Accessories and Test Gear

Five Digit Frequency Counter

Probably the most useful addition to any rig! It has two input channels which are normally connected to the rig's VFO (60+ MHz) and CIO; these can be read individually, added or subtracted so making it suitable for additive or subtractive superhets with any IF, or for direct conversion rigs or test gear. The five displays show the MHz & KHz with a normal LSB of 1 KHz. This counter is recommended for multi-band superhets. It uses high efficiency small 0.3" wide by 0.5" high common cathode 7 segment LED displays; these are driven by DC signals from the electrically quiet CMOS counting logic using a crystal reference oscillator whose harmonics are outside any amateur band. The individual LED segment resistors are mounted immediately behind their display and are separate from the driving logic PCB. The high impedance input circuits have a typical sensitivity of 50 mV RMS at 20 MHz. PCB size 160 x 50 mm. The kit includes parts for one input buffer and suits all Somerset Range rigs. Five digits £45.

Antenna Matching Unit

This unit includes a resistive 50R matching bridge, a broadband RF transformer and the T configuration matching parts. Power is limited to around 20W by the variable capacitors! When selected for tuning up the matching controls, the bridge will always present a safe load to the TX no matter what the antenna looks like! The bridge uses a high intensity LED for indication but can also work with conventional meters or an S meter, or the Audio Extras kit. When the bridge is out of circuit the LED indicates output RF voltage. The broadband RF transformer allows the matching section to work with balanced or unbalanced feed lines. The T matching section (two variable capacitors and a tapped inductor) is very adaptable, dealing with load impedances from 10 to 2000 Ohms. To avoid winding the main inductance, it is made from a printed circuit flexible strap with switch selected taps for 10m upwards - for 160m there are toggle switched extra capacity and inductance. Price £32

This is the AMU, with the matching bridge resistors on the extreme left:-

Dual Low Pass Filter

The LPF kit contains a pair of relay selected 6 element 50 Ohm half wave low pass filters for removing unwanted transmitter harmonics. There are normally two relays, one to select each band, so that several of these kits can be cascaded for multi-band operation. Alternatively, a single relay can be fitted in a slightly different position to select directly between just two sets of filter parts, without the cascading for other bands. The standard kit of parts allows either band to be selected from 20, 30, 40 or 80m using a combination of series or parallel connected capacitors, with the inductor values being changed by altering the number of turns on the powdered iron toroids. Size 50 x 80 mm. On request, parts for any other HF band are available. £19.

Linear Amplifier

This is a broadband design for any of the 1.5 Watt transmitters of Somerset Range rigs. Producing 10W on all bands, 20 to 160m on 13.8 volts, it can be used on the higher bands with reduced output. With higher supplies to 22 volts, it can produce appreciably more output power! By connecting it just after the driving rig's output transistor(s), it makes use of the rig's existing TR switching and low pass filtering, which reduces the total complexity. The bias supply is also controlled from the main rig. The PCB is only 80 x 100 and has two 50 mm high upright heatsinks. It goes very well with the Fivehead - see photo there. Price £24.

Mini Mixer kit

This design has two main uses; firstly for obtaining stable local oscillator signals on the higher bands, and secondly to act as a receiving converter. It has a SA602 mixer with crystal oscillator followed by a buffer stage, and a double tuned RF filter using toroids and trimmers. For crystal mixing LO applications, an external VFO feeds one mixer input with the other being provided by the mixer chip's internal oscillator using an appropriate crystal to achieve the desired higher frequency VFO range - by addition or subtraction. For receiving converter use, the aerial feeds the RF filter which then feeds the mixer and outputs to the main receiver. The standard kit includes parts for all bands 20 to 80 of the RF filter; the crystal is chosen to suit either the external VFO or the receiver input band - these would often both be 80m. The kit can easily be used with other crystals and RF filter frequencies. It occupies about three-quarters of the 50 x 80 mm PCB. Remember you need to define the application when ordering! Price is £14.

Speech Processor

This kit increases the average output level of an audio signal from a microphone by clipping off the excessive signal peaks. Because it is the peak signal which determines the maximum allowable drive to a phone transmitter, by lowering the peaks in proportion to the average level, a higher average output level can be attained with an associated increase in intelligibility under difficult conditions. The kit works by modulating a 5 MHz carrier which is then clipped, the resulting harmonics (of 5 MHz) are filtered out and then demodulated back to audio. It is set up easily without special equipment and has a preset for a compression control which can be changed to a normal pot if preferred. It can be used with any make of rig - requiring only signal connections to and from the normal mike socket. It uses a single sided 50 x 80 mm PCB working on nominal 12 volts. Price £24.

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Audio Amplifier

This is a general purpose audio power amplifier for driving loud speakers or other low Z loads; it can be used for boosting the output of a headphone output receiver, or acting as a low power AM modulator. It has a low level pre-amplifier with a gain control (preset with shaft and knob) between the amps. Both amps have a bandwidth of about 3 KHz; the pre-amp voltage gain is x20, the output amp voltage gain is x50. On 12 volts, the power amp will deliver a peak power of 1.5W into 8R or 4W intermittently on 22 volts. The pre-amp input is suited to low impedance dynamic microphones. When used as an AM modulator, it will need a suitable modulation transformer for the  transmitter - consider using a small mains one! PCB size 50 x 80 mm. £14.

Two Tone signal source

This kit comprises two sine-wave audio oscillators that can be used singly as a source of audio test signals or together as a pair of unrelated tones for accurate setting up of single sideband transmitters. The two fixed frequencies are nominally 725 Hz and 1590 Hz. A three position slide switch allows either oscillator run separately or their outputs to be combined. The outputs (approx 1 volt) are balanced with a preset using an ordinary AC multi-meter and then a further optional resistor is added to bring the level down to about 10 millivolts - a bit higher than most modern microphones, with actual output level being set by a further shafted preset. This low level signal is input to the transmitter under test while the RF output is monitored with an oscilloscope. The presets can easily be changed to normal potentiometers if required. Size is 50 x 80 mm on a single sided PCB. It can use a 9 volt battery (to avoid hum loops) or nominal 12 volt supply. Price £19.

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