Airborne Phased Array Radar Transmit Aperture Design for a Two-Way Pattern

The Airborne Phased Array Radar (APAR) is being developed by the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR). Four of these 4368 element C-band arrays will be placed on a C 130 plane. Its mission requires a two-way antenna pattern with a 3 dB beamwidth of less than 2.2^o, sidelobes less than -40 dB, and an integrated sidelobe ratio (ISLR) less than -35 dB.

To obtain the lowest possible peak sidelobe level (PSLL) and integrated sidelobe ratio (ISLR) in the two-way pattern, the first null in the transmit pattern should be at the location of the first sidelobe of the receive pattern. This same concept was used to lower the peak sidelobe level of the two-way pattern by thinning the transmit pattern while applying a low sidelobe Taylor taper to the receive pattern.

Thinning approximates a low sidelobe taper. This one-bit amplitude approximation limits the PSLL and causes an increase in the root mean square (RMS) sidelobe level (SLL). Its advantage on transmit is that the power amplifiers (PA) can be operated at their peak efficiency. Whereas a low sidelobe taper based on a Taylor window would have the PAs operating in the linear region of the amplifier where the efficiency of the PAs is reduced. In a system such as APAR that is operating on an Airborne platform there are available power limits. It is desired to operate the PAs at peak efficiency to reduce wasted power and reduce the amount of heat that must be dissipated. This allows the power budget to be more efficiently used for parameters like increased duty cycle.

Since prime power availability and cooling capacity are important factors for active electronically scanned arrays (AESAs), we examined the projected peak power consumption for the two different transmit apertures. We assumed that the elements have two Qorvo QPA1019 power amplifiers (PAs). Each PA has an output of 10 watts peak power when fully driven. The thinned transmit array has 76% of the elements fully driven, with the remainder powered off. A six-bit, 31.5 dB attenuator at each element set the quantized amplitude weights for the 20 dB Taylor taper in 0.5 dB steps. The Gallium Nitride (GaN) technology in the QPA1019 has a power added efficiency (PAE) of 42% over our desired frequencies of operation. The PAE is reduced significantly (~8% near the edges of the array) when the amplifier is operated in its linear range, as in the case of the Taylor tapered transmit array.

A 20 dB Taylor implemented with 6-bit attenuators had better PSLL and ISLR performance than a similar 1 bit (on/off) thinned transmit aperture. However, due to the decreased efficiency in the PAs a 72% thinned aperture was able to have a nearly ~11% (~0.5 dB) higher peak transmit power given the same input power.

This work was supported by National Science Foundation through the Mid-scale Research Infrastructure Program under grant #AGS-215337 and by the National Oceanic and Atmospheric Administration through the Oceanic and Atmospheric Office of Weather and Air Quality under grant #NA23OAR4590125.