Radio electronics cookbook

of switch that you want to use for the PTT function. Figure 1 Vertical support made from ball-point pen shell Figure 2 The microphone stand base A simple desk microphone 281 Some epoxy resin such as Araldite Rapid is used to mount the separate parts. Mix only a small amount at a time, and glue each part of the base assembly and allow to harden before moving on. Be sure to read and follow the instructions for the use of the glue to prevent accidents. Firstly, glue the end cap to the base, feeding it underneath and pushing it up through hole C. Then, the vertical pen body is placed over the protruding end cap and glued into place, making sure that the top (shaped) end is pointing in the right direction to hold the other plastic tube carrying the microphone. While the glue hardens, place the DIN plug sheath on the second pen body. Place and glue this pen body on the shaped top of the vertical support, making sure that the whole assembly is supported while the glue sets. What you should have now ought to resemble Figure 3, but without the microphone and wiring. Leave everything to set for 24 hours. Any switch of the press-to-make variety can be used for your PTT switch, but choose one which does not apply too much force to operate! If you find the whole assembly is too light and slides over your table, some plasticine can be pushed into the base when all the wiring has been completed. Figure 3 Complete microphone stand assembly Radio and Electronics Cookbook 282 The circuit A small electret condenser microphone is used, powered by a DC supply. Most hand-held radios use a single screened lead for both the PTT line and the microphone audio lead. In such cases, the circuit of Figure 4 will operate the PTT function. Note that the PTT switch is in series with the audio lead from the microphone. If you find that the PTT switch does not switch your radio into transmit, reduce R1 from its normal value of 33 k to 27 k. You should find that the original value works with most hand-helds. For radios with a separate PTT lead, the circuit of Figure 5 is used. However, a power supply is needed for the electret microphone. This can be a PP3 battery, or can (in most cases) be derived from the microphone socket on the transceiver. You will need to consult the makers’ handbook for this information. Figure 5 shows the PTT switch wired in a ‘ground- to-transmit’ configuration, which is correct for most base-station trans- ceivers. If you’re in any doubt about what your radio needs, consult an experienced friend. Figure 4 Wiring diagram for hand-held radios Figure 5 Wiring diagram for radios which use separate PTT lines A simple desk microphone 283 The last connections The microphone element should be connected with screened cable. Tape (masking or insulating) should be wound around the electret insert until it is a snug fit inside the DIN plug sheath. Feed the screened cable through the barrel of the pen until it emerges from the far end. Poke it through hole B and make the connections under the coffee jar lid. The components can all be mounted on the PTT switch or on a small piece of Veroboard mounted under the top surface. The output lead emerges through hole A and is of sufficient length to reach your transceiver. A suitable plug needs to be fitted to it. Parts list Resistors: all 0.25 W carbon, 5% tolerance R1 (Figure 4) 33 000  (33 k) R1 (Figure 5) 1000  (1 k) Capacitors C1 (Figure 4 and 5) 0.1 microfarads (0.1F) disc C2 10 microfarads (10F) electrolytic Additional items S1 Push-to-make Mic Electret microphone (Maplin type FS34W) Screened cable As required Plug As required by your radio Veroboard If required PP3 battery and clip If required Two old plastic ball-point pens Lid of coffee jar Plastic sheath from DIN plug Araldite or similar epoxy resin glue Radio and Electronics Cookbook 284 80 Morse oscillator Introduction This is not the simplest Morse oscillator to build, but it differs from the simple circuits in that it produces a pure note, not a coarse, rasping sound. People who have practised Morse using a non-sinusoidal oscillator sometimes find that they have trouble copying Morse code with a pure tone. As the pure tone is the correct way to receive Morse code, it is important that you should learn to listen to the code from a pure oscillator – so here’s one! The twin-T There are many oscillator circuits, and there are many variations of the twin-T oscillator that we are going to use. Figure 1 shows one version of a very useful circuit. All oscillators must have positive feedback in order to work. The feedback determines the frequency of the note produced by the oscillator. Figure 1 Circuit diagram of the sinewave oscillator and amplifier Morse oscillator 285 Here, the feedback circuit looks like two letter Ts. If you look at Figure 1, one T is formed by R1, R2 and C3, the other by C1, C2 and R3 – hence the name ‘Twin-T’. Notice that the two Ts are connected in parallel between the collector and base of TR1, so any signal appearing at the collector is fed into both Ts. What emerges is then fed back into the base, producing in turn a signal at the collector. And so it goes on, producing a sine wave output. The oscillator output is fed into an integrated circuit amplifier for output via a small loudspeaker. Putting it together The prototype was constructed on plain matrix board (the type without the copper strips), as shown in Figure 2. The components have their leads pushed through the holes in the board, and connections are made underneath. Build the amplifier circuit first, using a socket for IC1. Connect the 9 V supply and touch pin 3 with an ordinary piece of wire. If a buzz is heard from the speaker, all should be well. If not, check your circuit and make changes until it does. Build the oscillator circuit, and connect its output to the volume control VR1 via C4. Set VR1 half-way along its travel and switch on. A note should be heard from the speaker when the Morse key is depressed. The component values making up the twin-T determine the frequency of the note. Try varying them if you