by David Fiedler and Edward Farmer
Historically, to increase transmitted signal strength, coverage area and point-to-point ground distance for tactical very-high-frequency (30-88 megahertz) radio systems, the Army and Marine Corps turned to elevated (30-foot mast-mounted) ground-plane and biconic antennas such as the widely used RC-292 and OE-254 (following figures). Both antennas perform acceptably electrically but are much too heavy and complicated for today�s fast-moving operations, particularly for light infantry, Special Forces and airborne/airmobile units. (See related information on RC-292 and OE-254 characteristics and problems.)
The RC-292 is a general-purpose, stationary, ground-plane antenna used to increase the transmission/reception range of tactical frequency-modulation radio sets. The radiating and ground-plane elements must be adjusted to the proper length for a particular operating frequency. The RC-292�s technical characteristics are: frequency range, 20-76 mhz; planning range, about twice the planning range of a radio set using a quarter-wave whip antenna; height when erected, 11.28 to 12.56 meters (37 to 41.2 feet); and weight, about 19.5 kilograms (43 pounds).
|The OE-254 is a general-purpose, stationary, broadband, omni-directional antenna used to extend the range of tactical FM radio sets. Under normal field conditions, the antenna will be mast-mounted. Once installed, the OE-254 doesn�t have to be taken down for adjustment when a new frequency band is assigned to the radio net. The OE-254�s technical characteristics are: frequency range, 30-88 mhz; planning range, 57.9 kilometers (36 miles) over average terrain or 48.3 kilometers (30 miles) over difficult terrain; radio-frequency power capability, 35 watts nominal; antenna-erection time (one person), 15 minutes; height when erected, 12.8 meters (42 feet); input impedance to radio, 50 ohms; and weight, 20.4 kilograms (45 pounds).|
Also, they both contain many separate parts that are easily lost. Both require much too much time to erect and tear down. Assembling and adjusting the mast, and assembling the multisection screw-together antenna elements that form both antennas, take most of the installation time.
The Marines have found another answer; now we need a better way.
A new antenna is available that has ground-plane and biconic radiation characteristics but is designed specifically to improve both electrical and tactical characteristics. The Marine Corps and other government agencies are procuring this antenna from Atlantic Microwave Corporation.
Commercially named the COM-201 (NSN# 5985-01-450-3798, USMC PN 960-15A 1008), the antenna is a 30-88 mhz, vertically polarized, omni-directional, ground-plane type. It�s unique because it�s designed so it can mount directly on the ground using a built-in �snap out� tripod that�s also the antenna�s complete ground-plane structure. The COM-201 is also fitted for mast-mounting on standard antenna masts if a more elevated antenna is needed. The eye fitting at the antenna�s top facilitates suspending it from buildings or trees when a mast isn�t available but more height is desired.
One of the COM-201�s best features, however, is that 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 COM-201 gets its broadband characteristics (variable standing-wave ratio less than 3-to-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 the COM-201 and OE-254 antennas are modeled using the EZNEC-PRO implementation of the NEC-4.1 antenna-analysis software, they show similar frequency response, gain and antenna patterns when both are elevated at 30 feet (figures below).
Comparison of COM-201 to OE-254 at 30 mhz.
|Comparison of COM-201 to OE-254 at 45 mhz.|
|Comparison of COM-201 to OE-254 at 60 mhz.|
|Comparison of COM-201 to OE-254 at 75 mhz.|
|Comparison of COM-201 to OE-254 at 88 mhz.|
At the high end of the frequency range, the OE-254 shows some overhead modes that waste useful signal power for desired 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 COM-201 with its lower takeoff angles delivers more energy (gain) at the radio horizon to make it more useful in ground-to-ground operations.
Why should the COM-201 be considered to replace existing antennas? Part of 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-structures that serve as both mount and electrical ground plane allow the antenna to be installed directly on the ground or atop roofs, shelters, bunkers, etc. (Care should be taken to keep the radiating element vertical to the ground to avoid distorting the antenna-radiation pattern.)
When time and situation permit, the antenna has the fittings required so it can be mounted on standard or makeshift masts, or roped into trees and buildings to gain the advantages of increased height. The antenna can be moved assembled, partially assembled or broken down. The antenna�s active element has a threaded interconnect at the midpoint to reduce its disassembled length to only 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 about 10 pounds. If deployed on its tripod/ground plane, the COM-201 needs only a few feet of coaxial cable to connect it to a radio. This is a feature not available with other antennas.
At platoon and company level, the load reduction of about 30 pounds (when compared to the OE-254) is a very attractive feature that will allow units who couldn�t previously afford to carry a more efficient antenna to do so by using the COM-201. Communications distance loss (if any) generated by locating antennas close to the earth is less of a factor at these lower echelons where distance requirements are shorter to begin with.
In most cases, overall results will be much better because more effective ground-mounted COM-201s can replace the far less efficient 10-foot or three-foot standard-issue manpack vertical antenna with minimal additional effort.
Part of the answer also lies in the COM-201�s electrical- and signal-radiation characteristics. The first thing one notices about the COM-201 antenna�s construction is that the radiating element has a much wider diameter than anyone familiar with our current antennas might expect. This is because increasing the diameter at a constant length (increasing the distance/length ratio) has the effect of decreasing the electrical reactance of the antenna�s elements, which, in turn, increases the frequency range (bandwidth) over which the antenna can be efficiently operated.
