Military tactical communications require transportable radio systems that can be set up quickly in the field, under difficult conditions, to establish communication links. These radio systems are mounted in tactical operation centers (TOC), vehicles and shelters. Legacy antennas used in these systems operate primarily in the VHF or UHF frequency bands and have omni-directional radiation patterns. They are set up on the ground, mounted on vehicles, or attached to telescopic masts when the terrain requires additional height to establish a line-of-sight link.

To complement the latest generation of software-defined, multi-band/multi-mission radio designs utilized by the military, new tactical communication antennas are needed, featuring wider frequency bandwidth than was previously available. The need for dual- and tri-band antennas is also becoming more commonplace, as the number of antennas mounted on military vehicles continues to grow. The ability to accomplish multiple missions with a single aperture has become highly desirable. Chelton Microwave Sensor & Antenna Systems – Atlantic Division is designing wideband and multi-band antennas to address these requirements.


Legacy Antenna Designs

In the 30 to 88 MHz Single Channel Ground and Airborne Radio System (SINCGARS) band, there are a number of legacy antenna designs that have been used over the years. These are mainly biconic antennas for mast-mounted applications and whip-style antennas for vehicle applications. Improved alternatives to these conventional approaches are the COM201B and COM231 antennas designed for military tactical SINCGARS applications (see Table 1). The COM201B is a ground-based antenna that can be deployed in a stand-alone, mast-mounted, or suspended configuration (see Figure 1). The COM231 model is a vehicle-mounted antenna that includes a flexible spring mount that enables the antenna to survive the rigors of driving on rough terrain. These antennas provide an omni-directional radiation pattern in the horizontal or azimuth plane and a cardioid radiation pattern in the vertical or elevation plane, as shown in Figure 2.

The COM201B 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 on rooftops, bunkers and shelters. These tripod/ground-plane radials telescope and can either be removed or folded up parallel to the active element for transport.

Physical properties designed into the COM201B have 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 COM201B antenna’s physical construction is such that the diameter-to-length is optimized to produce radiation across the 30 to 88 MHz frequency band, using a structure that has a reasonable size as well as effective radiation-efficiency characteristics. To make the antenna even more effective, the COM201B also includes a broadband, lumped element, impedance matching network. The network is located in the base of the lower radiating element near the feed point of the antenna. The network maximizes efficient transfer of power from the radio to the antenna field across the desired frequency range with a VSWR of less than 3.5:1, as shown in Figure 3.

The COM231 vehicle-mounted antenna uses the same design techniques as the COM201B, but whereas the COM201B is equipped with the tripod metal-leg structures to provide a ground plane, the COM231 model uses the vehicle it is mounted to as its ground plane. The impedance matching circuitry of the COM231 antenna is similar to that which is used in the COM201B antenna, but because the COM231 is used in a mobile environment, additional lightning protection and high voltage protection circuitry is added. The COM231 is supplied with a standard military vehicle mount, as well as with a side-mount kit for attachment to shelters and some military vehicles.

Since the COM201B and COM231 antenna structures were specifically designed to deliver maximum low angle signals, their power on the horizon is greater than for other antennas of its type. In addition, the low loss impedance matching network and the higher length-to-diameter ratio of the antenna both work to increase the power gain on the horizon even further. Therefore, radio systems using these antennas can be expected to perform at distances greater than what is normally accepted to be radio line-of-sight using other antennas, and this has been confirmed in field trials by the US Army.

Next Generation Multi-band Designs

The COM201B and COM231 antennas were used as a starting point for the development of a new series of antennas to cover the ultra broadband frequency range of 25 to 2500 MHz, as required for the next generation communication and electronic warfare applications. Using technology developed from these legacy antenna designs, new antennas have been designed for both dual- and tri-band applications.

One such new antenna is the COM237 model, which operates over the 30 to 512 MHz frequency band and complements the latest software-defined radios that are currently being procured by the military (see Table 2). This is a vehicle-mounted antenna that has the same mechanical footprint as the COM231 vehicular antenna (see Figure 4). It has a base section 32" long, which is 3" in diameter and the remainder has a 1.75" diameter. The overall length of the antenna is 75". This broadband operation is accomplished by integrating a UHF radiator into the same space as a VHF radiator and employing techniques to reduce the overall antenna length. The VHF section is an end-fed monopole element operating in the 30 to 100 MHz range. The UHF section is a sleeve dipole operating from 100 to 512 MHz. The COM237 can be supplied in either a one-RF or two-RF port configuration. When supplied as a one-RF port antenna, the COM237 can interface to a software-defined radio and take full advantage of its wideband operation. When configured with two RF ports, this antenna can support both VHF (SINCGARS) and UHF (EPLRS, for example) bands of operation simultaneously, thus minimizing the number of apertures installed on the vehicle.

Another next generation antenna is the COM235, a multi-band antenna that covers the VHF (30 to 100 MHz), UHF (100 to 512 MHz) and L-band (900 to 2500 MHz) frequency bands. The design of this model builds upon the COM237 dual-band design by adding an additional bi-cone radiator on top to address the L-band requirement. This can be accomplished in a physical package size that is 3.5" in diameter, with an overall height of 70". This antenna can be configured with three separate RF ports, or two of the elements can be combined through a diplexer to reduce the number of RF ports. When configured with three RF ports, the single antenna can be used for three distinct missions such as SINCGARS, EPLRS and JTIDS. The isolation between the bands of operation is sufficient to operate this way. When the elements are combined to a single RF port, the antenna can be used for wideband detection and jamming applications.

A COM236 model has also been developed for JTRS. The COM236 again features a VHF (30 to 100 MHz) radiating element, a UHF (100 to 512 MHz) radiating element and an L-band radiating element covering 900 to 1850 MHz. The COM236 outline is similar to the COM201B, featuring an upper radiating diameter of 1.5", with an additional seven inches in height to accommodate the L-band radiating element.

Conclusion

New multi-band antenna designs are available to address wideband software-defined radio applications as well as multi-mission requirements. These multi-band antennas feature improved bandwidth, radiation efficiency and power handling performance in the same basic outline as legacy single-band antennas. The antennas are used in military tactical communications applications, as well as in some broadband detection and jamming applications.

Chelton Microwave Corp.,
Sensor & Antenna Systems – Atlantic Division
Bolton, MA
(978) 779-7000,
www.cheltonmicrowave.com.