This is a three band direct conversion receiver for 20, 40 and 80m but with an optional extra kit that gives chosen sideband suppression. The basic Mendip uses a crystal controlled converter ahead of a DC receiver operating near 5.5 MHz. A MC1496 mixer is used in the converter with double RF tuning, and low gain, for least potential overload from unwanted BCI! This is followed by the DC receiver, which has the option of adding the extra parts for sideband suppression. Although the audio stages have a bandwidth for phone signals, they are followed by a switch selectable narrow filter specially for CW. The main VFO runs at 5.5 MHz and uses a JFET with PolyVaricon main tuning, with Fine tuning by LED or varactor. The converters use three different band crystals to mix to the 5.5 MHz nominal frequency of the DC RX. The supply range is 9 to 18 volts and the kit includes all parts to build it 'open style' as shown below; a blank 100 x 160 PCB is included for you to lay out as you like for your front panel, but it can be easily boxed if you prefer. This approach allows a basic price of £59 for the 3 band RX.
By adding the optional extra phasing circuits, the DC RX is much improved by removal of the unwanted sideband. This kit adds the parts for a second channel to the 5.5 MHz DC RX together with the phase changing circuits for the audio and VFO signals. This phasing approach effectively eliminates most of the unwanted 'other sideband' signals that would be present with a simple direct conversion approach. The phase shifting uses 'all pass' filters specially developed for this rig which are based on the centre frequency of the CW filter. The two audio channels are combined just prior to the audio filters to remove the unwanted sideband. With careful adjustment, the suppression of the unwanted sideband is about 30 dB below the wanted one over all of the CW filter's bandwidth; this means that for most signals, you will not hear the unwanted sideband! Either (or both) sidebands can be selected by the sideband toggle switch. The optional extra kit costs £11, making a total of £70 when ordered with Mendip.
The matching CW transmitter is the Polden (see below) and can be used irrespective of whether the phasing options have been included. For a frequency readout, use the Five digit counter.
This the matching 3 band CW transmitter to go with the Mendip. It can be used on 20, 40 and 80m. Together they are operated as a TCVR with control of the RX's RIT from the Polden. The Polden uses the VFO and crystal oscillators of the RX to drive the transmitter's crystal mixing scheme which provides good stability and avoids any potential problems with chirp. The bandpass filters of the mixer and the RF output low pass harmonic filters are controlled by relays selected by the RX band switch. The output stage uses an IRF510 mounted on a generous heatsink and provides a nominal 5 Watts of CW on a 13.8 volt supply. TR operation is full break-in when used with the Mendip. The TX has 725 Hz sidetone which is fed to the RX along with normal signals for muting and control of the RIT. The Polden price is £49. When ordered with a plain Mendip the discounted price for both is £99, or if the optional phasing extras are required £110.
This is a sensitive phone superhet RX for any single band 20 - 80m. You decide which band to use it on when you get the kit! It has a double tuned RF filter, SA602 first mixer followed by a simple IF amp and the 6 MHz IF ladder filter, which uses five 6 MHz crystals. This feeds another SA602 product detector, with two stages of audio amplification separated by the AF gain control. The second audio stage has a discrete buffer so that it can drive low impedance phones or a small loud speaker. The RX uses a proper VFO whose frequency is chosen to suit the desired band but, to maintain stability, it is followed by a stage which is either a buffer, or a frequency doubler (with LO tank) for those bands where the LO frequency would be too high for decent VFO stability. Tuning is by PolyVaricon with three band segments provided by a toggle switch to keep the tuning rate low enough for easy use. Both SA602 mixers are controlled by electronic switches so that on transmit, the direction of signal flow in the whole IF strip and front end can be reversed to generate single sideband signals for transmission. Although this adds complexity to the RX, it considerably simplifies the associated Burnham transmitter so reducing overall cost! The RX has supply protection and the kit includes a small PCB front panel as shown below - this is an early 20m version. The main PCB is 100 mm square and can be easily boxed if you prefer. If you want to use it for CW, then add either the Mini CW kit or the Audio Extras which also provides AGC. For AGC alone, use the Mini AGC kit. The RX only price is £59.
The Burnham
This is the transmitter for the Bridgwater RX. Together they form a single sideband phone TCVR for any single band 20 - 80m. The TX has a speech amplifier suitable for dynamic microphones; the speech amp output is fed to the Bridgwater's product detector which acts as a balanced modulator on transmit. The TR relay control circuits also reverse the signal flow through the Bridgwater's IF strip so that it generates SSB out of its aerial terminal! This is then used to directly drive the low level RF stages of the Burnham, which use BS170s and an AD8055 op-amp with adjustable RF gain. The Burnham output stage uses a robust IRF510 (with heatsink) that feeds the transmitter low pass filters and provides up to 1.5W pk of SSB output. There is provision also for driving the 10W Linear amplifier if you wish to add that. Like the RX, the TX can also be used with either the Mini CW kit or the Audio Extras. The Burnham is a 100 x 80 mm PCB which must be mounted immediately behind the Bridgwater. This TX can only be used with the Bridgwater RX, so there is only one (discounted!) price for both - £84.
I regret to say that the Mk 2 version was not a success and I am already working on the Mk 3! It will eventually be a 5W phone TCVR, probably for any band 20-80m with an optional extra kit for other bands up to 10m. Be patient please and keep watching this space!
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