In practice, the radiation efficiency of antennas is often measured by the anechoic chambers. For this method, it takes plenty time and cost. In order to bring down the measurement costs, another alternative method was proposed and published by H.A. Wheeler in 1959. The Wheeler Cap method can measure the radiation efficiency of the antenna quickly and easily during the design and development stage. It has the many benefits, such as simple construction, easy and fast to implement, lower cost and accurate enough compared to other methods. Moreover, it is a reliable and extensively used method for measuring the antenna’s radiation efficiency. The basic concept about Wheeler’s method is to use the "radiansphere" which is the boundary between the near field and the far field of any small antenna to measure the radiation efficiency of antenna under test. The radiation efficiency is obtained by performing measurement in two steps i.e. measuring the antenna under test (AUT) without and with conducting radiation metal shield. The main objective of this thesis work is to design and construct a Wheeler cap test set-up. This thesis is performed to study a modified Wheeler cap method which is based on constant power loss principle and according to this principle the loss resistance of AUT remains constant whether the Wheeler cap is placed or removed, it means the AUT has zero or finite radiation resistance. Thus, the radiation efficiency can be measured using the return loss magnitude with or without Wheeler cap. The equivalent RLC circuits of AUT in the free space and with a Wheeler cap have been theoretically reviewed and analyzed. At the same time, the mathematical expressions of radiation efficiency were presented in terms of return loss using equivalent RLC circuits. In this thesis, four types of antenna have been selected to find their radiation efficiency by the Wheeler cap method using HFSS software. To compare the simulation and measurement results of radiation efficiency, lossless wire monopole antenna and lossy loop antenna are simulated, fabricated and measured. It is found that the measurement results correlates with the simulation results.

In many wireless devices, antennas occupy the majority of the overall size. As compact device sizes become a greater focus in industry, the demand for small antennas escalates. In this thesis, detailed investigations on the design of a planar meandered line antenna with truncated ground plane and 3D dipole antenna at 2.4 GHz (ISM band) are presented. The primary goal of this research is to develop small, low coast, and low profile antennas for wireless sensor applications. The planar meandered line antenna was designed based on a study of different miniaturization techniques and a study of the ground plane effect. The study of the ground plane effect proved that it has a pivotal role on balancing the antenna current. The study of the miniaturization process proved that it affects directly the gain, bandwidth, and efficiency. The antenna efficiency and gain were improved using the truncated ground plane. This antenna has a measured gain of -0.86 dBi and measured efficiency of 49.7%, making it one of the efficient and high gain small antennas. The 3D dipole antenna was designed using a novel method for efficiently exploiting the available volume. This method consists of fabricating the dipole on a cube configuration with opening up the internal volume for other uses. This antenna was tested, and it was found that this antenna has good radiation characteristics according to its occupied volume. Ka of this antenna is 0.55, its measured gain is 1.69 dBi with 64.2% measured efficiency. Therefore, this design is very promising in low-power sensing applications. A Wheeler Cap was designed for measuring the efficiency and the 3-antenna method was used for measuring the designed antennas gain.