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The 'Super-Sixty' - a 5 MHz Transmitter & Receive Converter Contents: 1. Features Summary
This picture shows the completed Super-Sixty on top of the companion Elecraft K2 transceiver. Here, the K2 is being used to receive 5 MHz via the receive converter section of the Super-Sixty. The keying line of the K2 is connected to the 'Keying Out' line from the Super-Sixty, thereby providing sidetone on transmit (see below).
This transmitter and receive front-end combo has been designed for use on 5 MHz and has
the following features: |
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Between August and October 2002, several hundred UK radio amateurs applied for permits allowing experimentation on 5 MHz (60 m). Most applicants were successful in gaining the special authorisation (or 'Notice of Variation') required to carry out experiments on the five 3kHz-wide channels made available under these arrangements. I've had an interest in building a simple VXO transmitter for many years, but was unsure that the limited tuning range usually associated with VXOs would be useful on the busy HF bands. However, when the channelised frequencies became available on 5 MHz, I thought that this would be an ideal application for a VXO TX/Receive Converter project to be used alongside my Elecraft K2 transceiver. While much of the design is based on the 'Marathon' 136kHz QRP transmitter, described in the Autumn 2001 issue of SPRAT (SPRAT Nr. 108). I am grateful to G-QRP Club member Dave Sergeant G3YMC (Bracknell, Berkshire) for his ideas and advice during the design of the transmitter. Those not having access to 5 MHz will find that all the circuit elements can be adapted for similar projects on other HF bands. As things turned out, my first QSO using this TX design was with G3YMC - who was also using a very similar 5 watt VXO TX. You don't need high power to work around the UK on 5 MHz! For details of Dave's VXO TX, click here. The transmitter uses: a bipolar VXO; FET buffer; 2N2222 pre-driver + BC212 keying transistor; 2SC2166 driver; and a pair of 2SC2166 transistors in parallel in the 5 watt power amplifier (PA). The receive converter uses a single-ended FET mixer, and a 24 MHz oscillator to up-convert the 5 MHz band to 29 MHz. With portable operation in mind, the Super-Sixty was ‘shoe-horned’ into a small diecast box measuring only 85 x 155 x 50 mm.
Variable Crystal Oscillator (VXO) and Buffer For the transmitter, channels ‘FA’ (centre frequency of 5.260 MHz) and ‘FB’ (5.280 MHz) were chosen. The 50 pF tuning capacitor provides a tuning range of approximately 2 kHz above the crystal frequency. It is easier to tune a crystal to the high side of its intended frequency of operation, so a 5.25900 MHz crystal is used for channel 'FA' (tuning 5.259 to 5.261 MHz); and a 5.27900 MHz crystal is used for channel 'FB' (tuning 5.279 to 5.281 MHz). The VXO drives a very effective FET buffer, which presents a chirp-free signal to the driver stages.
The pre-driver stage uses an untuned 2N2222 common emitter
amplifier, keyed by a BC212 PNP transistor. In my experience, many simple TX designs
fail to provide adequate shaping of the keying waveform, resulting in very hard keying. To
reduce the likelihood of transmitting key clicks, the keying circuit in this transmitter
provides rise and fall times of 4 ms. The result is a very pleasant T9 note. The KEYING OUT line is for use with an external sidetone generator. In my case, I wired the KEYING OUT line to a panel-mounted phono socket, and then made up a connecting lead to the key jack of the K2. (When using the K2 in this way, it is wise to inhibit CW transmit on the K2 by selecting the 'test' CW mode!)
Power Amplifier (PA) and Low Pass Filter (LPF)
L1 – 14 turns,
26 SWG enamelled copper wire on T50-2 ring core. The VXO stage was screened from the other stages within the diecast box using 'walls' of copper-clad board soldered to a 'floor' of the same material. Earth (or 'ground') connections were made directly to the floor, keeping all earth connections as short as possible – especially around the driver and PA stages. If this is done consistently, Mr Murphy will be more forgiving if you end up with longer connecting wires elsewhere. Each PA transistor draws 400 mA on key down. This can be checked by measuring the voltage across one of the 1 ohm emitter resistors: 400 mV corresponds to a current of 400 mA. Increasing the value of the 10 ohm emitter resistor in the driver stage will reduce the drive level to the PA. The 'RX' antenna terminal should be connected to the 5 MHz input terminal of the receive converter. Alternatively, the 'RX' terminal may be connected directly to a separate 5 MHz receiver. The neat thing about using a switched receive converter is that you don’t have to keep winding back every gain control on the main receiver each time you switch to transmit. It’s well worth the trouble of incorporating a receive converter for that reason alone! When using a separate 5 MHz receiver with the Super-Sixty, the ‘+12 R’ line could be used to generate a ‘mute’ signal for the receiver. Although I made over 30 contacts using the first prototype without fitting heatsinks to the PA transistors, these devices will require a small heatsink to survive extended periods of key-down. Note that the tab of the 2SC2166 is internally connected to the collector, so be sure to use a TO220 insulating kit! Although the driver transistor does not need a heat sink, I found it convenient to mount all three 2SC2166s on the inside wall of the diecast box. Initially, the VXO coil was tuned for maximum signal, and then off-tuned very slightly (to a point where adjustment of the core had less effect upon oscillator frequency). The top of the adjustable core ended up being about 2 mm below the top of the coil former.
This simple two transistor design is similar to one that I have used for many years in a homemade 80 m transceiver, and performs very well indeed. I have included some modest protection at the output (the pair of 1N4004 diodes), in case the associated 29 MHz rig is keyed accidentally. To align the converter, initially set the top of all adjustable cores about 2 mm below the top of the coil former. Then, using a weak off-air signal, peak all coils. As with the VXO, I suggest off-tuning the 24 MHz oscillator coil very slightly.
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| Introducing 'Ugly' Construction Those with lots of experience of building homemade rigs will have no trouble building the transmitter into a smart equipment case. Those with less experience would, perhaps, benefit by starting with a simpler approach. A 'bird's nest' version can be constructed quickly on copper-clad board using 'ugly' construction techniques. This is exactly what I did when I made the prototype transmitter section of the Super-Sixty (see picture below). It didn't look very pretty, but that really didn't matter - the important thing was that (even before fitting heatsinks to the PA transistors) it was possible to use the prototype to test the design - and also work a number of stations around the UK!
For such an approach, start by fixing some copper-clad board to a piece of wood or
'chipboard'. (Prior to fitting the copper-clad board, be sure to remove any oxidation
using metal polish, followed by a rinse with soap and water.) Odd scraps of aluminium
sheet can be used as the front and rear panels by screwing them to the front and back
edges of the chipboard. Start by getting the tuning capacitor; antenna sockets; TX/RX
switch; NET switch; 12 v connector; and PA transistors/heatsink mounted in convenient
positions, and in relation to their position in the circuit diagram. For more information about ugly construction techniques, click
here. The HC25 crystals and sockets may be obtained from:
Feedback from other constructors Use with the K2 (added 15/03/2003) Dave's configuration of the K2 subtracts the conversion frequency (in my case, 24 MHz) from the K2 receive frequency (say, 29261 kHz), and hence displays the actual received signal frequency of 5261 kHz. Neat, eh? All the details can be found on Dave's web site at: Note that on my K2, I set the 'rF' option to '4'; and the 'IF' option to '28' (thus shifting the readout by the required 24 MHz).
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