasing emphasis on light /medium forces and urban-terrain operations. Since the Cold War era has ended, so has the Army�s emphasis on "Fulda Gap" type of scenarios involving fast-moving heavily armored forces. The realities of light /medium force tactical communications over rough or urban terrain have set in. The Grenada, Somalia and Panama operations (as well as innumerable training exercises) have uncovered some basic problems when using the Army�s workhorse of tactical communications, the very-high-frequency frequency-modulation radio (single-channel ground and airborne radio system, AN/VRC-12, etc.) under these conditions.
Radio hardware isn�t the primary issue when addressing the light infantry/military operations on urban terrain communications problem. Current VHF-FM equipment is of reasonable weight, size and output power for the mission. Light fighters always benefit from equipment that�s less of a load or that radiates more signal, but the manpack and hand-held radio hardware we have in today�s inventory is what it is. It isn�t likely changes will occur soon. The challenge, therefore, is getting the best performance possible from our current radio equipment under conditions imposed by light/urban warfare. As always, this boils down to getting maximum performance out of the key components of any radio system: the antenna and the operating frequency(s).
The Fall 1995 and the Summer 1996 editions of Army Communicator include a description of some techniques that improve tactical groundwave VHF communications in all environments. After some thought and experimentation, the authors now have a few more suggestions that will result in further improvements.
Frequencies don�t offer many opportunities for innovation. Tactical VHF-FM operates from 30 to 88 megahertz. At these frequencies, propagation is line-of-sight and groundwave; there�s no dependable skywave propagation. This drives antenna selection.
A good radio antenna efficiently transfers radio-frequency energy (signal) from a source (the transmitter) into the antenna�s surrounding electromagnetic field. The RF energy is then used for communications. When receiving, the energy-transfer function is reversed. Obviously, the more efficiently the antenna does its job, the more effective the radio system will be.
An antenna for the Army�s LI/MOUT mission must also have more capabilities:
|First, it should concentrate the radio energy (signal) at low elevation angles � near or even below the horizon. Concentrating signal at low angles extends communication distances and aids in penetrating structures encountered in MOUT environments;|
|Second, the antenna�s pattern should be omnidirectional in azimuth, providing equal signal in all directions. This assures communications in all directions, which is usually needed in an infantry environment;|
|Third, it must operate across the tactical-frequency (30-88 mhz) band with a consistent radiation pattern;|
|Fourth, it must provide good electrical impedance match across the frequency band. This reduces standing-wave reflections and passes the maximum amount of the transmitter�s energy into the antenna�s field while operating in either fixed-frequency or frequency-hopping mode;|
|Fifth, it should be of a physical size, shape, weight and level of complexity that makes it practical for LI operations.|
After considering these requirements, the authors concluded there are viable options offered by the current inventory of Army tactical antennas. As we showed in our 1995 and 1996 articles, system efficiency can be increased for combat-net radios by using the longer standard vertical (whip) antennas; by building "ground planes" or counterpoises under the radiating elements; and by elevating the feedpoint. Keeping the whip antenna vertical off the operator�s back and away from energy-absorbing structures and vegetation also helps. In many applications, such as LI and MOUT, these actions still may not be enough to provide the circuit performance and reliability required � particularly for data communications. The question becomes, "How can we further improve performance with current equipment or materials readily available to field soldiers?"
Historically, to improve signal strength, and thus coverage area and over-ground distance, the Army has turned to elevated ground-plane and biconic antennas such as the RC-292 and OE-254. Both these antennas have a large number of parts, and their installation kits include 30 feet of mast supports required to elevate the antenna. These antennas get their improved performance from their height (which increases clearance of path obstructions) and because of their designs.
The RC-292 consists of a vertical element and a ground plane. The integral ground plane improves efficiency markedly over a vertical monopole (for example, a whip antenna) operating over real ground. It also ensures the pattern will be concentrated at low-elevation angles, more or less independently of the earth over which the antenna is installed. To produce an acceptable impedance match at any particular frequency, it�s necessary to physically lengthen or shorten the antenna elements to a point near electrical resonance.
The OE-254 is based on a different idea. The SINCGARS frequency-hopping concept requires operation over the entire tactical-frequency range (30-88 mhz) without physical adjustment of the antenna. The OE-254 is based on a frequency-independent design consisting of two cones (biconic) arranged apex-to-apex on a common axis. In the OE-254�s case, three pairs of elements arranged symmetrically simulate the upper and lower cones. The feedpoint impedance is stable over the VHF-FM band but requires a matching network in the central assembly to assure a proper match to our radios and transmission lines.
