System and Circuit Models for Microwave Antennas and Communication Channels

Antenna characterisation for modern communication systems involves obtaining the reflection coefficient (S11) using a three-dimensional (3D) electromagnetic (EM) simulator and conducting experimental verification using a network analyser. The gain and radiation patterns are then obtained at certain frequencies from the simulation or measurement. These parameters are insufficient to completely characterise the antenna as a two-port network, however, because radiation pattern simulation and measurements require a large number of frequencies. The conventional design also does not provide essential antenna parameters such as S11 phase and the complex transfer parameter, S21. These parameters are important in digital communication systems (DCSs), which are typically simulated and studied in the time domain. The aim of this thesis is to derive circuit and system models for antennas and to then use these derivations in different applications. These derivations are useful for characterising antennas in point-to-point (P2P) and domestic applications. Initially, wideband Vivaldi and narrowband patch antennas are characterised in this work using three techniques: (1) deriving an equivalent circuit model (ECM), (2) processing the measurements between two identical antennas in an anechoic chamber, and (3) applying the Hilbert transform to the amplitude of S21. These techniques are then used to obtain the S21 of the antenna. The S21 values obtained through these techniques are then compared to verify their validity and to show their difference when realising a low S11 in the conventional design. The ECM is also derived to characterise the antenna behaviour in the frequency domain. The thesis also presents the characterisation of antennas and channels from only one S21 measurement in a multipath channel, which will allow designers to obtain the S21 of an antenna without using an anechoic chamber. The antenna and channel responses in the time domain are separated from the time response, which is derived from the antenna's frequency measurement. The procedure is then validated using wideband and narrowband antennas to establish measurement limitations in multipath channels. The antenna's frequency response can be characterised in terms of effective aperture and gain. This characterisation is important in P2P communications, as the frequency response can vary due to changes in the radiation pattern in the physical channel. This thesis presents a process to investigate the frequency response of a wideband antenna to identify the best orientation of the antenna for P2P communications. During this process, the antenna's effective aperture and gain are predicted for each orientation. The frequency-variant radiation pattern is ascertained from the S21 phase obtained from the ECM for the antenna. For each orientation, the S21 phase is analysed based on the linear, minimum, and all-pass phase components, which then enables the derivation of an ECM. The process is validated using a non-minimum-phase monopole ultra-wideband (UWB) antenna. An antenna is also modelled as a system to predict its effects when used in a DCS. A time-domain system model is derived to enable estimation of the antenna effects in a DCS. The results show that antennas cause symbol scattering and contribute to the error vector magnitude (EVM) and the bit error rate (BER). The technique is applied to different types of antennas with varying orientations and environments and with added noise. The modelling of an inverse antenna system to de-embed the antenna is also presented in this work for P2P communications to characterise channels and to compensate for antenna effects within DCSs. A stable inverse antenna system is derived for a Vivaldi antenna to be applied to the measurements within two different office environments to characterise the two channels. The technique is also applied to a commercial antenna to improve system performance and to reduce the BER caused by the antenna in DCSs. The results show that the inverse antenna system can be useful for several applications in DCSs. Finally, an antenna's total radiated power from measurements in a reverberation chamber built with aluminium walls is computed using an empirical equation, which enables the prediction of antenna losses and antenna characterisation for domestic applications.