Microstrip Rotman Lens
Image of the Microstrip Rotman Lens.

The Rotman lens (developed by W. Rotman and R.F. Turner in the 1960s) is a type of beam-forming network used in wide-angle scanners to feed antenna arrays. It is popular in RADAR applications such as collision avoidance systems in the automotive industry. As a true time delay device, the Rotman lens has wideband scanning capabilities, however it is mostly used in narrowband applications. The main advantages of the Rotman lens are its simplicity, robustness and cost-effectiveness when compared to other beam forming networks.

As a beam forming network, the Rotman lens creates a specific phase taper that can be used to feed an antenna array. The elements of the array are connected to the output ports of the lens with transmission lines of specific length. For this reason, the output ports of the lens, which lie along the ‘array contour’, are often called the array ports. The input points of the lens are known as the beam ports and they are located on the ‘beam contour’. The array and beam contours are designed so that a signal illuminating the antenna array from a specific direction will be focussed on a particular point along the beam contour. The Rotman lens is designed for 3 focal points and beam ports that do not lie exactly on one of these focal points will cause phase errors across the array aperture. The beam contour on which the beam ports lie is therefore designed to minimise the phase error caused by these inter-focal ports.

The Rotman lens is most commonly realised in either waveguide or microstrip. While the waveguide implementation provides higher power-handling capabilities, the microstrip version (implemented in Antenna Magus) has wider bandwidth, is generally lighter and less expensive and also easier to reliably manufacture.

Typical array factor at 0.9f0, f0 and 1.1f0 with beam port 1 excited
Typical array factor at 0.9f0, f0 and 1.1f0 with beam port 2 excited