JF1RNR／今井さん設計のシンプルな3石DSBトランシーバには惹かれるものがあります。子供の頃のゲルマラジオや超再生トラに通じるためでしょうか。

(1)AF部をreflex化し、トランジスタを一個省略（つまり2石に）できないかトライしました。しかし送信出力は10dB低下し、受信感度も同様に10dBほど悪化。RFゲインは25dBで変わらないのに、理由はよく分からないがAFゲインが10dB下がってしまう(33dB→23dB)模様。残念ながら実用性は乏しいという結果です。

(2)RFトランジスタの選択は、受信感度優先で実施。ｆT≧500MHzの手持ちトランジスタの中では2SC784の受信感度が他より10dB良く、-10dBμV以下のキャリアが十分判別できます (スーパー並み！)。それで試しにNF測定用のノイズソースをつないでみたら、小さな音だがちゃんとノイズが聞こえるではありませんか。これには感動しました。ENR10dBのノイズジェネレータを付けて仮に3dBのYファクタが得られたとすると、NFは概ね10dB。つまり「サッサッ」と断続ノイズが聞こえること自体が、かなり高感度であることの証左だからです。

(3)イヤフォンは最近のスマホ用が好適です。昔のマグネチック型は感度が悪く、上記N/Gのノイズはまったく聞こえません。実測比較したところ、前者の音声変換効率は後者より10dB以上高く、それがゲインに乏しいこのリグの実用感度を押し上げる確実な要因になっています。

　回路部は70年代に使った東光の4回路プッシュSWを中心にして組み立て、それを菓子の空き缶に組み込みました。送信時のMixer(SBM)に入るAFレベルは-10dBm以下が妥当なので、この3石トラのRF出力は+10dBmが平均的なところです。つまり片サイドバンドの出力数ミリワットのQRPPPですが、見通しの良い高地移動運用ならスリリングに楽しむことができるでしょう。Sメータが振れない様な弱い信号でも内容は100%コピーできるので、かなりのDXもできると思います。たった3石の原始的なセットですが、無駄が無い分余計に奥深いものを感じさせるリグです。

Total gain of 55dB, 50.253～50.160MHz, DC12V25mA, carrier suppression -32dB, W80×H120×D38 mm



The simple 3-transistor DSB transceiver designed by JF1RNR/Imai-san has something that attracts me.

(1) I tried to make it "reflex" and omit one transistor. However, transmit power was reduced by 10 dB and receive sensitivity deteriorated by the same amount. RF gain remains the same at 25dB, but for some reason AF gain is 10dB lower (33dB to 23dB). Practicality seems to be poor.

(2) I selected RF transistors with priority on receiving sensitivity. Tested with on-hand transistors of fT≥500MHz, the reception sensitivity of 2SC784 is 10dB better than the others, and -10dBμV or less carrier was well discernible (as good as super-het!). And I hooked up a noise source for NF measurement to try it out, and I could hear the noise. I was very impressed. If a noise generator of 10dB ENR is attached and a Y-factor of 3dB is obtained, the NF is approximately 10dB. In other words, the fact that intermittent noise can be heard is in itself proof that the sensitivity is good enough.

(3) Recent earphones for smartphones are excellent. On the other hand, old magnetic type of 20th century has poor sensitivity and the above N/G noise was not audible at all. A measured comparison shows that the audio conversion efficiency of the former is more than 10 dB higher than the latter, which is a definite factor that boosts the practical sensitivity of this rig of small gain.

The circuit part was assembled around a Toko 4-circuit push SW used in the 70's, which was then built into an empty candy can. Since an AF level of -10dBm or less into the mixer (SBM) during transmission is reasonable, an RF output of +10dBm is average for this rig. I was able to QSO with local stations 6-15 km away, but there were times when I was on edge due to urban noise. Since the output power of one sideband is a few milliwatts, it is suited for thrilling portable operations from high land of good visibility. Even with a weak signal that the S-meter does not swing, it is able to copy 100%, so I think DX is quite possible. This is a simple set of only three transistors, but it is a fascinating rig for QRP enthusiasts.

Total gain of 55dB, 50.253～50.160MHz, DC12V25mA, carrier suppression -32dB, W80×H120×D38 mm

 

 

 

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