初めてEMEを聞いた当時は狭帯域のCWフィルタがなかったため、0dB以下の弱い信号に長時間集中すると耳がひどく疲れた。AFフィルタが欲しくなって最初に88ｍＨのインダクタでＬＣフィルタを作り(1983)、そのあとOPアンプのアクティブBPF(1985)やJM1MCFのスムージングフィルタ(1989)を製作したが、最近は46ｍＨのハイＱコイルを使ったパッシブフィルタを再び試したりしている(2006)。
今回良い機会到来とばかりに、この四つの同時評価を試みた。SGの微弱信号とノイズジェネレータの雑音出力を混合して受信機に入れて人為的に0dB以下の状況を作り、聴感による比較を行ったところ・・・・・ベストだったのはJM1MCFのスムーザ(アナログ相関器によるアベレージングフィルタ)で、背景ノイズは明らかに最も静かになる。次はハイQインダクタを使ったパッシブフィルタで、中心周波数は300Hz付近のものが了解度に優れている、という結果だった。
既にデジタルフィルタが主流の時代だが、簡単に製作できる特にアナログLCフィルタの価値はまだ十分ある。46mHコイルを使ったパッシブLCフィルタのレスポンスを実測したので、回路図とともにデータをアップします（追加2020.1.16）。
When I heard EME signal for the first time, I had no CW filters but SSB
filter of 2.7kc. Therefore the concentration on a weak signal less than
0 dB for a long time made me very tired. I came to want an AF filter, and
the passive LC filter (88mH) was made soon in 1983. An active BPF (1985)
and then smoothing filter of JM1MCF was manufactured (1989) after that.
I have made a passive filter again using high Q coil of 43mH recently.
These four filters were compared and estimated at the same time this time.The
output of a noise generator was mixed with weak signal of SG, it was put
in a receiver, less than 0 dB of situation was made artificially and it
was compared by audibility. JM1MCF noise smoother (analog correlation-type
of averaging) met that it was best. Background noise becomes quietest clearly.
The next was a passive filter using a 43mH high Q inductor, and the center
frequency of around 350Hz seemed to be excellent. A digital filter is in
the mainstream already, I think it's still worth an analog LC filter in
particular enough, as it can be made easily.
The actual response of the LC filter was measured, so the data as well
asthe circuit diagram are raised. (added on Jan.16th of 2020)