think your note is too high or too low. Whatever changes you make, either to the resistors or the capacitors, always ensure that R1 = R2 and that C1 = C2. Figure 2 Component layout and interconnection diagram Radio and Electronics Cookbook 286 Parts list Resistors: all 0.25 W carbon film R1, R2 18 000 ohms (18 k) R3 4700 ohms (4.7 k) R4 10 000 ohms (10 k) R5 10 ohms (10 ) Capacitors C1, C2 10 nF (0.01F) C3, C8 47 nF (0.047F) C4 100 nF (0.1F) C5 22 nF (0.022F) C6 10F electrolytic, 16 V WKG C7 22F electrolytic, 16 V WKG C9 220F electrolytic, 16 V WKG Semiconductor TR1 BC109 Integrated circuit IC1 LM386 Additional items VR1 10 k log potentiometer with switch LS 8  loudspeaker Matrix board 3.5 × 9 cm Small jack socket for key input Box PP3 battery and clip A simple 6 m beam 287 81 A simple 6 m beam Introduction The attraction of building your own aerials is an abiding feature of our hobby. You can buy almost any shape or size of aerial, but one you have made yourself can often work every bit as well as a commercial device costing ten times as much. The design This aerial, designed for use on the 6 m band, is essentially a two-element Yagi, with the elements bent in order to reduce the physical size. It is known Figure 1 Plan view of the complete 50 MHz VK2ABQ antenna Radio and Electronics Cookbook 288 as the VK2ABQ beam, and was designed originally for the 20, 15 and 10 m bands, principally because of its space-saving qualities. It is made using a wooden frame and wire elements, and is ideal for portable operation. Tools ready? The beam is shown in Figure 1. The driven element is the one whose centre is fed by the coaxial cable, and lies between the two insulators marked A and the feedpoint at B. The reflector is also anchored at the points A, and lies over the upper half of the frame. The wooden centrepiece is used to support the cross-pieces and to mount the aerial on the mast, using a common shelf bracket. The cross-pieces, known as spreaders, can be wooden canes or dowelling, and are mounted to the centrepiece using cable clips and adhesive. Figure 2 shows how this is done. If the aerial is to be a permanent installation, the spreaders should be weatherproofed using a good-quality exterior varnish. The wire elements are PVC covered and fed through holes in the spreaders. Figure 2 Centre support piece A simple 6 m beam 289 The end insulators are made of drilled perspex, and the wire passed through the two holes and twisted, as shown in Figure 3. Note: if you have not drilled holes in perspex before, take care! The drill bit must be well-lubricated because it generates a lot of heat (enough to melt the perspex and jam the drill). Never turn the drill for more than a few seconds at a time, and moisten the bit in between. Start with a pilot hole and use bits of increasing size until the hole is the size you want. Another perspex insulator is used to secure the feeder to the aerial, as shown in Figure 3. The feeder then passes directly to the centrepiece, where it is fastened with a cable clip and then passes down the mast. Figure 3 Details of insulators Radio and Electronics Cookbook 290 Adjustment The driven element (A–B–A) in Figure 1 is fixed to the end insulators in such a way as to have ‘pigtails’ which are about 10 cm long. Using an SWR meter between the aerial and the transmitter, trim the pigtails equally at each end for minimum SWR in that portion of the 6 m band in which you plan to work. Make sure that the transmitter is off when you trim the ends, as high voltages can be present there. Always listen on the frequency before you transmit and, when you do, ask if the frequency is in use and identify yourself. Use as little power as possible. The prototype had an SWR of 1.2 at 50.2 MHz and performed well. If you look at Figure 1, which is a view of the aerial from above, you will immediately see that when the aerial is horizontal, it radiates with horizontal polarisation in a direction from the top of the page to the bottom. A metal pole or mast can be used for horizontal polarisation, but if you intend to use the aerial for vertical polarisation, it is much better to use a wooden or fibreglass pole. Portable use If you plan to operate portable with this aerial, the only real modifications you need are to the centrepiece and how it supports the spreaders. Instead of using glue and cable clips, nuts and bolts through the spreaders and centrepiece would allow the spreaders to be ‘hinged’ closed for transport. Materials Centrepiece Hardwood, 15 × 15 × 25 mm 4 spreaders 110 cm long (cane or 6 mm dowel) Wire 1.5 mm PVC-covered copper 50  coaxial cable RG58 or similar Cable clips 6 mm (12 off) Varnish Polyurethane for waterproofing Tape Self-amalgamating, to waterproof all soldered joints Insulators See text An integrated-circuit amplifier 291 82 An integrated-circuit amplifier Introduction A simple audio-frequency amplifier is a very useful building-block in many more advanced electronic circuits. It is also a very useful piece of test equipment. To have one spare in the shack can be a life-saver at times. Planning Get into the habit of planning your project. How do you want it to look when it is finished. Do you want it in a box? Do you want it ‘open-plan’, with all the components on view? The ‘minimalist’ approach to any project is simply a front panel and a baseboard, on to which all the components are fitted. In this case, the front panel would accommodate the loudspeaker, the volume control and the input and output jack sockets, and the baseboard would support the circuit board. Once you have decided these things, and know the size of the board, speaker and volume control, you can decided how big the panel and baseboard should be. The amplifier Instead of building the amplifier from discrete components (i.e. transistors), as was done in the project An audio-frequency amplifier (which you will find elsewhere in this book), we are going to use an integrated-circuit (IC) amplifier. External resistors and capacitors are still needed but, compared with the number of components inside the chip, these are