Background - Several antenna configurations can be designed to cover one or more octaves. Some configurations are inherently directional such as the conical spiral and log periodic antennas. Others are omni-directional such as the bicone and tapered-blade antennas. Most broadband antenna configurations are difficult to design for low voltage standing wave ratio (VSWR) over more than one octave. Low VSWR is not a design necessity as long as the antenna is an efficient radiator. If the VSWR is high (more than 3:1), some form of matching network must be placed between the transmitter and antenna to minimize mismatch loss. Such designs are costly and make automation of the matching function much slower than is possible with lower power, solid-state tuning elements. SwRI began investigation of a simple antenna configuration that exhibits low VSWR over a wide bandwidth while providing linear and circular polarization at certain frequencies. This configuration may be suitable for many military and commercial requirements.

This antenna configuration can best be described as a tapered area small helix (TASH). Physically, the TASH antenna can be constructed by rolling a right-triangle-shaped conductive material into a spiral having a specific length and diameter. The key physical parameters affecting antenna performance are the height-to-base ratio, base feed point, turns spacing, and spacing above a ground plane. A three-turn TASH monopole above a ground plane with simulated current distribution (color pattern) is shown below.

Approach - The primary objectives of this research and development effort are to determine the parameters that affect the characteristics of the TASH antenna by simulation; determine the limitations on bandwidth, radiation efficiency, and antenna pattern; develop design tools and aids that would allow design of a TASH antenna with particular characteristics; and assemble and test a practical prototype TASH antenna to validate the simulations and design tools and aids.

The TASH antenna configuration was investigated using simulations based on method-of-moments analysis of the tapered area. The simulations allowed optimization of the antenna configuration (turns spacing, taper, height/diameter ratio, and other characteristics) and development of aids and tools usable for design of practical antennas. Assembly and test of a prototype TASH antenna followed as a verification of the simulation and a test of the design aids and tools. The tests were structured to verify the radiation efficiency, pattern, and VSWR characteristics of various TASH antenna configurations as simulated and designed.

As a result of the simulations, nomographs and empirical equations were developed for determination of VSWR, pattern, and material characteristics for a particular TASH antenna configuration. This development was followed by verification of nomographs and equations. Verification was via design, assembly, and test of a TASH antenna using the tools and aids for a specific application.

Accomplishments - The program simulated the TASH monopole and other single-element monopoles for comparison. Physical models were assembled and verification tests of VSWR were performed to determine the validity of the simulations. The verification tests showed excellent correlation with the simulated antennas. A large series of simulation runs was then performed on variants of the TASH monopole to establish design guidelines for TASH monopoles with low VSWR over at least one octave above first resonance.

One physical model TASH monopole was provided to the U.S. Army Communications and Electronics Command (CECOM) for field-testing. Army test results of the model verified the low VSWR and, in addition, gave useful data on the measured field pattern of the TASH monopole. The field pattern data showed that the TASH monopole is omnidirectional as expected for a monopole, but is less sensitive to ground plane variation than conventional monopoles. As a result of the project and CECOM test results, a proposal was submitted to the Army for an airborne version of the TASH antenna. To date, no award decision has been made regarding the proposal.

Simulated current intensity on a three-turn TASH monopole antenna

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