High Power Microwave Generation with Pulse Compression

Chirp was first used by Bernard Oliver in an internal Bell Lab memo, Not with a Bang, but with a Chirp, in 1951. It describes how radar detection ranges are increased without loss of resolution or increase of peak power. Transmitted radar pulses, with long periods T and low peak power Chirp have Frequency Modulated FM carriers that sweep frequencies across bandwidth B during each period. The pulses are backscattered from buried ancient towns, buried water pipes, or other structures to radar receivers and compressed with matched filters into sin X X with high peak powers Bang. Pulse compression ratios are ratios of period T to collapsed pulse width 1B, or BT. Typical BT ratios vary from values of 10 to 100,000. The product BT is the increase in peak power due to pulse compression. With BT equal to 20, 100 watts before compression increases to 2 kilowatts after compression. Chirped pulse compression is achieved in transmit modes with wideband antennas, such as log periodic dipole arrays LPDA, where the matched filters are the antennas. Computer generated curves of amplitude and phase spectra are presented for variable Chirp signal pulse width, reduced periods after compression, and frequency intervals during a period of the original pulse. Pulse compression ratios BT from 20 to 120, in increments of 20, are seen with one microsecond pulse periods T, and frequency bandwidths B of 20 to 120 megahertz in 20 megahertz increments. The compressed pulse period decreases from 50 nanoseconds BT 20 to 8 nanoseconds BT 120. Differences in these and previously published curves were found because use of more accurate computer generated Fresnel Integral values. The effect of Doppler frequencies on Chirped pulse waveforms is seen with 10 to -10 megahertz Doppler frequencies in 2 megahertz increments. This family of Doppler shifted outputs of the matched filter are like the ambiguity function for the linear FM signal.