RC-292 and OE-254 characteristics, problems

When compared to manpack or vehicular vertical monopole antennas (whips), both the RC-292 and OE-254 get improved performance (signal gain) due to several factors:

Physical height, which increases clearance of path obstructions, lengthens distance to the radio horizon and reduces terrain masking;

Electro-mechanical construction that places a more perfect radio signal ground (ground plane) under the antenna�s radiating element to increase antenna efficiency and remove the effects of less-conductive �real ground�; and

Higher antennas with good ground planes tend to produce more energy on lower angles that enhance surface-wave radio propagation.

Unfortunately for radio operators, both these standard antenna designs have a large number of parts, and each includes up to 30 feet of cumbersome mast supports required to mount and elevate the antenna. Tactically, this makes them hard to install, operate and maintain in a combat environment.

RC-292. The RC-292, in addition, consists of a single vertical element and three ground-plane elements that form the antenna �ground� or �counterpoise.� The integral ground plane improves efficiency markedly over a vertical monopole (for example, a whip antenna) operating over real ground. The ground plane also ensures the pattern will be concentrated at low-elevation angles, more or less independently of the earth over which the antenna is installed. Effectively, the ground plane electrically raises the earth to the antenna�s height to complete the antenna circuit while also gaining the wave-propagation advantages of elevating the antenna.

For the RC-292 to produce an acceptable impedance match to the radio and transmission line at any particular frequency, however, it�s necessary to physically lengthen or shorten the antenna�s elements to a physical point near electrical resonance. That means that the antenna is �tuned� physically by adding or removing metal elements to/from the radiating and ground-plane assemblies so the antenna-radiation impedance will be close to that of the radio and the transmission line, and therefore the maximum amount of signal will be radiated from the antenna. Resonance also implies that most of the energy applied to the antenna is radiated as signal, and very little is reflected back down the transmission line toward the radio. This being the case, an antenna near resonance will have a low voltage standing-wave ratio. This indicates low levels of reflected signal and high antenna performance.

No electronic matching network is provided in the RC-292 antenna.

Changes in operating frequency of more than about 20 percent will cause the RC-292 to �de-tune� since each physical configuration only provides a good impedance match over a small portion of the reduced frequency range. Obviously, this isn�t a good design to use when frequency hopping across the entire 30-88 mhz tactical radio spectrum, but it may be OK when hopping over a narrow band if the band is aligned to be near the range of one of the RC-292 physical/electrical configurations. When signals of a frequency outside the resonance band of a configured RC-292 are used, the result is that a large amount of signal energy is reflected back down the transmission line and very little energy is radiated from the antenna as signal.

Very few RC-292 antennas are now left in the Army�s inventory because the mechanical adjusting and reduced bandwidth of each antenna configuration makes the RC-292 unsuitable to use with now-common frequency-hopping-type radios such as the Single-Channel Ground and Airborne Radio System. This antenna may still be found in some units that use single-channel operation for certain applications.

OE-254. 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 physically adjusting the antenna elements. The OE-254 is based on a frequency-independent design consisting of two simulated 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 feed-point impedance is stable over the VHF-FM band because of the biconic design, but it�s significantly greater than the 50 ohms our radios and transmission lines are designed for.

Consequently, a broadband matching network is provided in the antenna central-feed assembly to provide a proper impedance match to both the radio and transmission line. A small amount of signal energy is lost in the matching network, but it effectively adjusts the antenna�s complex impedance characteristics to eliminate reflections of energy (standing waves or VSWR) back down the transmission line. Broadbanding the antenna in the frequency sense is produced by the antenna's biconic construction. Biconic structures are inherently broadbanded if they are large (long) compared to the lowest frequency with which they are used.

dividing rule

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