007: 432MHz BPF (4-pole Butterworth)

-3dB passband 432±1.5MHz, insertion loss 6.5dB, -20db band width 424MHz - 437MHz

Some data of the transmitting spectrum was requested by the Radio Regulatory Bureau in Nagano when I applied for 500W license of 432MHz EME. As I knew that the spurious suppression of my transmitter was not perfect, then I made this filter soon to get a clean 20 Watts to drive the final power amplifier of K2RIW. The passband that I needed was 432MHz±500kc, and steepness was necessary. It’s because I believed that a good BPF contribute not only to a clean spectrum of transmitter, but also to the improvement of receiving capability in the cross-modulation point of view.

First a copper wire of 2mm dia. was used as a resonance element, but it wasn't possible to get a steep resonation and I was not satisfied. After all, the expected good resonation was obtained in case of adopting 75 Ω line of the copper strip which can maximize the unloaded Q. An electrical coupling of an adjacent resonator is done with a small copper tongue at the hot end, and I remember the setting and the adjustment was rather critical than a 144MHz BPF. The pass band of the completed filter was narrow and steep enough. It was very good to know the signal of FM repeater of my local has disappeared, which was coming through on the band edge till then. (The data which was taken recently shows that there is more than 30dB of attenuation on 439MHz.) The spurious signal really disappeared in the output of my transverter as expected. Though there were 6dB of insertion loss on 432MHz, the level adjustment was easy because there was originally ATT of 10dB being set between my transverter and the driving amplifier (M57716). Making was in 1989.
-3dB passband 432±1.5MHz, insertion loss 6.5dB, -20db band width 424MHz - 437MHz





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