The Shipham Mk 2 Receiver
This project arose from a visit to The Nation Radio Centre at Bletchley Park, where staff suggested that very simple introductory projects need to be followed by something more advanced that would lead into amateur radio. The Yeovil Club kindly expressed an interest and have successfully built several of an earlier version. The receiver is initially built for the high power broadcasting stations that operate in the Medium Wave band below 1.5 MHz using Amplitude Modulation; this requires just two transistors for the RF amplifier and detector, with an integrated circuit audio power amplifier to drive a small LS or modern ear buds! With a few metres of wire aerail (ideally outside), it should be possible to comfortably receive 3 stations anywhere in the UK. The MW parts are mainly on the left hand side and include a volume control with ready wound inductors to avoid difficult hand wound high values! The instructions are very detailed for novice builders.
The next stage is to add a band switch, one more transistor and the parts on the right that turn it into a Regenerative RX for short waves – typically the 40 or 80m amateur radio bands depending on the setting of the trimmer capacitor. This enables it to receive morse (CW), phone single sideband, or amplitude modulated signals with much increased selectivity and sensitivity depending on the setting of the extra Regeneration control. Two or three evenings should be enough to build it! You can even add the Ilford CW transmitter (or possibly the AM Bradney TX) but these do have to be operated as ‘seperates’. The price of the two band Shipham is £27.
The Stathe RX
This is another new project but it is not quite ready yet. It is a very simple direct conversion receiver for any of the main HF amateur radio bands. It will be in the flat format like the Shipham above, but being a DC RX, it will be rather easier to operate! Watch this space!
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 £31.