Low Pass Filter

While the exciter does have an internal low pass filter on its output, the 90 watt power amplifier does not. To assure adequate harmonic filtering, this tubular low pass filter was employed after the PA. This particular filter (courtesy of Mark, KD6WLY) was removed from a Motorola PURC5000 high-power paging transmitter.

Network analyzer plots of the filter showing a 0.2 dB insertion loss and 80 dB rejection at the second harmonic.