very few indeed! The IC we are going to use is the TBA820M. The circuit may look complicated, but with the use of a matrix circuit board it becomes quite simple. Figure 1 shows the circuit diagram and Figure 2 the layout on the board. The external connections to the PCB are shown in Figure 3. To avoid having a separate switch to switch off the power supply when the amplifier is not being used, a volume control which incorporates a switch is used. Radio and Electronics Cookbook 292 Figure 1 Audio amplifier, circuit diagram Figure 2 Audio amplifier, component layout Figure 3 Connecting the circuit board to the speaker, volume control/switch and battery A novice ATU 293 What power supply? If the amplifier is to be used only for short periods, as a test instrument, for example, then it can be run from a PP3 battery, which can be mounted on the baseboard. If you intend to use it often, then a connection to an external power supply is preferable. For this option, you may want to consider fixing two terminals to the baseboard for this connection. 83 A novice ATU Introduction Having an aerial tuning unit (ATU) is always useful. It is used for adjusting the aerial impedance, as ‘seen’ by the transceiver, to be the same as that of the transceiver itself, usually 50. This process is called matching, and ensures that the receiver and the power amplifier (PA) stage of the transmitter work efficiently. This design is due to the late Doug DeMaw, W1FB, and uses readily available components. It will handle up to about 5 W, and operates over the frequency range 1.8 to 30 MHz. Circuit evolution The basic circuit of one type of ATU is shown in Figure 1. On transmit, the input signal at L1 is coupled into L2, which forms a resonant circuit with C1. The signal across the resonant circuit is fed to the aerial by C2. The combination of L2 and C1 helps to remove signal harmonics, because they are not at the resonant frequency and are shunted to earth. On receive, only those signals which are within the pass-band of L2 and C1 will pass into the receiver. This improves receiver performance by rejecting many out-of-band signals which the simpler receiver doesn’t like. Radio and Electronics Cookbook 294 For this design, the all-important resonant frequency is chosen to be in the 80 metre band. If we want to make the circuit operate on other bands, we need to change either L2 or C1. It is easy to change L2 by using a set of switched inductors (L3–L5) as shown attached to S1 in Figure 2. Forget about S2 and L6 for the moment. When S1 is in position 1, the circuit reverts to that of Figure 1. S2 should be brought into circuit only when S1 is in position 1. Looking at the way the circuit is drawn, you can see that when S2 is in position 2, we have two coils in series, which is equivalent to adding more turns to L2. More turns means more inductance, which lowers the resonant frequency still more, and gives coverage of the 160 m band. Rather than having to remember to flick S2 to the correct position and move S1 to position 1 when we want to operate on 160 m, both functions can be combined into one rotary switch with two wafers. This circuit is shown in Figure 3. Figure 1 Basic circuit of the ATU Figure 2 L6 added for 160 m A novice ATU 295 In all positions of S1a except position 1, the extra coil, L6, is shorted out. The two halves of the switch, S1a and S1b, move together, as the shaft is turned, so both halves are in position 1 at the same time, position 2 at the same time, and so on. The two switches, S1 and S2, are said to be ganged. Construction L1 is formed by winding four turns of 22 SWG enamelled copper wire over L2, as in Figure 4. L2 already exists on the purchased former. After scraping the enamel off the ends of the wire (with a sharp knife or sandpaper), one end of L1 must be soldered to one end of L2, as shown in Figure 4. The free end of L1 goes to the transceiver aerial socket. All components except the capacitors are assembled on the switch. Note that the rotor of C1 is earthed, but neither side of C2 is earthed. This means that the metal shaft of C2 is not earthed, and touching it will detune the ATU. Using a plastic knob for C2 will minimise this effect. If you decide to use a metal box, take precautions to ensure that no part of C2 is in electrical contact with the box. Figure 3 Two-pole switch for all bands Figure 4 L1 is wound over L2 as shown Radio and Electronics Cookbook 296 The capacitors are mounted using M2.5 screws. Make sure that the screws do not foul the vanes of the capacitor. If your screws are too long, a few washers between the box and the capacitor will solve the problem! In use The best indication of a good match is obtained with a standing-wave meter between the ATU and the transceiver. The controls are adjusted alternately to ‘feel’ your way to a better and better match. For receive-only use, the same alternate adjustments are used, watching for the maximum signal strength on the S-meter or, for a very weak signal, making the signal from the loudspeaker as large as possible. Parts list Capacitors C1, C2 350 pF variable Inductors L1 See text L2 27H L3 10H L4 2.2H L5 1H L6 65H Additional items SO239 sockets (2 off) S1 2-pole 6-way rotary Box as required Plastic knobs (2 off) Stick-on feet (4 off) M2.5 screws for capacitors Screws, nuts and washers for mounting the input and output sockets. CW QRP transmitter for 80 metres 297 84 CW QRP transmitter for 80 metres Introduction This is a relatively simple transmitter design having an output of 1 W. The design is not new, having been described before in other amateur radio publications. The components are all available new and the total cost should not exceed £15. The circuit Like other simple transmitters (see An 80 Metre Crystal-Controlled CW Transmitter and A Breadboard 80 m CW Transmitter elsewhere in this book) this one is crystal controlled. This assures frequency stability, but limits the usefulness of the transmitter. The key to increased frequency coverage without a conventional Variable Frequency Oscillator (VFO) is the use of a low-cost 3.58 