Reflections in microwave antennas and their harmful effects

In a microwave antenna of the focusing type, the reflecting or refracting element is illuminated by power from the waveguide assembly. At any boundary surface of the focusing element, some of this power may be reflected and return to the waveguide assembly. Here, this reflected power may cause troubles such as instability of the transmitting oscillator. Formulas for computing this reflection are presented, and techniques for its reduction are reviewed. There is also reflection from the waveguide assembly when it receives power by way of the focusing element. This reflection is composed of two components: that caused by incompletely matched plumbing elements, and that caused by scattering from the feed. The feed reflection is determined by the aspect presented by the feed to the incoming focused wave. The combination of the reflections from the focusing element and from the waveguide assembly causes a sinusoidal variation of antenna gain with frequency, at a rate dependent on the distance between the two reflections. In a lobing antenna these reflection, if asymmetrical, formation, which likewise varies sinusoidally with frequency. Formulas for computing these effects are presented, and techniques for their reduction are reviewed.