The main topic examined in this thesis is the development of planar antennas for observations at W band frequencies. A large portion of this analysis is based on the specific development of devices as part of an ESA research package entitled “New Technology High Efficiency Horn Antennas for CMB Experiments and Far-Infrared Astronomy”. The development of W band focal plane pixels (planar antenna with small lenses) was part of the research in this work package. Several planar antenna designs are modelled and optimised in CST Studio Microwave Suite, a commercially available computer modelling software used extensively in order to predict the device performance around 100 GHz. The final designs were manufactured and their beam patterns were measured in the Vector Network Analyser (VNA) setup. The main body of this research consisted of the development and analysis of patch antennas. The design of back-fed and side-fed patch antennas are modelled in the CST work environment, manufactured in-house and measured with the VNA. A number of lenslet (small lenses for each planar antenna in the array) designs constructed from High Density Polyethylene (HDPE) are developed as part of this ESA contract in order to develop a lens array for a planar antenna array. A particular focus was put on reducing the potential crosstalk between neighbouring pixels and optimising the lens shape. The lenslets examined included a hemisphere, a cylindrical and a plano-convex lens. A novel truncated plano-convex lens was also analysed for the task of reducing crosstalk between neighbouring pixels. Plano-convex lenses with cleaved sides (referred to as a truncated lens) were manufactured and tested with the VNA. The crosstalk signal level caused by these lenslets between neighbouring pixels were considered and measured. Additional topics developed include the analysis of a multi-moded terahertz horn antenna measured in an experimental setup in Cardiff University using GBMA (Gaussian Beam Mode Analysis). This 2.7 – 5 THz pyramidal horn antenna couples to Transition Edge Sensor detectors (TESs) and was placed in a cryogenic chamber to measure the farfield pattern. The antenna was illuminated by a terahertz source through a window in the cryostat. A GBMA model was extended to include truncation of the beam at this window in order verify no loss of signal and to ensure all power propagated though this window.