MHz ceramic resonator. The ‘pulling’ range of a 3.58 MHz ceramic resonator covers the UK novice 80 m sub-band and some of the CW segment below 3.525 MHz. A ceramic resonator is like a crystal, but not quite as stable in frequency. Its main advantage is its large pulling range. The block diagram is shown in Figure 1. It is very similar to a crystal- controlled transmitter, and includes an oscillator, buffer and final amplifier. Figure 1 Block diagram of the ceramic resonator controlled 80 m CW transmitter Radio and Electronics Cookbook 298 This amplifier is keyed, the oscillator remaining running all the time. This improves frequency stability because the oscillator is not being continuously stopped and started by the key. It is switched off while receiving, though, to avoid interference with the received signal. Transmit/receive switching is accomplished by a panel-mounted switch controlling both the aerial, oscillator and buffer switching. Figure 2 shows the transmitter circuit diagram. An unusual aspect of this transmitter is the use of a digital CMOS integrated circuit (IC) type 4069 for the buffer and oscillator stages. The IC houses six inverters, four of which are used in the circuit. One is used as the oscillator, two are used for the buffer stage, and the fourth provides an output for a direct-conversion receiver, should one be added at a later date. The frequency of the oscillator is changed by varying the capacitance in the ceramic resonator circuit. This is provided by VC1. The power amplifier (PA) is a small MOSFET (metal oxide semiconductor field-effect transistor), TR1. This is capable of providing an output power of 2 W but, in this circuit, it is run conservatively to give 1.5 W. The output can be varied by changing the resistance (R5 + R6) in the gate circuit. Attempts Figure 2 Circuit diagram of 80 metre transmitter CW QRP transmitter for 80 metres 299 to raise the output power by decreasing the values of these resistors may result in immediate MOSFET failure. A pi-network (C8, RFC, C9) provides impedance matching to 50 , together with harmonic suppression. Like all inductors in this transmitter, the pi-network inductor is a pre-wound RF choke. A pi-network is so called because the components are arranged in the shape of the Greek letter pi (). Keying is carried out by a pnp transistor switch, TR2. Closing the key earths the base and supplies 12 V to the collector of TR2 and to the drain of the MOSFET, TR1, allowing the PA to operate. Construction You must house your transmitter in a metal box, to avoid hand-capacity effects and the radiation of spurious frequencies. Size is not important, provided it is large enough to accommodate the transmitter without cramping the components. You may want to allow space for future additions such as a direct-conversion receiver, break-in keying, sidetone or a small power amplifier. A good size is 5 × 15 × 15 cm. You can make your own box, buy it, or even use a biscuit tin! Front and rear panel connectors can be fitted first. The choice of these is a personal matter, but a good working choice would be: (a) Power socket – 2.1 mm panel socket – centre pin positive. (b) Key socket – 1⁄4 inch jack socket. (c) Aerial and receiver connectors – panel-mounting SO239 type. Particular attention must be paid to the mounting of the variable capacitor, VC1. Make sure the hole for the shaft is amply big enough, and if you use screws to mount the capacitor on the front panel, then make sure they are not too long, otherwise they will touch the vanes of the capacitor! Mounting can be by means of glue, sparingly applied and kept well away from the shaft. A board size of about 6 × 10 cm is adequate. Component layout on the board is suggested in Figure 3. The prototype used ordinary matrix board, which is preferable to stripboard for a design like this; stripboard has undue capacitance between adjacent strips. Component leads are fed through holes in the board and are connected underneath. Make sure that leads and connections are rigid because, if they can move, there is always the danger of short-circuiting, and capacitance changes. To facilitate construction, servicing and testing, it is advisable to use Veropins for connections to the variable capacitor, transmit/receive switch, Radio and Electronics Cookbook 300 aerial and power sockets. Use screws and spacers to mount the circuit board to the box. Mounting the board horizontally assists troubleshooting. Use a socket for the IC, and observe the CMOS handling precautions given in An Electronic Die, elsewhere in this book. When soldering the leads to the ceramic resonator, do it quickly – excessive heat damages the device. The earth lead running acoss the bottom of the board must be connected to the metal case by a short length of stout wire. Testing After carefully checking your wiring, both against the circuit diagram and the layout diagram, it is time to test your circuit. You will need a multimeter, an 80 m SSB receiver and a 50  dummy load. A good design of dummy load can be found in the project A Switched Dummy Load, also in this book. An RF power meter and frequency counter will also be useful, although if your receiver has a digital frequency readout and S-meter, the latter two items are not really necessary. You will also need a 12 V 1 A power supply unit (PSU) to power the transmitter. Figure 3 Component layout of the 80 metre transmitter. The transmit/receive switch is not mounted on the board and is not shown in this diagram CW QRP transmitter for 80 metres 301 Switch the transmitter to receive and switch on the transmitter. No current should be consumed. Switch to transmit and check that pin 14 of IC1 is 6.8 V positive. With the dummy load connected to the aerial socket, press the key. The voltage on TR2 collector should now be 12 V, dropping to zero when the key is released. Now check the operation of the oscillator. In transmit mode, you ought to be able to find a strong carrier signal with the receiver, even though the dummy load is connected. Adjusting the variable capacitor should change the frequency. At