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Ideal project for novices... 3 BAND AMATEUR RECEIVER Want to listen in on the most popular HF amateur bands? Perhaps you own a short wave radio but are disappointed with its ability to receive amateur radio signals? This inexpensive and easy to build receiver is just what you need. Short wave listening is a fascinating pastime and you only need a cheap receiver to listen to transmissions from around the world. Most short wave broadcast stations are very powerful and because they use amplitude modulation (AM), a receiver used to tune their signals only requires modest sensitivity and a simple AM detector. Unfortunately it is not this easy to listen to Amateur band transmissions, which are transmitted with much less power and generally use single sideband (SSB). Therefore, receivers used for amateur signals must have high sensitivity and selectivity, and an SSB demodulator. Just as important, amateur bands occupy only a tiny segment of the overall short wave bands. Typical low priced short wave radios, while they may tune over the amateur frequencies, do not have the sensitivity and selectivity to pick up these low level signals and generally cannot resolve SSB transmissions. Of course you can buy receivers that will do a good job receiving both general shortwave and amateur transmission but these are quite expensive and can cost many hundreds, perhaps thousands, of dollars. 80, 40 & 20 metre bands This receiver, while being reason- By LEON WILLIAMS VK2DOB 28  Silicon Chip Fig. 1: The block diagram of the three band receiver. It covers the most popular high frequency amateur bands. ably simple, has adequate sensitivity and selectivity and can receive SSB, CW, RTTY and SSTV signals. It tunes three 500kHz wide sections of the HF spectrum which include the 80, 40 and 20 metre amateur bands. The 80-metre amateur band covers 3.5MHz to 3.8MHz. During the daytime only local signals will be heard, although at night both local and interstate signals can be picked up. The 40-metre band goes from 7MHz to 7.3MHz and is excellent for daytime local and interstate reception and at night it is possible to hear stations from around the world. The 20-metre band, which extends from 14MHz to 14.35MHz, is the best to hear long distance (DX) transmissions from all parts of the world, day and night. This band is affected more by the changes in the ionosphere than the other two bands. Sometimes, only SPECIFICATIONS 80m Band: 3.5 to 4.0MHz 40m Band: 7.0 to 7.5MHz 20m Band: 14.0 to 14.5MHz Power: 12V DC (nom) <at> 250mA maximum – from a regulated supply or high capacity battery (not plug-pack) Antenna: 50Ω impedance Output: 8Ω speaker or headphones signals from certain parts of the world can be heard or even no signals at all and yet at other times the band will be crammed full. With this receiver you can listen to amateur transmissions at almost any time by selecting the band that is best suited to the time of day and the propagation conditions. It was designed to be inexpensive and easy to build, while offering good performance. To this end, the whole receiver is constructed on a single PC board and housed in an inexpensive case. One aim of this design was to eliminate the need to wind coils, as this appears to be quite a challenge for the newcomer to radio construction. Most of the coils used are pre-wound RF chokes; only two coils need to be wound. Power requirements The receiver can be powered from any suitable DC voltage source between 9 and 15V. At 12 volts, the receiver draws 40mA with no signal and about 250mA at full volume. A regulated 12/13.8V power supply capable of about half an amp would be ideal. A diode in the positive supply line protects the receiver from inadvertent reverse polarity connection. Note that most DC plug packs have quite high hum levels and probably won’t be suitable because the hum will make its way into the audio stages. The receiver does not have an internal speaker. This is done for a couple of reasons. The case used is not really big enough and it is likely that there would be some mechanical feedback between the speaker and the oscillator coil. Anyway an external speaker or headphones will provide much better sound than a small internal one. The front panel has the main