Abstract
The present dissertation deals with electrically small loop antennas, namely antennas which are small compared to the wavelength. The single turn loop antenna is an effective ‘small’ transmitting antenna. Small being defined as having a maximum physical dimension, which is less than the wavelength of operation. Electrically small antennas are those whose overall length is less than one-sixth of a wavelength. Most applications of loop antennas are in the HF (3-30 MHz) VHF (30-300 MHz) and UHF (300-3000 MHz) bands. The fundamental limitations, namely the antenna Q, of small antennas derived in the past by several authors, are reviewed and unified, yielding simple formulae for both the antenna Q and bandwidth of an antenna as a function of its size. This theoretical work enables us to understand deeply the physical phenomena occurring in small antennas, such as the importance of the stored reactive energy or the increase of losses when an antenna is miniaturised. An important point regarding electrically small antennas is that their performance is closely related to their electrical size. The product of the bandwidth and the gain is a function of the size of the antenna, so that the gain can only be increased at the expense of the bandwidth, and vice versa. Furthermore, an electrically small antenna is highly dependent on the environment in which the antenna operates, which must be taken into account. The environment comprises both the device on which the antenna is mounted and the surroundings. This thesis describes a technique to determine the radiation efficiency of an electrically small antenna, primarily by measuring the radiation Q of the antenna at the input terminals at a number of frequencies. This is called the Q-bandwidth method. From this all antenna mode radiation resistances, the antenna loss resistances, the antenna efficiency, and input impedance can all be derived from the measurements. The results show that traditional formulas for antenna Q and radiation resistance do not predict what is measured in the laboratory or in the field. The measurements show that the fundamental formula for electrically small antennas has been breached. Furthermore thermal heat balance and field strength measurements are used to confirm the Q-bandwidth method. Computer simulation in general agrees with current theory but not with the measurements as the current theory does not predict the extra radiation modes described in this thesis. This thesis addresses some of the discrepancies.