the lower end of the frequency range, you may find that the oscillator is unreliable in starting, because the circuit is attempting to pull the resonator too low in frequency. If this is the case, set the trimmer at the back of VC1 to minimum capacitance. If your version of VC1 has two trimmers, and you don’t know which one to set, set them both to minimum capacitance. If there is still a problem, reduce the value of RFC1 to 6.8 or 4.7H. In all probability, the unmodified circuit of Figure 2 will not require any of the changes outlined here. A coverage of 3.518 to 3.558 MHz should be possible, while preserving good frequency stability and reliable oscillation. A signal probe (see An RF Signal Probe, elsewhere in this book) is useful for checking the operation of the oscillator and PA. Alternatively, an RF power meter or the receiver’s S-meter can be used. With the PA running, the unit should draw between 200 and 300 mA. If TR1 becomes too hot to touch after a few seconds of transmitting, increase R5 or R6 to limit the transistor’s heat dissipation. A small 6.3 V bulb connected across the aerial output is a simple way to check that the PA is working. An orange/white glow when the key is pressed is indicative of correct operation. The final test is to monitor keying ‘quality’. With your dummy load connected, press the key and listen to the note on the receiver’s loudspeaker. Then operate the key, sending a string of dits, for example. What you hear should be free of chirps and clicks, as well as being stable in frequency. This test is sometimes better performed with no aerial connected to the receiver, thus preventing receiver overload and its associated plops. No problems should be encountered here. Frequency tuning A peculiarity of ceramic resonators is that, every now and again, their frequency changes abruptly by 100 Hz or so, then remains stable for some time. This is certainly noticeable in the received signal, but does not detract from the QSO and no characters are lost as a result. Try to keep the area around the ceramic resonator cool, to avoid temperature variations. Radio and Electronics Cookbook 302 Parts list Resistors: all 0.25 W, 5% tolerance R1 10 megohms (M) R2 2200 ohms (2.2 k) R3 270 ohms (270 ) R4 1 megohm (1 M) R5, R6 1.5 megohms (1.5 M) R7 1000 ohms (1 k) Capacitors C1, C7 100 nF C2 100 pF C3 47 pF C4 1 nF C5, C6, C10 10 nF C8 560 pF C9 820 pF VC1 10–160 pF variable Inductors RFC1 8.2H RFC2 10H RFC3 2.2H Semiconductors IC1 4069 TR1 VN10KM TR2 BC640 D1 6V8 Zener Additional items Matrix board (see text) 14-pin DIL socket Pointer knob Sockets (see text) An audio booster for your hand-held 303 85 An audio booster for your hand-held Introduction The audio output from many hand-held transceivers and receivers usually leaves much to be desired, so this little amplifier was designed to increase the output at minimal expense. All that is needed is a separate amplifier and bigger loudspeaker. This is accomplished using a single integrated circuit (IC), a few components, and a loudspeaker from the junk box. This circuit will enable the output from your hand-held to be heard easily in a car. The circuit This is shown in Figure 1. It uses only those components necessary to operate the IC amplifier. VR1 is the preset volume control, and varies the signal coming from the ‘External speaker’ jack on the hand-held before feeding it into the IC for amplification. C1 blocks any constant voltage present on the input. Figure 1 Circuit diagram of the audio amplifier. The power is derived from the cigarette lighter socket and the fuse is in-line with the lead Radio and Electronics Cookbook 304 The IC output comes from pin 4 and is fed via the electrolytic capacitor, C4, to the loudspeaker. The circuit is provided with an on/off switch, fuse and LED to indicate when the circuit is switched on. Construction The box is made of aluminium. This is necessary to help to dissipate some of the heat generated by the IC. Do not build the circuit inside a plastic box unless you take special precautions! The IC has a metal mounting tab with a hole, specifically designed to be mounted to a metal box or other metal heat sink. Apply plenty of heat sink compound between the tab and the box, tighten the nut and bolt, and then wipe off any excess compound. The box will get slightly warm in operation. The size of the box is not specified. You may want to decide on this when you find a loudspeaker. Choose one which will be able to handle 6 W Figure 2 Layout of the components within the box A grid dip oscillator 305 output. Drill all the holes in the box first. Holes for the speaker, input phono socket and the LED. The amplifier can be constructed on ordinary matrix board, which can be mounted inside the box with screws and spacers. The layout is shown, for your guidance, in Figure 2. The components are mounted by pushing their leads through the holes in the board and making connections on the underside. The preset volume control, VR1, is set such that the hand-held’s volume control is sufficient to control the final output over a good volume range. Use a screened lead from the ‘External speaker’ jack socket to the phono plug. An external power supply is needed for this circuit. The normal dry battery which we usually use for small projects in this book will not work here, so you will need a proper mains power supply producing a stabilised 12 V. If you are going to use the unit principally in a car, then the cigar lighter socket can supply this voltage easily. Do make sure that the polarity is correct before you switch on! When you plug the jack plug from your booster into the ‘External speaker’ socket on your hand-held, its internal speaker will be muted, so don’t think that something dire has gone wrong! Adjust VR1 for a good volume range on your booster, when the volume control is turned on the hand-held. Parts list Resistors: all 0.25 W, 5% tolerance, unless otherwise stated R1 15 ohms (15 ) 1 W R2 220 ohms (220 ) R3 2.2 ohms (2.2 ) R4 470 ohms (470 ) VR1 1000 ohms (1k ) Capacitors C1 10F 25 V C2 470F 25 V C3, C5 220 