Tune control with a calibrated dial. The main Tune control does not have a vernier mechanism and so a Fine Tune control is provided to make it easier to accurately tune in signals. Also on the front panel is the volume control and an RF attenuator. The final front panel control is a 3-position band switch. The antenna connection is made via an SO239 socket. The antenna should be one cut for the bands of interest and have an impedance of 50Ω for maximum signal pick-up. If the antenna is simply a long piece of wire, an antenna tuner or matcher will probably improve the performance, especially on the 20m band (see separate panel). Block diagram The overall block diagram of the receiver is shown in Fig.1. The receiver can be divided into two parts: a Direct Conversion receiver tuning from 2 to 2.5MHz, and a switchable 3- band frequency converter. The job of the converter section is to convert or translate the frequency of the signals from the three bands to a common 2 to 2.5MHz Intermediate Frequency band. The direct conversion receiver then converts the Intermediate Frequency signals to audio frequencies, filters and amplifies them. September 1996  29 30  Silicon Chip Signals from the antenna are fed to the RF attenuator, included to reduce the level of very strong signals which could cause the receiver to overload. This is especially true of short wave AM broadcast stations which unfortunately frequent the 40M band at night. The signals from the antenna then pass through the selected bandpass filter and appear at one input to the mixer. The Band switch also activates the relevant crystal oscillator and its output is applied to the second input of the mixer. A 2 to 2.5MHz bandpass filter selects the difference between the signal and oscillator frequencies at the output of the mixer and passes it onto the product detector. A variable frequency oscillator (VFO) is tuned by the main Fine Tune and the Fine Tune controls between 2 and 2.5MHz. The VFO signal is applied to the second input of the Product Detector and audio is recovered at the output. The low level audio is amplified and passed through a 2.3kHz lowpass filter which helps to eliminate adjacent channel interference found on a crowded band. Finally, the audio signal is fed to a power amplifier to drive a loudspeaker or headphones. Mixing The mixer used in this receiver is a double balanced type, meaning that the main outputs are the sum and difference of the two input frequencies. The two input frequencies themselves are largely suppressed. When the receiver is switched to tune the 20m band, 12MHz is injected into the oscillator input of the mixer, while it also receives signals in the range of 14 to 14.5MHz. The output of the mixer contains the sum frequencies between 26 and 26.5MHz and the difference frequencies between 2 and 2.5MHz. The filter connected to the output of the mixer passes only the 2 to 2.5MHz signals. The 14MHz signal has been converted to 2MHz and 14.5MHz to 2.5MHz. When 40m is selected the conversion is similar, where an oscillator frequency of 5MHz is mixed with the 7 to 7.5MHz signals to produce difference frequencies between 2 to 2.5MHz. The operation on the 80m band is slightly different in that the mixing frequency of 6MHz is above the input frequency of 3.5 to 4MHz. This means that this band tunes backwards compared to the other bands. 3.5MHz is converted to 2.5MHz while 4MHz is converted to 2MHz. This is a small price to pay for the simplification it provides. 12, 6 and 5MHz crystals are low cost common items. To make the 80m band tune forwards we would need to use a 1.5MHz mixing frequency which has two problems. Firstly crystals at this frequency are not common and more expensive, and secondly the image frequency lies in the AM broadcast band. This image could not be easily eliminated with the input bandpass filter. Circuit description The circuit diagram for the receiver is shown in Fig.2. Signals from the antenna pass through the variable RF attenuator (VR1) to three bandpass filters. Each filter is a double pole type using capacitive coupling. The inductors are standard prewound RF chokes and are brought to resonance by a parallel combination of a fixed capacitor and a variable trimmer capacitor. The filters are designed with a bandwidth wide enough