Both antennas perform well but are much too heavy and complicated for today�s LI/MOUT operations. They contain many separate parts that are easily lost during fast-moving operations, and they require too much time to erect and tear down.
After much thought and experimentation, we�ve identified three options to improve the Army light/medium fighter�s situation. They are:
|Modify the OE-254 (very few RC-292s remain in the inventory) to better fit the situation;|
|Change to another available ground-plane or biconical antenna designed to be more compatible with light/MOUT operations;|
|Construct a field-expedient antenna from commonly available components.|
The OE-254 has been in the Army inventory since the 1970s. It replaced the RC-292 as we fielded SINCGARS. This was necessary because SINCGARS� frequency-hopping features use the full 30-88 mhz tactical-frequency range, and the OE-254 provides acceptable impedance across that entire range.
The OE-254 consists of three pairs of opposing elements, each eight feet long arranged 30 degrees from the vertical and 120 degrees apart (see the following figure). Elements arranged this way and fed in the center electrically simulate two solid cones fed at their apexes. This structure produces omnidirectional low-angle radiation across the frequency range. The feedpoint impedance is fairly constant across the band at about 160 ohms. To provide a low standing-wave ratio and match a 50-ohm transmission line and radio, the antenna incorporates a 4:1 balun (balanced to unbalanced transformer) with a small capacitor (24 pf) across the low-impedance (feedline) side.
|The Army's OE-254 biconic antenna for frequency-independent operation between 30 and 88 mhz. This antenna is usually mounted about 30 feet aboveground on a pole included in its kit. There are three pairs of eight-foot-long elements which simulate two cones arranged apex-to-apex. The center section includes a balun and simple matching network. This antenna provides a low SWR, omnidirectional pattern and low angle of radiation over the entire 30-88 mhz band.|
Maximum energy at the lowest angle possible is important for ground-to-ground communications, since energy on angles above the radio horizon goes off into space and isn�t useful for communications. Polarization is parallel to the cones� axis so elements are arranged up and down to produce vertical polarization. Vertical polarization is preferred for ground tactical communications because it�s less affected by terrain, vegetation and manmade obstacles than horizontal polarization. (See Army Communicator Summer 1996 for an explanation of this phenomenon.)
The light fighter�s complaints about the OE-254 usually include its size, weight and complexity of the full OE-254 package � not its communications performance. Transportation of the full kit in units that only have one or two humvees to carry cargo is painful. The time required to deploy the full antenna can also be a problem. Even well-trained soldiers can take 15 minutes to assemble the antenna and install it on its 30-foot manually erected mast. Damage to the mast and antenna is not uncommon during the installation process. The antenna contains many parts that are easily lost, particularly when in a hurry or at night.
Knowing the theory of biconic antennas and the drawbacks of the OE-254 in light/MOUT operations, we decided to see what could be done to improve conditions in the field. We first discarded the most cumbersome part of the OE-254 (the mast and coaxial cable) and merely set the antenna on its lower elements on the ground (refer to above figure). The results weren�t bad. Since the antenna is inherently balanced due to its construction, the earth�s effects didn�t cause serious problems. The pattern remained omnidirectional with good low-angle radiation, and impedance characteristics remained stable. Certainly the low elevation limited maximum-circuit distance (due to a nearer radio horizon), but signal strength to the horizon was good due to the low-angle radiation pattern.
This is excellent for MOUT operations, where distances involved tend to be well short of the horizon anyway. A ground-mounted OE-254 will give better performance than the long (10 feet) or short (three feet) whip antennas normally provided with SINCGARS, even with a ground system under them (see Army Communicator Fall 1995). Raising the antenna off the ground by means of ropes or a short substitute mast will, of course, increase communication distance as LOS increases. Elevated terrain features or convenient structures encountered in MOUT operations can also be used to raise the antenna without using a mast. Particularly for battalion-or-below echelons moving quickly across urban or broken terrain, a ground-mounted OE-254 offers decent performance without the need to transport a heavy load.
A second alternative available to improve LI/MOUT communications is an antenna designed specifically for these types of operations. The U.S. Marine Corps is procuring such an antenna from the Atlantic Microwave Corporation (following figure). Nomenclatured as the CH-201 (NSN# 5985-01-450-3798 USMC PN 960-15A 1008), the antenna is a 30-88 mhz, vertically polarized omnidirectional ground-plane type. It�s unique because it�s designed so it mounts directly on the ground using a built-in tripod leg /ground-plane structure. The antenna also includes a ring atop the vertical element to facilitate suspending it from buildings, trees, etc.
|The Atlantic Microwave CH-201. This antenna may be used on the ground (as shown), pole-mounted or suspended using the eye fitting at its top. This design bears marked similarity to a vertical monopole over a ground plane. Broadbanding is achieved by the large-diameter elements and an internal matching network. This antenna also provides a low SWR, omnidirectional pattern and low radiation angle over the entire 30-88 mhz band. The antenna breaks down into five parts that can be assembled in less than a minute.|
While the OE-254 gains bandwidth by simulating frequency-independent biconic construction, the CH-201 gets its broadband characteristics (variable SWR less than 3:1 across 30-88 mhz) by using large-diameter elements and a very well-designed broadband matching network built into the antenna base at the feedpoint.
