Spatial power combining techniques for semiconductor power ampliiers

Growing demand on special signal modulation schemes in novel radars and ability to transmit relatively long pulses cause the Travelling Wave Tubes (TWT) to be constantly replaced by new concepts of power amplifiers. Solid-state power amplifiers appear to be a good candidate, however, the output power from a single transistor module is still relatively low. The only available solution is that of combining output power from a number of semiconductor amplifiers. To accomplish this, one can use, classical and well-known, twoway power combiners (like Willkinson type) or specially-designed new type of multi-input combiners. Current requirements for radar working modes imply using active antenna arrays, thereby providing multifunction ability. The active antenna concept assumes the use of transmit-receive modules (T/R), each comprising a power transistor. The overall transmitted power is then a function of the sum of the output powers from each T/R module, and the power summing operation is performed in free space. On the other hand, in some radar applications (or generally, where a power amplifier is needed, be it electronic warfare or jamming), the central power transmitter is still desired. The older applications are based on TWTs, and although they give enough power, they carry a number of disadvantages. The main are as follows: TWTs generally offer low duty factor (although some of them are approaching up to 100%), they need special power supplies, which are dangerous due to tube working voltages in the range of kVolts reliability is limited due to erosion of inner electrodes inside the TWT reliability system is two-state, a tube works or does not; any failure results in a complete malfunction of the radar. Additionally, in higher bands there are no solid-state power sources with enough power. The conclusion and current trends are that there is a constant need for combining power from a number of sources. 8