Design of an electronically small wearable antenna with a reactive impedance substrate operating at 433 MHz

Abstract

The primary objective of this investigation was the development of a design procedure for a compact wearable antenna operating at 433 MHz that can be used for finding lost miners in underground mines in emergency situations. To accomplish this objective the antenna was to have a relatively high gain of 5 dBi, and a compact size at less than 0.5 λ0 by 0.5 λ0, where λ0 is the wavelength of 433 MHz in a vacuum. The chosen solution was to use a basic antenna with a reactive impedance surface (RIS) reflector. As it was known beforehand that the final product would be large, and therefore expensive, it was decided to use a low-cost material to manufacture the antenna. FR-4 was selected as the antenna’s dielectric material because it is inexpensive and readily available, at the cost of having high dielectric losses. Various RIS designs were investigated, and the square patch RIS was found to be most suitable for this application. The final RIS design was a 2 by 2 square patch RIS. It was deemed unnecessary to use a complex antenna design, due to the fact that the RIS was already rather large. It was decided to use a basic planar monopole antenna, because integrating even a basic antenna with an RIS would result in a very complex model. The final antenna was developed by first designing an RIS unit cell to have a zero-degree reflection coefficient phase at the design frequency, then the antenna was designed to resonate at the design frequency. Both the RIS unit cell and antenna were first optimised for the design frequency before being combined to form the integrated antenna. The integrated antenna was then optimised according to the design goals. Two antennas were designed, one for optimal performance, and the other to be as compact as possible. The first of the two antennas, the standard planar monopole antenna, had a wide practical -10 dB impedance bandwidth of 25.1 %, with more than 5 dBi boresight gain over the same frequency band. The second of the two antennas, the loaded planar monopole antenna, used the standard planar monopole antenna as a starting point before optimising for as compact a size as possible, whilst maintaining practical performance. The loaded planar monopole antenna achieved a practical impedance bandwidth of 5.28 %, with at least 5 dBi boresight gain over the same frequency band. The final size of the standard planar monopole antenna was 0.4 λ0 long, 0.4 λ0 wide, and 0.069 λ0 high, and the final loaded planar monopole antenna was 0.346 λ0 long, 0. 346 λ0 wide, and 0.107 λ0 high. Concerning wearability, human loading did not detune either of the antennas in such a way as to render the design frequency of 433 MHz outside the respective impedance bandwidths. The front to back power ratio, i.e. the ratio between the average power radiated into the front hemisphere and the average power radiated into the rear hemisphere, was larger than 10 dB in the two antennas’ respective -10 dB reflection coefficient bands. This means that if a human were to wear one of these antennas, the antenna’s performance would not degrade, and the majority of the radiated power will be radiated away from the human. In summary, a design procedure was developed, two antennas were designed through said procedure, and the final designs were manufactured. The manufactured antennas verified the design procedures and proved that they are practical. The final designs achieved the goals for this investigation by being compact, wearable, and relatively inexpensive.