When both antennas are modeled using the EZNEC-PRO implementation of the NEC-4.1 antenna-analysis software, they show similar frequency response, gain and antenna pattern when both are elevated at 30 feet (following figure). At the high end of the frequency range, the OE-254 shows some overhead modes that waste useful signal for ground communications. Energy at these high angles is generally produced at the expense of radiation on the much more tactically useful low angles, and therefore it�s detrimental to good communications. The CH-201 with its lower takeoff angles therefore delivers more energy (gain) at the horizon that may make it more useful in ground-to-ground operations.
|Left, superimposed elevation pattern plot of OE-254 mounted at 30 feet with the frequency varied from 30-90 mhz in five-mhz steps. At the high end of the frequency range, useless overhead lobes begin to appear at the expense of useful radiation at low angles near the horizon. Right, superimposed elevation pattern plot of CH-201, same height and frequency conditions. While overhead lobes can be seen developing at the high end of the frequency, they're less pronounced than those of OE-254.|
When ground-mounted, both antennas exhibit similar performance, with the edge going to the CH-201. The differences are minor at most frequencies and significant over only a narrow portion of the frequency range. The OE-254 performed better below 50 mhz, which may be an advantage when operating in the continental United States where frequency assignments above 50 mhz are rare. Across the 30-88 mhz frequency range, neither antenna had a clear performance advantage.
Why then should the CH-201 be considered for use as a LI/MOUT antenna? The answer lies in its mechanical design. The antenna is designed with quick deployment and ease of operation in mind. The unique tripod metal-tube leg structure allows it to be installed directly on the ground (although care should be taken to keep the radiating element vertical with respect to the ground). When time and situation permit, the antenna can be mounted on standard or makeshift masts, or roped into trees, buildings, etc. The antenna can be moved assembled, partially assembled or broken down. The active element has a threaded interconnect at the midpoint to reduce disassembled length to 36 inches. The tripod/ground plane radials telescope and can either be removed or folded up parallel to the active element. This results in a package 36 by 10 inches weighing in at about 10 pounds.
If deployed as intended, the CH-201 needs only a few feet of coax cable to connect it to a radio. Longer cable runs or radio remote units are needed if the antenna is to be located away from the operating point.
At the LI platoon and company level, the load reduction of about 30 pounds (when compared to the OE-254) is an attractive feature. Distance loss (if any) generated by locating antennas close to the earth should be less of a factor at the lower echelons where distance requirements are shorter to begin with. Battalion/brigade/regiment echelons may need to install masts to achieve communications with distant units using OE-254 or CH-201 antennas. This shouldn�t be as serious a problem at these levels, since they�re equipped for it.
In most cases, overall results will be much better because more effective ground-mounted OE-254s or CH-201s can replace the far less efficient 10-foot or three-foot standard-issue vertical antenna with minimal additional effort.
Another option is to construct a field-expedient antenna that will do the job with a minimum of effort. Typically, these homemade structures are made from readily available bits of wood, plastic, rope and electronic parts available from local civilian sources.
A common method of "broadbanding" an antenna is to feed two identical wire dipoles from the same transmission line. Spreading the dipoles on the ends makes the antenna look like a "bowtie," from which the name bowtie antenna is derived (following figure). The spread dipole elements on each side of the transmission-line feedpoint have the effect of making a two-element cousin of the biconic (OE-254 type) design. A broad frequency response is obtained in exactly the same way that�s achieved in the OE-254. To produce the vertical polarization required, the antenna must be suspended vertically.
Feedpoint impedance for an antenna of this type depends on the cones� apex angle. To match a 52-ohm transmission line, the apex half-angle would be about 65 degrees. At this half-angle, a 15-foot cross-member is required, which is a bit clumsy for this application. A more physically practical antenna would have a half-angle of perhaps 30 degrees, at which the feedpoint impedance is about 150 to 200 ohms. Such a design requires a matching network. For this application a 4:1 torroidal transmission-line transformer, also known as a balun transformer, is connected across the feedpoint. Depending on the particular balun used, the impedance match may be improved by a small-value (for instance, 10-24 pf) capacitor placed across the low-impedance side of the transformer. The balun�s use has the additional advantage of reducing current flow (and consequently radiation) on the outside (shield) of the antenna transmission line. This reduces pattern distortion and lowers SWR.