The �why� of this takes a bit of explanation. At radio frequencies, all conductors (for example, antennas) inherently have resistance, capacitance and inductance. The resistance is made up of two components which we call �loss resistance� and �radiation resistance.� Loss resistance comes from the flow of radio-frequency electrical current through the antenna�s elements and connections. This energy is dissipated as heat, isn�t useful for communications, and is negligible small in antennas such as these. Radiation resistance accounts for the portion of the energy we apply to the antenna that actually does what we�re trying to do: produce an electromagnetic field and get a signal into the air.
Capacitance and inductance present in an antenna structure produce an effect similar to resistance that we call �reactance.� Reactance opposes the flow of current, as does resistance, but doesn�t result in lost energy since the energy is stored in the inductive field or as electric charge on the capacitive structure.
Taken together, an antenna�s resistance and reactance are called its �impedance.� By definition at the resonance frequency, the inductive reactance and capacitive reactance are equal in magnitude and opposite in phase; consequently they cancel each other, and the antenna presents a pure resistive load (the radiation resistance) to the transmission line and radio. At the resonance frequency, the pure resistive electrical load causes the antenna to be the most efficient radiator of signal possible for that structure.
As the frequency of operation strays above the resonant frequency for the antenna, the antenna begins to have inductive reactance in addition to its resistance. When the frequency strays below the resonant frequency for the antenna, the antenna begins to appear capacitive. Capacitive or inductive reactance not canceled in an antenna circuit reduces the effective radiated power of the signal the antenna generates.
All this becomes particularly interesting when frequency hopping with radios such as Single-Channel Ground and Airborne Radio System because the bandwidth of an antenna � that is, the range of frequencies over which it operates efficiently � is proportional to the ratio of its resistance to reactance. The smaller an antenna�s reactance, the wider its frequency bandwidth.
The COM-201�s physical construction is such that the diameter-to-length ratio is optimized to produce radiation (signal) across the 30-88 mhz frequency band using a structure that has reasonable physical size and radiation-efficiency characteristics. This optimized construction produces more signal (gain) and lower takeoff angles for the COM-201 when compared to an OE-254 at the same height, as the plots show. This means better electrical performance as well as better mechanical performance can be expected from the COM-201 under tactical conditions.
Another part of the answer lies in how the COM-201 electrically �matches� its complex antenna impedance (resistive, capacitive and inductive components) to the transformed 50-ohm impedance of the radio and transmission line to reduce reflected power (voltage SWR) and produce maximum radiated power. To do this, a matching network is incorporated at the antenna�s feedpoint. It provides the inductive and capacitive reactance necessary to compensate for the antenna�s inherent reactance as the operating frequency varies over the 30-88 mhz frequency range. As you can see from the following figures, the network transforms the antenna�s complex impedance into a 50-ohm resistive impedance that closely matches the radio and transmission-line impedances and produces a VSWR of less than 3-to-1 across the frequency range.
The network transforms the antenna's complex impedance into a 50-ohm resistive impedance.
|Comparison of tuned COM-201 vs. untuned antenna.|
Since deploying the OE-254 antenna in 1978, the Army has had to live with the mechanical and electrical shortfalls inherent in its design. The OE-254 antenna was clearly the best available at that time, and it did well as we moved the Army from the AN/VRC-12 family of single-channel radios to the SINCGARS family of frequency-hopping equipment. The OE-254�s 24 years of service and the huge number of antennas fielded prove the antenna worked well; however, time marches on. Just as we�re not driving the same vehicles and shooting the same weapons we did in 1978, we shouldn�t be using the same field antennas.
The authors believe the Marine Corps made a good decision to adopt the COM-201, but we�re not advocating change for change�s sake. There have been real advances in antenna technology over the last 24 years. The Marines� successful use of the COM-201 shows that if nothing else, the antenna will provide, at the least, a better physical package since the COM-201 is quicker to deploy and has far fewer small parts to lose or break. A soldier�s ability to set up the COM-201 without need of a mast is a great tactical advantage � particularly in the type of mobile warfare, urban warfare and homeland-defense operations we�re now conducting. Another bonus the COM-201 provides is the higher gain and the lower takeoff angles to help improve tactical combat-radio operations. The Army needs to very seriously consider following the Marines� lead when replacing our aging stocks of these types of antennas.
Mr. Fiedler � a retired Signal Corps lieutenant colonel � is an engineer and project director at the project manager for tactical-radio communications systems, Fort Monmouth, N.J. Past assignments include service with Army avionics, electronic warfare, combat-surveillance and target-acquisition laboratories, Army Communications Systems Agency, PM for mobile-subscriber equipment, PM-SINCGARS and PM for All-Source Analysis System. He�s also served as assistant PM, field-office chief and director of integration for the Joint Tactical Fusion Program, a field-operating agency of the deputy chief of staff for operations. Fiedler 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 a master�s degree in industrial management. He is the author of many articles in the fields of combat communications and electronic warfare.
Mr. Farmer is a Vietnam-era Signal soldier and former lieutenant colonel in California�s State Military Reserve, where he ran intrastate emergency communications. He�s a professional engineer, has an extra-class amateur radio license and is president of EFA Technologies, Inc., in Sacramento, Calif. He has a bachelor�s degree in electrical engineering and a master�s in physics, both from California State University. He has published three books and more than 40 articles, holds four U.S. patents and is a frequent guest speaker at communications and antenna-oriented conferences.
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