Miniature UWB Antenna with Embedded Inductive Loading

Many commercial and military applications require small low profile UWB antennas that operate from 50 MHz to 2000 MHz. Using conventional designs to cover such a vast frequency range with a single antenna would require an aperture size and profile which are too large for practical applications. Antenna miniaturization techniques such as dielectric [1, 2] or reactive loading [3, 4] are commonly used to increase the antenna’s electrical size without increasing its physical size. However, each of these miniaturization techniques by itself faces important performance trade-offs for large miniaturization factors. In this paper, a hybrid approach that involves both dielectric and reactive loading is used to maximize the miniaturization factor while minimizing any adverse effects. Our approach to miniaturizing an UWB antenna involves the use dielectric material on both sides of the antenna (substrate and superstrate) to maximize the miniaturization factor for a given dielectric constant [5]. In addition, the thickness of the dielectric material is tapered to suppress dielectric resonance oscillation (DRO) modes and surface waves as well as to maintain high-frequency performance [2, 5]. To maximize the miniaturization factor while minimizing the negative effects of dielectric loading, reactive loading or the artificial transmission line (ATL) concept [3] is also used. This allows us to minimize the dielectric constant which results in less impedance reduction, a minimal antenna weight and reduces possible surface wave effects. The following sections will discuss some of the issues associated with dielectric loading, the implementation of reactive loading for the spiral antenna and the miniaturization limit for the spiral antenna. 2. SPIRAL ANTENNA MINIATURIZATION VIA MATERIAL Dielectric material loading for the purpose of spiral miniaturization has its limits [5]. Specifically, while the low frequency gain is usually improved by dielectric material loading [2, 5], high frequency gain tends to decrease if high contrast material is used. To demonstrate this, we chose to simulate a four-arm spiral antenna that is 2″ wide and 0.5″ high above an infinite ground plane, with dielectric material the same size of the antenna sandwiched between. Specifically, we extract the broadside circular-polarized gain at two different frequencies and plot them as a function of dielectric constant (Figure 1). As can be seen, there exists an optimum value of dielectric constant of the loading material, above which high frequency gain starts to decrease.