Abstract
Microwave ultra-wideband technology has been widely adopted in instrumentation and measurement systems, including ground-penetrating radar (GPR) sensors. Baluns are essential components in these systems to feed balanced antennas from unbalanced feed cables. Baluns are typically introduced to avoid issues with return signals, asymmetrical radiation patterns and radiation from cables. In GPR systems, these issues can cause poor sensitivity due to a reduction in radiated power, blind spots due to changes in the radiation pattern and additional clutter from common mode radiation. The different balun technologies currently available exhibit a wide variation in performance characteristics such as insertion loss, reflection coefficient and phase balance, as well as physical properties such as size and manufacturability. In this study, the performance of two magnetic transformer baluns, two tapered microstrip baluns and an active balun based on high-speed amplifiers were investigated, all up to frequencies of 6 GHz. A radio frequency current probe was used to measure the common mode currents on the feed cables that occur with poor performing baluns. It was found that commercially available magnetic transformer baluns have the best phase linearity, while also having the highest insertion losses. The active balun design has the best reflection coefficient at low frequencies, while, at high frequencies, its performance is similar to the other baluns tested. It was found that the active balun had the lowest common mode current on the feed cables.
Highlights
When looking at the reflection coefficient, the long tapered balun outperformed the short taper, especially at low frequencies. This was as expected because the performance of these tapered microstrip baluns depends on the length of the structure compared to the wavelength
The phase linearity was best through magnetic baluns, as the gradient of the phase is almost constant across the operating band, with the
It was clear that the tapered microstrips cause the highest magnitude of common mode current on the feed cables
Summary
Microwave ultra-wideband (UWB) technology is commonly used in ground-penetrating radar (GPR) systems. GPR has been successfully used in a large variety of applications, such as archaeological surveys, buried pipe and cable detection, road surface inspection and planetary investigations [1]. It has been deployed to aid in the detection of buried landmines, where its use can lead to a significant reduction in the rate of false alarms [2]. Microwave UWB measurements are widely used in instruments and measurement devices, for varying applications. Other applications for UWB in the field of instrumentation and measurement technology include the use of time-domain reflectometry (TDR). One of the difficulties with the use of GPR systems, and UWB instruments in general, is the electrical feed for the antennas [1,10,11]
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