Development of a Band-Selectable Smart Antenna for Software-Defined Radio, 10-R9543

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Principal Investigators
Mike Pilcher
Travis R. Thompson

Inclusive Dates:  04/01/05 – 09/30/06

Background - During the last decade, digital radio systems have begun replacing analog radio systems as programmable hardware components have become more capable, inexpensive, and available. Consequently, an evolving technology known as "Software-Defined Radio" (SDR) has become more realizable. A digital radio system typically uses application-specific integrated circuits (ASICs) and digital signal processors (DSPs) to implement the digital portion of the intermediate frequency (IF) section and baseband section. In an SDR system, many of these operations are implemented with software. As hardware components evolve, future SDR systems may also allow programmable radio frequency (RF) bands within the RF section. Manufacturing companies are developing hardware components for use in future SDR systems. These components include improvements in RF front-ends (smarter antennas, better isolation, lower noise amplifiers, more flexible filters, additional programmability) and analog-to-digital and digital-to-analog converters (wider bandwidths, higher dynamic ranges, faster sampling rates).

Approach - This project has focused on identifying new and innovative techniques for using current technology to meet the military Joint Tactical Radio System (JTRS) requirements. The JTRS communications program, initiated in 2002 by the JTRS Joint Program Office, defines a family of software-programmable radios that will eventually replace legacy radios. Common modular software contained in JTRS devices will implement waveforms, protocols, encryption, and communications processes. Although the JTRS program has been reorganized, there have been significant delays in the fielding of JTRS radios, and the military is mulling other options for future radio systems like DirecNet and Tactical Targeting Network Technology. However, two things are clear. The first is that whatever mobile radio communication technology is adopted by the military, it will be based on SDR. The second is that the continual crowding of the radio spectrum will necessitate more flexible, reconfigurable technology including antennas.

Accomplishments - After experimentation with several variants of the SwRI patent-protected cylindrical meander topology, the team found two promising topologies for prototyping. The first adds coils (linear loading) to some number of meander loops. The number of active coils is controlled by shorting around the others. The second topology shorts half-loops using mechanical or diode stubs. By changing the number of shorting stubs, the resonant frequency of the new, still electrically-small, smart antenna can be controlled. The VSWR results for the coil-loaded meander were generally in the range of 2 to 3 and could be improved with matching circuitry. The resonant frequency was slightly lower than the standard cylindrical meander and could be controlled in the 75- to 80-MHz range, which would normally require a dipole at least 10 times as long as the meander. The primary tradeoff for this reduction in size is usable bandwidth, but this can actually be an advantage when interfering signals are close in frequency.

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