nF (0.22F) Mylar C4 1000F 25 V C6 100F 25 V Semiconductor LED 5 mm Red Integrated circuit IC1 TDA2003 Radio and Electronics Cookbook 306 Additional items Heat sink compound Nuts and bolts Loudspeaker 4  6 W 3 A fuse On/off switch (SPST) Matrix board 4 × 6 cm Solder tags Plugs and screened cable for connecting lead Aluminium box 86 A grid dip oscillator Introduction When an inductor and a capacitor are connected, whether in series or parallel, they form a circuit with a natural (or resonant) frequency. The circuit stores energy, and this energy is being constantly shifted from the inductor to the capacitor and back again. The dip oscillator is a simple instrument used to measure the resonant frequency of a tuned circuit without having to make any direct connection to the circuit. The circuit is more commonly known as the grid dip oscillator (GDO), from the days when the active device in the circuit was a valve. The FET or Field-Effect Transistor operates in a way which is very similar to that of the valve, so it is not quite a misnomer to call this instrument a grid dip oscillator, too. The circuit The GDO uses a calibrated, tunable FET oscillator in the circuit of Figure 1. It has a frequency range of 1.6 to 35.2 MHz in four ranges using a set of plug-in coils, shown in Figure 2. When the oscillator coil, L1, is placed near an external resonant circuit, some of its RF energy is coupled into the external circuit. A gain in energy of the external circuit must mean a loss of energy in the GDO circuit, resulting in a change of current through TR1, which is measured by the meter, M1. The current through TR1 is of the order of 5 to 8 mA, but the change of current may be only a few microamps. To measure a very small A grid dip oscillator 307 change superimposed on a much larger standing current, the method of offset can be used. One connection to the meter goes to the source of the FET, while the other goes to a variable offset voltage set by VR1. M1 has a full-scale deflection (FSD) of 100A. If the current through TR1 changes, the voltage across R3 changes. When there is no resonance, the voltage at the wiper of VR1 is set to be very slightly greater than that across R3, and there is a 75% FSD meter deflection. When the voltage across R3 decreases very slightly, due to Figure 1 Circuit of an FET GDO. The coils are wound on DIN speater plugs, which provide both a plug-in base and a coil former Figure 2 Details of coil construction: Range 1: 1.6–4.0 MHz 55 turns of 30 SWG Range 2: 3.3–7.9 MHz 27 turns of 30 SWG Range 3: 6.3–4.0 MHz 14 turns of 26 SWG Range 4: 11.9–4.0 MHz 7 turns of 24 SWG. Radio and Electronics Cookbook 308 external circuit resonance, a significant ‘dip’ in the meter deflection is produced, hence the name of the instrument. The circuit is not difficult to make on standard matrix board. Provided you can follow a circuit and translate it into a good component layout, then this project is probably only an evening’s work. The most important part of the GDO is the tuning capacitor and its associated frequency-calibrated dial. New, air-spaced tuning capacitors can cost you up to £20, so it is worth delving around in junk boxes, or scouring the tables at a local boot sale or rally. The tuning capacitor from an old transistor radio should be perfect. It may even have a slow-motion drive and a dial which can be remarked for the project. Choose a coil plug and socket arrangement that is practical. Think about crystal holders or phono plugs and sockets. The prototype shown in Figure 1 and Figure 2 used 2-pin DIN plugs, with the coil wound on the outside of the plastic plug cover. Figure 2 shows the coil construction and the winding details. If you use a variable capacitor, VC1, with a value different from that shown in the parts list, then the frequency ranges will be different. This does not matter, as it will be taken into account during calibration. Position VC1 so that the dial will be easy to see and to operate, while locating the coil socket as close to it as possible. Figure 3 shows the traditional layout of the GDO. Calibration Because the GDO also radiates a very small amount of energy, a general coverage receiver can be used to calibrate the dial. Don’t try to aim for great accuracy and clutter the dial with marks and figures! If you include C10, R7 and R8 in your circuit, you can connect a frequency counter directly to the GDO and leave it in circuit all the time. The GDO in use Always try to place the external tuned circuit with its coil coaxial with the plug-in coil, as shown in Figs 1 and 3. If the coils are at right angles, the GDO may not produce any resonance. Set the offset control to give about 75% FSD and slowly tune L1 through its whole range. If no dip occurs, you may have the wrong coil plugged in. When you eventually find a dip, move the external coil further away until only a minute dip is seen. You may have to retune the dip meter as you do this, but it gives a much more accurate reading of frequency. Remember that you cannot ‘dip’ a coil by itself – there must always be a capacitor present. A grid dip oscillator 309 Figure 3 Layout of a typical GDO. The dial, meter and the location of the coil to the circuit under test can all be viewed at the same time. It is shown measuring resonance of a tuned circuit Radio and Electronics Cookbook 310 Aerial resonance Figure 4 shows how to check the resonance of a dipole aerial. Disconnect the coax feed at the aerial and place a short piece of wire, terminated with crocodile clips, across the centre insulator to short together the two ends of the aerial. By placing the GDO close to the shorting link, a dip should be seen on the meter while VC1 is turned. Alternatively, a loop in the element can be made around the coil, as in Figure 4, or the shorting link can be made long enough to loop over the coil. The latter method does not require tampering with the mounting and tensioning of the dipole wires. Figure 4 A wire antenna element can be looped into a single turn coil for increased coupling to the GDO A grid dip oscillator 311 Parts list