The directivity of a compact antenna: an unforgettable figure of merit

Abstract

Abstract. When an electrically small antenna is conceived, designed, simulated, and tested, the main emphasis is usually placed immediately on its impedance bandwidth and radiation efficiency. All too often it is assumed that its directivity will only be that of a Hertzian dipole and, hence, its directivity becomes a minor consideration. This is particularly true if such a compact antenna radiates in the presence of a large ground plane. Attention is typically focused on the radiator and its size, while the ground plane is forgotten. This has become a too frequent occurrence when antennas, such as patch antennas that have been augmented with metamaterial structures, are explored. In this paper, it is demonstrated that while the ground plane has little impact on the resonance frequency and impedance bandwidth of patch antennas or metamaterial-inspired three-dimensional magnetic EZ antennas, it has a huge impact on their directivity performance. Moreover, it is demonstrated that with both a metamaterialinspired two-element array and a related Huygens dipole antenna, one can achieve broadside-radiating electrically small systems that have high directivities. Several common and original designs are used to highlight these issues and to emphasize why a fundamental figure of merit such as directivity should never be overlooked.

Tunable, Electrically Small, Inductively Coupled Antenna for Transportable Ionospheric Heating

“…In addition to shifting the radiation pattern, ground plane size influences the maximum gain available with larger ground planes providing higher gain, as expected (Figure ), with a 20‐m ground plane being an acceptable compromise between size and available gain. There is a point of diminishing returns occurring at approximately 30 m, where the large ground plane is fully excited by the radiated fields (Ziolkowski, ).…”

“…Mounting the ESA on a steel barge partially submerged in seawater increases the directivity of the antenna with a narrower HPBW as a result of having a more substantial ground plane with a larger amount of power being directed upward. This is in part due to the longer path that any reverse current must travel, in addition to the radiated fields no longer having sufficient magnitude to excite the large plane (Ziolkowski, ). The barge itself provides much of the additional gain being 32 m wide, while the seawater's contribution to the overall gain is small.…”

Improved Signal-to-Noise Ratio, Bandwidth-Enhanced Electrically Small Antenna Augmented With Internal Non-Foster Elements

“…The simulated (measured) resonant frequency is f res = 455 (456) MHz with a narrow fractional bandwidth (FBW-10dB): 0.73% (0.77%). The calculated electrical size at the resonant frequency is ka = 0.44, where a is the radius of the smallest sphere that completely encloses the radiating element (the ground plane radius R 1 is much larger and, hence, has little impact on the impedance bandwidth [26]) and can be calculated as the outer radius R 2 of the CLL element. The Wheel-Chu minimum antenna quality factor corresponding to this ka value is Q Chu = (ka) -1 + (ka) -3 = 14.01 [2,4,6].…”

Metamaterial-inspired Electrically Small Platforms: Enhanced Directivity Properties

“…An Egyptian axe (electric) dipole (rectangular version) and a 3D magnetic EZ antenna were combined into a compact array to study its directivity properties [4]. This configuration is shown in Fig.…”

“…Several NFRP designs have explored the ability to achieve higher directivity from ES platforms. These include the use of electromagnetic band gap (EBG) structures as structured ground planes [2], [3] and compact arrays [4]. Combinations of electric and magnetic multipole NFRP elements have led to ES Huygens dipole [5]- [11] and multipole systems [13].…”