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.
Rockwell Regenerative Receiver
This is a simple Regenerative Tuned Radio Frequency (Regen TRF) receiver design for those just embarking on electronic construction projects; it is also suitable for Radio Club group building projects and as an introduction to short wave radio. You should be able to build it in a couple of evenings! It has two general coverage bands which can be chosen from MW (near 160m), or near the 40 or 80m amateur bands. These are chosen by alternative fittings for the four ready wound inductors. Because it is a Regen TRF, it is ideal for the powerful broadcast stations (using Amplitude Modualtion) of the Medium Wave (and the ‘short’ wave) bands, which can be easily heard with a short aerial; so you start by building it for the MW as a good confidence booster! Because it is a Regen TRF it can also copy the common modes of Morse, and phone Single Sideband used by amateur radio enthusiasts. It has three controls – the Main tuning, and RF gain control and the Regen control. It uses four transistors arranged as an RF amp, the Regen stage and two for the audio amplifier. The audio amplifier is designed specifically for modern 32R stereo phones which turn the RX on when plugged in and there is also very smooth control of the critical point of oscillation. The kit includes 5m of aerial wire for an initial ‘throw-out’ aerial which is usually enough for MW reception. The price is £20.
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.