Resistors: all 0.25 W, 5% tolerance R1 100 000 ohms (100 k) R2 56 ohms (56 ) R3 1000 ohms (1 k) R4 5600 ohms (5.6 k) R5 560 ohms (560 ) R6 4700 ohms (4.7 k) R7 1000 ohms (1 k) R8 5600 ohms (5.6 k) VR1 1000 (1 k) linear Capacitors C1, C2 22 pF C3, C4 100 pF C5, C7, C9 100 nF (0.1F) C6, C8 1 nF (1000 pF) C10 6.8 pF VC1 2 × 365 pF Semiconductors TR1 J304 or similar D1 1N4148 LED Additional items L1 See Figure 2 RFC 1 mH S1 SPST M1 100A Source Components are available from Maplin. Radio and Electronics Cookbook 312 87 A CW transmitter for 160 to 20 metres Introduction This very small transmitter is designed to work on any band from top band (160 m) to 20 m, with an RF output of 1 W. It will work on higher frequencies but with a reduced output. The circuit The three-transistor circuit is shown in Figure 1. It comprises a crystal oscillator using a BC182 transistor which drives a 2N3866 power amplifier (PA) keyed by a ZTX750 PNP transistor. The oscillator and PA are coupled by a capacitor and resistor; this provides a very small amount of positive bias to the PA. The oscillator can be used as a basic crystal oscillator but, by including a variable series capacitor as shown in Figure 1, the crystal frequency can be ‘pulled’ slightly, making the oscillator a variable crystal oscillator (VXO). Construction The PCB layout is shown in Figure 2 and in the photograph. Although the prototype was built around a PCB, this circuit is equally amenable to Figure 1 Transmitter circuit diagram A CW transmitter for 160 to 20 metres 313 construction on a matrix board. Populating the PCB is very simple, and you can expect to be able to do this in about one hour. The radio-frequency choke (RFC) is made by winding 10 turns of 33 SWG wire on a ferrite bead. The enamel coating of the wire is intended to vaporise when soldered into the board, thus obviating the need to remove the enamel manually with a knife or sandpaper. However, if you do have problems with the PA either not working or keying intermittently, it is suggested that you investigate the RFC connections immediately! If you decide to make the VXO version, you will have to cut the track between the crystal and earth, and connect the variable capacitor (250 pF) across the break. In use After performing the usual checks on the accuracy of your circuit building and the wiring of the external components, it is time to connect a 12 V battery between the points shown in Figure 2. Do not switch on yet. An aerial needs to be connected to the output via an ATU and a crystal, of frequency matching that of the aerial, fitted. The variable capacitor should Figure 2 Component placing on the PCB. The external connections are also shown Radio and Electronics Cookbook 314 give you a tuning range from about 14.058 to 14.064 MHz. ‘Netting’ (the process of tuning your transmitter to the same frequency as that of a received station) is achieved simply, because the oscillator is always running, and the leakage of the signal (despite the fact that the PA is not powered) is sufficient to bring the two frequencies to zero beat. Matching the end-fed random-wire aerial 315 Warning Note that the transmitter has no filtering; harmonics are not suppressed. It is strongly recommended that you use this transmitter in conjunction with the excellent low-pass filter described in the project A 7-element low-pass filter for transmitters, described elsewhere in this book. 88 Matching the end-fed random-wire aerial Introduction Many amateurs who do not have the space (or money) for a multi-band beam aerial, make use of the simplest possible alternative – the longest piece of wire that they can erect, with its end connected to the transceiver or receiver by an aerial tuning unit (ATU). The length of the wire is not of major importance. Any length between 10 m and 80 m, with bends if necessary, will suffice. A good earth connection to the radio is just as important. Bends in the aerial wire can have some interesting effects on the directional properties of the aerial; V- or L-bends, or even a square shape are permitted. The only thing not permitted is to fold the wire back on itself in a tight hairpin bend! Longish wire aerials The term ‘long wire’ is usually used (incorrectly) to describe an end-fed aerial. How long is a piece of string? It depends what you mean by ‘long’. In aerial parlance, it means ‘long with respect to one wavelength’. Again, this depends on the band you are using. A long wire at 20 m is somewhat different from a long wire at 160 m. However, if you have sufficient real estate for a long wire on the 160 m band it must, by definition, be a long wire on all the other bands, too! Radio and Electronics Cookbook 316 This should put you on your guard when analysing published data about feed-point impedance and the directional properties of a long wire aerial. Such theoretical data relate to a real, ideal long wire which is straight, horizontal, very high above perfect (conducting) ground, and not obstructed in any way. So your aerial doesn’t quite match these criteria? Join the club! Don’t let this dampen your ardour when it comes to evaluating what the longish wire can do for you. The following should explain why. Feed-point impedance The impedance at the end of a longish-wire aerial can vary from a few tens of ohms to several thousand ohms, depending on the frequency in use and the wire length. It is also affected by factors such as bends, height above ground, proximity to buildings and wire diameter. The actual value doesn’t matter, provided we can make the aerial appear to have a 50  impedance at the aerial socket of the transmitter. This process is what we call impedance matching, or simply matching. It maximises the power transfer from the transmitter to the aerial, and from the aerial to the receiver. That is why an aerial tuning unit, or ATU, is almost (but not necessarily) obligatory. The ATU Many commercially produced HF receivers and transceivers have single 50 coaxial sockets as their one and only means for connecting an external aerial. This means that an external aerial should have a 