Determination of the amplitude and phase of the fundamental and harmonic excitation components of active phased array antennas

The Office of Telecommunication Policy Manual of Regulations and Procedures defines the spectral quality of the emission from radars. Design and evaluation of active phased array antennas to comply with spectral emission requirements involves a determination of the amplitude and phase of the fundamental and harmonic excitation components of the array elements. A study has been made of microwave amplifier modules designed for phased array antenna service. Output amplitude and phase behaviour has been measured under mismatched load conditions at fundamental and harmonic frequencies. A bridge has been used that is capable of being balanced at fundamental, second and third harmonics. Reference signals at these frequencies are adjusted in amplitude and phase to cancel the corresponding components produced by the amplifier under test. The amplitude and phase variations of the waves travelling out of the amplifier are measured as a function of load mismatch. A standard test load is used to present to the amplifier output, the impedances that lie on a selected series of VSWR circles on a Smith chart. The test load is not a replica of the dynamic impedance of any particular chosen element in a phased array but the data gathered can be used to provide amplitude and phase data for radiation pattern calculations if the dynamic impedance of each antenna element in known. Results obtained using the load are presented as equi-power and equi-phase contours in the overall region of load variation. - and -band bipolar transistor modules designed for phased array antenna service and various experimental -band TRAPATT amplifiers have been studied. In the modules it is found that harmonic levels are below 35 dB down on the fundamental and are subject to large variations with load change. In some cases the variation of harmonic output is a function of the load impedance at the fundamental but more frequently it depends upon the impedance presented at the harmonic. In the case of TRAPATTS it has been essential to operate them in a two circulator assembly. Less variation of harmonic level is observed with load change. Levels only 23 dB down on the fundamental are found in some units whereas in others greater than 50 dB has been observed. In the for- mer higher level case it was found that behaviour was not affected greatly by changing the diode in the circuit and it is inferred from this that behaviour regarding harmonic levels in the output is mainly determined by the microwave circuit.