Simple Projects

Bramwell Regenerative Receiver

This is a simple Regenerative Tuned Radio Frequency (Regen TRF) receiver design that is the modern equivalent of the sort of receiver that was used in the early days of radio communication.  It is suitable for group building projects run by radio clubs and as an introduction to short wave radio. The technique has very good performance for such a fundamentally simple circuit. It has two bands, primarily for amateur radio reception – the 40m band with its nearby broadcast stations, and the 80m amateur band. They use the same inductor which is adjusted for the desired part of the 40m band, and then the slide switch adds extra capacitance (with the trimmer) for the 80m band. Because it is a Regen TRF, it can be used for the powerful AM broadcast stations near 7 MHz as well as the common amateur modes of CW (for Morse code) and SSB (for phone) which are used on both bands. It has three main controls – the Main tuning with the large knob, the audio gain control (AFG) on the left and the very smooth Regeneration control on the right. There are also two preset controls for the RF gain and a master ‘oscillation’ control – neither of which need frequent adjustment. It uses three junction type field effect transistors in the RF section; these are arranged as an RF amp, the Regen stage and a buffer stage to drive the strong, but sensitive, full wave diode detector that feeds the integrated circuit output audio amplifier which can drive a LS or modern stereo 32R phones. This combination of controls provides excellent control over the receiver’s operation to suit radio conditions. There is a power on/off switch for the 9v PP3 battery (not supplied). The price is £24.

The West Pennard DC RX

This is a very simple direct conversion receiver for those just setting out on building analogue radio equipment. It can be built for either the 40m or 80m amateur bands. It has a simple RF filter that feeds the twin junction Field Effect Transistor (JFET) product detector and audio pre-amplifier; this is followed by the AF gain control potentiometer and the audio output power amplifier, than can drive phones or a small loud speaker. The product detector is also driven by another JFET in the tunable local oscillator that determines the reception frequency; this also make it easy to adjust with a general coverage RX or another previously adjusted similar RX. To make it easier to tune accurately with the single variable capacitor control, the tuning is usually restricted to a section of the chosen band – say that for the CW section or the phone section as you prefer. For ease of construction, it uses a single sided PCB and a pictorial approach in the instructions to help you locate the correct position for individual parts. Although designed for use with a 9 volt PP3 battery (not supplied), it can use a nominal 12v – this is relevant when a transmitter is added later! (The matching simple crystal controlled East Pennard CW transmitter is being proved.) The price of the West Pennard is £25.

The Ivel Receiver

This is a direct conversion receiver for any single band 20 to 80m using the classic combination of an SA602 mixer chip followed by audio stages with filtering for phone and CW; and finally an audio power amplifier output stage, which can drives phones or a small loud speaker. It is normally built in the small upright format which is easy to use and ‘calibrate’ with a vertical front panel. The PCB is single sided to keep things easy to locate the correct holes for parts and for ease of soldering. The photo above shows how simple it is! The RX design is suitable for serious contacts and incorporates several features like twin tuned RF bandpass filters and an audio filter for CW, that would normally belong to a more elaborate rig; for tuning, it has both main Tuning by a PolyVaricon capacitor, as well as a Fine control so that resolving SSB becomes much easier. The rig’s product detector uses the SA602 mixer/oscillator chip but with an external Variable Frequency Oscillator using a discrete JFET to provide better stability than is possible with simpler designs using an oscillator in the SA602. The mixer chip incorporates a balanced Gilbert cell transistor arrangement for good rejection of unwanted signals. The mixer is followed by an audio pre-amplifier, using one section of a TL072 dual op-amp with a bandwidth suitable for normal phone (SSB or DSB) contacts; the other section of the TL072 is a second order CW filter for Morse. This has a peaked low pass response with its maximum at about 725 Hz and sharp attenuation for high frequency signals. This suits the typical CW beat note pitch that many operators like to use in the 500 – 800 Hz band. The CW filter is selected by a front panel switch which feeds the front panel AFG control and then the LM380-8 output power amplifier for phones or a speaker. Adding a transmitter (Ilford for CW or the Ilton for DSB phone) is relatively easy but does take the project into the Intermediate category! Start with the receiver alone and then add the transmitter later if you wish. If you are unclear which band is best to build it for, the 40m band often a good compromise with signals from UK and European stations. Price is £29.

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