50 feed impedance if it is to work efficiently, and it rules out most of the aerials being used by amateurs on the HF bands. Some means is necessary to change the aerial feed impedance to ‘match’ that of the transceiver. Such impedance-matching, or Z-matching (because Z is the symbol for imped- ance, just as L is the symbol for inductance) is the roˆle of the ATU. These can be bought and will accommodate either an end-fed or a coax-fed aerial. They can be bought ready for use or in kit form. Whether you want one for receiving only, or for use with a low-power (QRP) or high-power (QRO) transmitter, will determine what you need and how much you pay. Matching the end-fed random-wire aerial 317 A simple single-band ATU The simplest form of ATU is shown in Figure 1. It is simply a parallel LC (coil and capacitor) circuit, resonant at the chosen frequency, with taps on the coil for the aerial and the coaxial feed to the transceiver. If we assume the circuit is resonant, a high impedance exists at the top of the coil, and a low impedance at the bottom. We said earlier that the end-fed longish wire presented an impedance which was high (or at least higher than 50). This explains why the aerial is tapped to the coil near the top, where the impedance is high, and the 50 coax is tapped near the bottom, where the impedance is low. Because the feed-point impedance of the aerial changes with frequency, so must the point at which the aerial is tapped to the coil to achieve impedance matching. The value of C must be changed also, to ensure that the circuit is resonant, and the 50 tap will require tweaking also. Setting up an ATU is quite simple. Make up an LC parallel-tuned circuit consisting of 50 turns of enamelled copper wire on an empty 35 mm film plastic container (or similar), tuned with a 500 pF variable capacitor. Make sure the enamel is removed from the ends of the wire before soldering. Solder the inner wire of the coaxial cable from the radio to the first or second turn of the coil from its grounded end. Then solder the braid to the grounded end. Connect the aerial about one-third of the way down the coil from the top, removing the enamel at the connection point. Adjust the variable capacitor for maximum noise or signal strength in the receiver. Then, try different tapping points from the aerial, to maximise the signal again. This matches the aerial impedance to that of the tuned circuit. Repeat the process with the coax tap, thus matching the impedance of the radio and the cable to that of the tuned circuit. Figure 1 Parallel tuned circuit as single-band ATU for end-fed longish-wire antenna Radio and Electronics Cookbook 318 You will no doubt find that the tapping process on the coil was not easily accomplished, especially when the enamel must be removed at each tapping point without shorting adjacent turns together. It is therefore logical to produce a design where the taps have been prepared during the winding of the coil, and are selected with a rotary switch. To this end, the following multi-band design is described. A multi-band ATU The same type of coil design around a 35 mm film container is used (see Figure 2). The tapping points can be prepared in advance by a little judicious planning. Dismantle your original coil and measure the length of wire on it. It will be a little more than a calculation of 50 × D would suggest (where D is the diameter of the film container), due to the lack of tension in the coil and the wire diameter itself. You will need aerial tapping points at turns 1, 2, 3, 5, 10, 15, 20, 25, 30, 35, 40 and 45, corresponding to all the bands from 28 MHz to 1.8 MHz. The coaxial cable tap is fixed at turn 2 (an acceptable compromise). All the turns are counted from the earthy end. Cut another piece of enamelled copper the same length as you used originally. Then, with the aid of an ordinary calculator, work out the positions of the points where the enamel must be removed for the taps. For example, turn 15 will have to be made 1550 of the way along the wire, turn 20 tap made at 2050 of the way along. So, if the length of wire is, say 4.7 metres, the two taps in question will be made at 1550 × 4.7 = 1.41 m and 2050 × 4.7 = 1.88 m from one end. This must be repeated for each of the tap positions, and the enamel removed ready for the wire to be soldered to it. With a Figure 2 Simple multi-band ATU for end-fed longish-wire antennas Matching the end-fed random-wire aerial 319 soldering iron, tin each tap point while the copper is shiny, thus ensuring a good, low-resistance connection. Do not solder on the taps yet. The coil can now be wound as before. The taps can be soldered on, taking the lead from each one to the wafer of a single-pole 12-way rotary switch, the pole being connected to the aerial. The tuning of the ATU is carried out by the same 250 pF capacitor, with a single-pole 5-way rotary switch used to select the band. Its tapping points will need to be chosen manually, using the method described earlier. Don’t attempt to make new tapping points on the coil for this – use the taps available on the wafer of the other rotary switch, and find which is optimum for each band. Notes  For the aerial, use PVC-covered stranded tinned copper, of size 16/0.2 mm or 24/0.2 mm.  Make the wire as long as possible, but anything over 10 m should be OK.  Keep the wire as high as you can, in the clear and away from obstructions.  Don’t worry about bends, but don’t use hairpin bends.  Use a good insulator to attach your aerial – anything plastic will do.  Anchor the wire near the point of entry to the building, but use a U-bend to prevent ingress of water.  The wire can be brought in through the corner of a window, the PVC acting as an insulator. If you must drill a hole in the brickwork, make sure it slopes upwards from outside, so that water is deterred from entering.  Use a good RF earth (as opposed to an electrical earth) such as half a dozen bare copper wires buried under the lawn in a fan shape. They should be joined together at the point of the fan and strapped to the earth connector of your ATU.