Regenerative receiver for AM band

Posted on Jul 18, 2012

The regenerative detector uses a field effect transistor (FET). Like with the better valve designs, feedback is controlled by a variable capacitor. A ferrite rod was used to allow reception of local stations without an external antenna. This FET stage forms a complete receiver on its own, but the audio output is quite low. The received audio is amplified by an NPN bipolar transistor. The gain of this transistor amplifier is sufficient to provide speaker reception of local stations in most areas. The 1k to 8 ohm transformer in the collector allows the set to be used with both low and high impedance headphones.

Regenerative receiver for AM band
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The aim of this project was to develop a simple receiver that could be built with readily obtainable parts. With the partial exception of the main tuning capacitor, this has been achieved. A 10 to 415 pF variable capacitor was used as the main tuning capacitor. These are found in valve radios and early transistor sets. They are rare new but are still common at hamfests. Their wide tuning range make it possible to cover the AM broadcast band and 160 metres without having to sacrifice coverage of the bottom end of the broadcast band. The long shafts of these capacitors also make them easier to use with vernier dial drives. Some constructors may wish to build their set now without waiting for the next hamfest. The first version of the Moorabbin used a 60/160pF plastic tuning capacitor (same as the regeneration control) instead of the 10-415 pF unit substituted later. Receiver performance with the plastic capacitor was good. The main difficulty encountered was coupling it to the vernier dial. This was overcome by extending the shaft with a 2.5 mm diameter screw and a spacer. To compensate for the lower maximum capacitance, more turns need to be wound on to the ferrite rod to cover the whole broadcast band. Details on this are given later. It is possible to get by without a vernier dial, but using the set will not nearly be as enjoyable, especially if you want to hear more than just the local stations. Though expensive, it is...

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