Miniaturized Antenna and Wave Propagation Studies Enabling Compact Low-Power Mobile Radio Networks at Low VHF

Abstract

Reliable tactical mobile networking in cluttered infrastructure-poor environments at conventional microwave frequencies is a very challenging task, which requires innovative and unconventional networking capabilities, due to very high signal attenuation and small-scale fading. At lower frequencies, such effects are significantly reduced, which makes these frequencies more appropriate for robust moderate-rate communication over longer ranges with low transmit power. However, the prohibitively large size of conventional antennas and lack of efficient small antennas have been a major bottleneck in realizing compact systems for applications such as autonomous networking among small robotic platforms. To enable compact, low-power, low frequency wireless mobile systems, empirical studies are first conducted to investigate the propagation characteristics of the low frequency channel among near-ground nodes. From rigorous studies via physics-based simulation and extensive measurements in complex environments such as non-line-of-sight (NLOS) indoor and outdoor settings, the lower-VHF band (30 MHz – 60 MHz) is chosen due to its favorable propagation properties (high signal penetration through multiple layers of walls and very low signal and phase distortion and delay spread) compared to higher frequency bands (e.g., upper VHF and UHF bands). The second key aspect of this thesis is the design of miniaturized antennas that enable the realization of compact low-VHF communication systems for mobile networking applications. Also, methods for its bandwidth enhancement and performance characterization are examined. A highly miniaturized (0.013λ in lateral dimension and 0.02λ in height at 40 MHz) and lightweight (98 grams) antenna is designed. The antenna provides an impedance bandwidth of 0.35 % and a vertically polarized omnidirectional pattern with the maximum gain of -13 dBi, which is more than 10 dB higher than state-of-the-art antennas with comparable size. In order to further enhance its bandwidth, a new design approach for a non-Foster matching technique utilizing a negative impedance converter is presented. This approach enhances 3 dB power bandwidth with a power efficiency advantage more than twofold compared to that of the passive one. Furthermore, a very effective characterization method for low frequency antennas is developed. This method comprises two procedures: 1) non-intrusive very-near-field measurements using an electro-optical system dispensing with costly large anechoic chambers, and 2) near-field to far-field transformation to compute a far-field radiation based on the reciprocity theorem and full-wave numerical simulations. In the third part of this thesis, a compact, low-power, low-VHF radio employing off-the-shelf ZigBee technology and an optimally designed bi-directional frequency converter (UHF ↔ low VHF) is introduced, in conjunction with the antenna described above, to investigate performance of such systems. The experimental studies show a highly reliable mobile ad-hoc network with a radio coverage of more than 280 m at low power (< 10 mW) in complex propagation scenarios. This work also facilitates multi-node mobile networking at low VHF applied to networking of autonomous vehicles carrying out collaborative tasks such as autonomous exploration and mapping.