to suit the Australian amateur frequency allocations. The filters are switched using diode switching and as each band operates the same way we will look at the 20m filter to see how it works. With the band switch in the 20m position, a current of about 3mA flows through each of the 1kΩ resistors, diodes D1 and D2 and the 470Ω resistors. The diodes provide a low impedance path for the RF signals when a few milliamps of DC current flows through them. The other diodes D3, D4, D5 & D6 will be biased off and provide a high impedance to the RF signals, effectively isolating the 40m and 80m filters. Using diodes eliminates the need to switch active signal leads and allows the switch to be located remotely. The only real drawback is some signal attenuation in the diodes. However this can be made up in the rest of the receiver. The output of the selected filter is connected to the primary winding of transformer T1. T1 matches the 50Ω impedance of the bandpass filters to the 3kΩ input impedance of the mixer. T1 also provides conversion from the unbalanced output of the filters to the balanced input of IC1 which is an NE602 mixer. September 1996  31 As you can see from this “opened out” photo, construction is almost entirely on one PC board. Since taking this photograph, we have added the reverse polarity protection diode, D8. The external mixing frequency is injected into pin 6 at around 0.5V peak-to-peak. Each band has its own crystal oscillator, formed with IC3, a 74HC00 and IC4, a 74HC10. This type of oscillator has a number of benefits over standard transistor oscillators. First, as they are made using NAND gates one of the inputs can be used to gate the oscillator on and off without switching power supplies or signal leads. Second, a 3-input NAND gate can be used to combine the oscillators into a single line and the output of the buffer stages will be a 5V logic signal. This means that we can use a simple 32  Silicon Chip voltage divider to provide the needed 0.5V peak-to-peak signal for all the frequencies. IC3a is the 12MHz oscillator with IC3b acting as a buffer stage. The oscillator is adjusted to exactly 12MHz by a trimmer capacitor in series with the crystal. Pin 1 of IC3a and pin 5 of IC3b are normally pulled low by a 10kΩ resistor, disabling the oscillator. When pins 1 and 5 of IC3 are switched to 5V by the band switch the oscillator is enabled. When one input of a NAND gate is low the output is forced to a permanent high state. The 5MHz oscillator uses IC3c and IC3d, while the 6MHz oscillator uses IC4a and IC4b. They both operate in the same way as the 12MHz oscillator. IC4c is the oscillator combiner. Only one oscillator will be operating at a time and the outputs from the other two oscillators will be high. When all the inputs to IC4c are high, pin 6 will be low. When the active oscillator’s output goes low pin 6 will go high. The 5V output signal is reduced to 0.5V by the resistive divider formed with the 1kΩ and 150Ω resistors. The 100pF capacitor across the 150Ω resistor provides some low pass filtering and reduces the level of harmonics. REG2 provides a regulated 5V for IC3, IC4 and the band switching diodes. The output of the mixer stage is applied to a 2 to 2.5MHz band pass filter. The PC board component layout, together with the PC board pattern. Take extra care when placing polarised components, such as electrolytic capacitors and semiconductors, to ensure they go in the right way! This filter is made up of two parts, a high pass filter using L7, two 56pF capacitors and a 150pF capacitor, and a low pass filter using L8, two 47pF capacitors and a 15pF capacitor. The 150pF and 15pF capacitors resonate with the inductors to provide deep notches of attenuation either side of the passband. The 2 to 2.5MHz signal goes to the product detector IC2 on pin 2. IC2 is another NE602 and mixes the input signal with a variable oscillator to produce an audio signal. The variable oscillator is formed with the second half of IC2. The os- cillator appears at pins 6 and 7. L9 is the coil for the oscillator and tuning is accomplished by a BB212 variable capacitance diode CD1. The 330pF capacitors provide the feedback path for the oscillator, while the 68pF capacitor in parallel with L9 acts with CD1 to set the frequency range. September 1996  