In systems like the magnetron or klystron, this is accomplished with another resonant cavity. The receiver was a super-heterodyne type with a low-power magnetron serving as the local oscillator. The first major improvement was the split-anode magnetron, also known as a negative-resistance magnetron. However, the two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency. This consisted of two radar antennas driven from a common magnetron transmitter. To overcome frequency instability of the magnetron, pulse modulation was used. Raytheon's research on the magnetron tube revealed the potential of microwaves to cook food. The Germans also recovered a cavity magnetron from a downed British bomber. Philips magnetron with better frequency stability was used. The cavity magnetron was also of interest to Yagi. H ions are generated with a magnetron or a Penning source. "Ku-Go" which aimed to employ microwaves created in a large magnetron. Another hazard is the resonance of the magnetron tube itself.