Intermediate Projects

The Ford

This is a single band Direct Conversion Receiver that has been designed for builders wanting their first serious amateur band receiver. Normally I would suggest that it is built for the 40m band as this often has stations that are national as well as much longer distances when the propagation conditions are good. It is laid out with plenty of space around all parts and can run from a 9 volt PP3 battery or from an external nominal 12 volt supply. Much thought has been given to making it easy to build; there is a very detailed Project Construction Manual kindly compiled by Steve G0FUW who has run many Buildathons in recent years. The Manual can be downloaded here! (You may need to minimise this website in order to open the pdf file.) The actual kits are supplied with other important information such as the circuit, parts list, and the options for 20 or 80m bands. The PCB is 100 x 160 mm and only has tracks on the underside. The design has an RF gain control followed by a double tuned RF bandpass filter which feeds the product detector or mixer. The other input of this mixer (twin JFETs) is driven by the Local Oscillator that runs at band frequency and has both a Main tuning control (with the large knob) and a Fine tuning control to make it easy to resolve stations. Being a direct conversion RX allows it to receive Morse or normal phone SSB or even DSB! An Audio Frequency Gain control follows the mixer and feeds a two stage audio amplifier that is designed for modern lightweight 32R stereo phones. The provision of both RF and AF gain controls makes it much easier to deal with BCI which often plagues simple designs! Also because it has an RF amplifier stage, there is far less LO radiation which much reduces unwanted hum effects that are often problematical with DC designs! (Please consult me first if you are contemplating this kit for a Buildathon!) The price is GBP 22.

The Culm

This is a new CW TCVR for any single band 20 to 80m. On 80m it uses a ceramic resonator for the VFO giving several 10s KHz tuning range. The RX is direct conversion with a PolyVaricon for the main tuning and a voltage controlled diode for the Fine tuning beat note offset, which is automatically cancelled when you go to transmit. The RX has an RF amp with an RFG preset, followed by a double tuned RF filter and then a pair of JFETs for the product detector. This is followed by the first audio amp and a humped low pass CW filter centred on 725 Hz , which feeds the AFG pot and the audio power amp for a LS or phones. For transmission, the VFO feeds digital gates for the RF keying which then drive a buffer stage and a pair of BS170 MOSFETs in the output stage for a nominal 1.5W (with a 13.8v supply) on any band to 20m. The transmitter output has twin pi low pass filtering and the TR control circuits automatically disconnect the LPF from the RX RF amp when transmitting for click, and thump-free full break in changeover! There is also a 725 Hz sidetone oscillator, with adjustable level, that feeds into the RX audio amp. The 3.58 MHz ceramic resonator can be replaced by a crystal of your chosen frequency for the higher bands where ceramic resonators are not suitable, or by a 4 MHz resonator that can drive the Sim-mix kit fitted with a crystal to for any band output (including 5262 KHz!) up to 20m - this has the advantage of giving approx 50 KHz tuning range instead of the very limited range with a crystal. The photo below has the Sim-mix piggy back on a 40m Culm. The plain 80m Culm is pictured on the Home page. The Culm price is GBP 42.

The Sparrow

This is a new crystal controlled transmitter that was originally designed for demonstrations of simple AM receivers in modern buildings, where reinforced concrete forms a Faraday cage preventing reception of external signals. But it can also be used as a normal transmitter with large external aerials working with Spaxton, Rockwell and maybe Ford RXs. It provides RF signals for speech using Amplitude Modulation; or Morse either by injecting a keyed 725 Hz audio tone into the AM speech amplifier for MCW, or ordinary plain interrupted carrier CW. The peak output power is nominally 1.5W on 13.8v supplies so the corresponding AM carrier power is 0.35W. The standard kit is for 160m using an 1843 KHz crystal with an IRF510 in the RF output stage; other bands are possible - please ask me about them. The switch changes between the MCW and ordinary CW modes.  The sidetone oscillator is activated for either type of Morse, with an optional audio output for feeding into an external amplifier, even when the RF parts are switched off for simple Morse audio demonstrations. The design includes a TR antenna relay and adjustable level muting for receivers where it is desirable to be able to hear a beat note from your own transmitter. It has socket inputs for a dynamic mic, with PTT switch, and a straight key. The RF output filter assumes a 50R load - real or dummy! The price is GBP 25.

 


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