Accessories

Three Digit Counter

A counter is probably the most useful addition to any rig! It has a single input channel that is normally fed from the rig’s Local Oscillator signal source. This much reduces the complexity (and hence cost), making it much easier to build. It shows just the important KiloHertz numbers of the input frequency – any MHz numbers overflow (normally) out of sight to the left! This means that it is suitable for direct conversion rigs, and for the superhets whose IF is an integer number of MHz – as in WE kits. It uses high efficiency small 0.3″ wide by 0.5″ high common cathode 7 segment LED displays with the least significant digit showing the units of KHz, and the top digit shows the hundreds of KHz. It uses low noise CMOS integrated circuits that count, store and decode the incoming signal for direct driving of the displays with all the advantages of low noise and only DC currents in the wires to the displays. For use with additive or subtractive superhets, it can count either up or down. It uses a crystal reference oscillator whose fundamental and harmonics are outside any amateur band. Input sensitivity is typically about 50 mV p-p at 10 MHz. The PCB size is 50 x 80 mm; supply required, 7 to 22 volts. The 21 segment resistors are mounted behind the display or end-on at the logic PCB. The kit can drive physically larger displays if required. The photo below shows the wiring between the displays and the three digit counter PCB when installed with a signal generator (an old retired kit!) that also has two extra counting chips to drive the extra MHz digits. The basic 3D counter cost is £26. Two extra displays with 2 counting chips are £5.

Antenna Matching Unit

The Mk 6 AMU uses switched toroids for the variable inductor to avoid the hassles of winding large multi-turn coils! It uses the T matching circuit, because this a very versatile circuit able to match a wide range of impedances and over a large frequency range. Winding toroids is easy and the switches are surprisingly easy to use too! The design also has a resistive 50R matching bridge, a broadband RF transformer as well as the T configuration impedance matching parts. Power is limited to around 20W by the variable capacitors! When selected for tuning up, the resistive matching bridge will always present a safe load to the TX no matter what the load from matching parts and the antenna looks like! The bridge uses a high intensity LED for match indication but can also work with a conventional meter or even in conjunction with an S meter. When the bridge is out of circuit the LED responds to RF output voltage as applied to the matching circuit. The broadband RF transformer (the white toroid) allows the matching section to work with balanced or unbalanced feed lines, and isolates the matching parts and antenna/feeder from the incoming unbalanced RF feed from the transmitter. The T matching section (two variable capacitors and the switched inductors) can match load impedances from about 25 to 2000 Ohms over the frequency range 2 to 30 MHz. The main inductor comprises seven switch selected inductors wound on T50-2 toroids so that the inductance can be increased in steps from about 0.1 /uH in a near binary sequence up to a maximum of about 20 /uH. For any particular band, feeder and antenna combination, usually only one or two (occasionally three) adjacent inductors will be selected which makes adjustment very easy by flicking probably just two adjacent switches. The price is £37.

Linear Amplifier kit

The High Power Linear has large heatsinks and uses IRF520 devices for about 15W out on 20m with 15v supplies while needing about 0.5W of drive for full output. 20m is intended to be the normal highest frequency band but on the lower bands, up to 30W is possible with about the same drive. The IRF520s really come into their own on higher supplies up to the maximum of 30v when 20W on 20m, or nearer 50W on 160m is possible (with care over heatsink temperature and that Walford Receivers are usually limited to 22v absolute max). The design includes two TR relays so that it is only activated during transmission; these relays and the bias circuits are activated when you ground your push to talk line. Following the amplifier there is a single double Pi harmonic filter for your band. The size is 100 x 180 x 35 mm over the topside of the PCB. The price is £38.

Simple Mixer kit

This Sim-Mix kit provides a simple approach for generating single band VFO signals on the higher bands where stability and chirp are often a problem. It normally takes in a low frequency VFO signal from the main rig (eg a Culm) whose original oscillator has been altered to use either a ceramic resonator (2 or 4 MHz), or a VFO operating typically near 5 MHz, to give about 50+ KHz of tuning range; this is mixed in a SA602 balanced mixer (for least unwanted output harmonics) to suit the desired output band, with the crystal oscillator frequency being selected for the desired out band and variable input frequency. After the mixer, there are double tuned filters for any band in the 20 – 80m group followed by an analogue amplifier to provide an AC signal biased at +2.5v DC that can directly drive the mixer and/or digital gates of the main rig. There are many options and please ask me about the intended use and the proposed frequency plan before ordering to ensure the correct crystals etc. are available! The Sim-mix kit costs £16.

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