Frequency-Agile Coaxial Magnetron
A frequency-agile coaxial magnetron differs from a standard tunable magnetron. The frequency agility (FA) of a coaxial magnetron is defined as the capability to tune the output frequency of the radar with sufficiently high speed to produce a pulse-to-pulse frequency change greater than the amount required effectively to obtain decorrelation of adjacent radar echoes. The frequency-agile magnetron, together with appropriate receiver integration circuits, can reduce target scintillation, increase the detectability of target in a clutter environment, and improve resistance to electronic countermeasures (ECM). The increase of the pulse-to-pulse frequency separation will improve the radar system performance. Furthermore, the greater the pulse-to-pulse frequency separation, the more difficult it will be to center a jamming transmitter on the radar frequency for effective interference with system operation.
The frequency-agile coaxial magnetrons are classified into three types:
1. Dither magnetrons: The output RF frequency varies periodically with a constant excursion, constant rate, and a fixed center frequency.
2. Tunable/dither magnetrons: The output RF frequency varies periodically with aconstant excursion and constant rate, but the center frequency can be manually tuned by hand or mechanically tuned by a servomotor.
3. Accutune magnetrons: The output RF frequency variations are determined by the waveforms of an externally generated, low-level voltage signal. With proper selection of a tuning waveform, the accutune magnetron combines with features of dither and tunable/dither magnetrons, together with a capability for varying the excursion, rate, and tuning waveform. Figure shows an accutune magnetron.
Figure: X-band accutune magnetron YMX-1430
The X-band frequency-agile coaxial magnetron VMX-1430 is a typical agile magnetron. Its pulse voltage is 15 kV, and pulse current is 15 A. Its maximum duty cycle is 0.0011, and accutune range is 1 GHz. Its center frequency is 9.10 GHz, and peak output power is 90 kW. The agile rate and agile excursion are shown in Figure 2. From Figure 2, it can be seen that the agile rate is the number of times per second that the transmitter frequency traverses the agile excursion and returns to its starting frequency. Similarly, the agile excursion is defined as the total frequency variation of the transmitter during agile operation. The number of pulses that can be effectively integrated cannot be greater than the number of pulses placed on the target during one scan of the antenna. Therefore, the antenna beamwidth and scan rate become factors that must also be considered in determining the integration period of the radar. Consequently, a design value for agile excursion can now be expressed in terms of radar operating parameters as
Figure : Agile rate and agile excursion