Simple Projects

The Bat Ultrasonic TCVR

The Bat transceiver is a simple electronic construction project using amateur radio techniques for two way communication – it is for individuals or clubs, schools, cadet forces, scout groups, etc. Because it uses ultrasonic transducers working at 40KHz, it does NOT need any special licence. Two Bats can communicate over 50 metres or more with messages sent in Morse code, or a single Bat can be used as a Morse practice aid when teaching or demonstrating Morse code to constructor groups. The transceiver can be split into separate receiver and transmitter sections which may appeal individual builders.

The transmitter is operated by a push button (or Morse key) and has a small preset to set the transmitter frequency. The receiver has its own supply switch and works on the ‘direct conversion’ principle with a tunable local oscillator for matching to the transmitting Bat’s frequency. The other control is for received audio level/volume with automatic gain reduction when your TX is operated for a comfortable sidetone level. The receiver output is suitable for modern stereo earphones, ear buds, or a small loudspeaker. The Bat uses a common digital chip and an analogue SA602 mixer with LM386 audio output stage – typical of simple radios – so it is a good educational aid! Individual kits have detailed pictorial step by step build instructions, which can be down loaded here, and there is an extra note for Supervisors running construction groups. The price is £20.

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 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 £27.

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