One of the authors (Farmer), with the California National Guard�s assistance, constructed a vertical bowtie and matching network to prove that a simple, lightweight structure made from inexpensive parts, wire and plastic pipe would work for LI operations. Farmer settled on a cone half-angle of 30 degrees, which reduced the cross-member length to eight feet and produced a feedpoint impedance of 158 ohms. While not perfect for a 4:1 balun, it works quite well. To match this antenna impedance, he constructed a balun by winding four bifilar turns of #20 AWG enameled copper wire on a Palomar BLN-68-61 core.
|The field-expedient wire antenna suspended from a light fixture in the California National Guard's Meadowview Armory.|
|The field-expedient antenna packs into a small, simple kit. The inset shows the matching network. This one used the center insulator from a GRA-50 antenna kit and a matching network from a broken OE-254 center section.|
|The matching network transforms the high (150-200 ohms) impedance of the antenna down to provide a better match to coaxial cables. A hand-wound balun was used for this design, but there's nothing magic about that. Any 4:1 balun suitable for the frequency range will work.|
For the wire portions of the "Farmer bowtie," he used eight feet per leg of the copper-braided nylon-core wire found in the standard AN/PRC-74 antenna kit. Phosphor bronze wire from the AN/GRA-50 antenna kit, field-telephone wire or any flexible stranded copper wire from a hardware store will also work. As a general rule, the wire should have as much copper in it as possible, and be of stranded construction and reasonable thickness (gauge).
Farmer used the insulator block from a standard AN/GRA-4 antenna kit for the center section because the connection hardware was already packaged for him. Any glass, ceramic or non-conducting insulating material can be used for this purpose. The non-conducting "spreader" for the antenna�s upper sections was made from sections of PVC pipe and threaded couplers. Any non-conducting stiff material such as wood or fiberglass can also be used.
The bowtie�s lower part is simply the eight-foot lower dipole wires spread at the same angle as the upper portion and attached to tent pegs. The pegs should be non-conductive, or insulators need to be installed between the pegs and the antenna wires. In the event the antenna is too high to terminate directly to the pegs, rope can be used bridge the gap so the 30-degree half-angle is maintained. An eight-foot length of rope can be used to check the lower spread.
While conducting field tests, Farmer discovered that slightly more gain and a more uniform pattern could be achieved from the antenna if the bottom half were rotated by 90 degrees from the top. (This may not always be physically possible when the antenna is suspended from walls, building windows, etc.) By turning the bottom half, a more uniform field pattern is created that gives a little extra benefit in signal strength (at the expense of being perfectly vertically polarized).
The field-expedient bowtie, while not the most sophisticated antenna structure around, will give performance comparable to an OE-254 at the same height. Its advantage is that its components are inexpensive and easy to obtain; it�s relatively immune to the effects of real ground; and it can be transported rolled in an infantry ALICE pack � or even in a battle-dress uniform�s cargo pocket!
While these three antennas are not the only possible answers to the LI/MOUT broadband antenna problem, they�re reasonable ways to go. We�re investigating more antennas that also appear promising. Significant results will be reported in a future article.
All the solutions to the LI/MOUT tactical-communications problem we�ve outlined are practical and are already being looked at by some tactical organizations in both the Army and the Marine Corps. They take advantage of techniques that will increase the effective radiated power of standard radio equipment when employed in the LI/MOUT environment. While past operational scenarios concentrated on the "take the high ground" hilltop solution for effective tactical communications, the picture changes considerably when fighting on urban terrain. If a situation requires more signal strength at lower angles for shorter-distance communications over highly obstructed paths, what we�ve shown will be easy to employ, with less exposure to the enemy, and at the same time provide more effective combat communications. When forced to fight in this environment, S-6/G-6 staff elements need to be aware of all these possibilities to engineer the best possible communications.
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 more than 40 articles and two books, and holds two U.S. patents.
Retired LTC Fiedler is employed as an engineer and special assistant to the project manager, operational tactical data systems, Fort Monmouth, N.J. He has served in Army, Army Reserve and Army National Guard Signal, infantry and armor units and as a Department of the Army civilian engineer since 1971. He holds degrees in both physics and engineering and an advanced degree in industrial management. He is the author of many articles in the fields of combat communications and electronic warfare.
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Army Communicator is part of Regimental Division, a division of Office Chief of Signal.