33 The 330pF and 68pF capacitors are specified as polystyrene types in the parts list. This type of capacitor, while more expensive than ceramic types, offers superior stability in oscillator circuits. The capacitance of CD1 and hence the oscillator frequency is dependent on the voltage which is provided by the tune control. As the voltage on the control pin increases, the capacitance of CD1 decreases and as a result the frequency of the oscillator increases. The Tune control VR2 is a dual gang potentiometer with both gangs in parallel except for a resistor in series with each gang. One gang has a resistor in its positive side while the other gang has a resistor in its earth side. This produces a differential voltage between the wipers and will be constant over the full movement if the resistors have the same value. VR3 is the Fine Tune control and sweeps over the voltage that exists between the two wipers. The wiper of the Fine Tune control provides the tuning voltage for CD1. Note that the 150Ω resistors can be altered to tailor the fine tune range if required. Decreasing the resistors would decrease the fine tune range, and increasing them would increase the range. The resistor values could be made different if more range was required at one end of the tuning range than the other. A 100kΩ resistor and 1µF capacitor isolate CD1 from supply noise that could otherwise modulate the oscillator. A 10kΩ trimpot, VR4, is used in conjunction with the slug in L9 to set the frequency range over which the Tune control operates. The oscillator in IC2 is sensitive to loading on pin 7 and makes it difficult to directly measure the oscillator frequency. To overcome this, a FET buffer stage is used so that a frequency meter can be connected without significantly loading the circuit. Q1 is a MPF102 and its high input impedance, along with the 5.6pF capacitor, provide light coupling to the oscillator. REG1 provides power for the two NE602’s and its output voltage has been increased to 5.6V by the inclusion of a diode in the common lead. This has been done because the NE602 has slightly better performance at this increased voltage. Recovered audio appears at pin 5 of IC2 and any residual RF is filtered out by a .01µF capacitor. The audio stages use an LF347 quad op amp. The first stage, IC5a is configured as a non-inverting amplifier with a gain of around 11 at 1kHz. The non-inverting input is biased to +5.5V by the two 10kΩ resistors connected to pin 3. IC5b and IC5c form a unity gain 4-pole low pass filter with a cutoff frequency of 2.3kHz. IC5d is another non-inverting amplifier and has a gain of around 13 at 1kHz. 470µF and 100µF capacitors provide decoupling for IC5 and help ensure stability and low noise. Both IC5a and IC5d have a tailored frequency response that rolls off the gain for high and low frequencies. The output of IC5d at pin 7 passes to the volume control via a 1µF coupling capacitor. The final audio stage is IC6, an LM386 power amplifier. The 10Ω resistor and the 470µF capacitor on pin 6 provide power supply decoupling. This stage has ample gain and power PARTS LIST 1 1 1 1 1 1 3 1 1 1 1 1 1 1 1 1 2 2 2 2 20 1 1 1 PC board code 06109961, 167mm x 95mm plastic case, 196 x 112 x 60mm (aluminium lid) black binding post red binding post SO239 panel socket, square 6.5mm jack socket 20mm knobs 35mm knob 500Ω linear potentiometer (VR1) 10kΩ dual linear potentiometer (VR2) 50kΩ linear potentiometer (VR3) 10kΩ log potentiometer (VR5) 10kΩ horizontal trimpot (VR4) 2 pole 3 position slide switch (S1) F14 balun former (T1) 5mm coil former assembly (L9) 2.2µH RF inductors (L1,L2) 4.7µH RF inductors (L3,L4) 10µH RF inductors (L5,L6) 100µH RF inductors (L7,L8) PC pins 5MHz crystal (X2) 6MHz crystal (X1) 12MHz crystal (X3) 34  Silicon Chip Semiconductors 7 1N4148 diodes (D1 - D7) 1 1N4004 diode (D8) 1 BB212 dual varicap (CD1) 2 78L05 +5V voltage regulator (REG1, REG2) 2 NE602 balanced mixer (IC1,IC2) 1 74HC00 quad NAND gate (IC3) 1 74HC10 triple NAND gate (IC4) 1 LF347 quad op amp (IC5) 1 LM386 power amp (IC6) 1 MPF102 FET (Q1) Capacitors 3 470µF 25VW electrolytic 2 100µF 16VW electrolytic 2 1µF 16VW electrolytic 17 0.1µF monolithic 1 .047µF greencap (metallised polyester) 1 .015µF greencap 2 .01µF greencap 1 .0047µF greencap 1 .0033µF greencap 2 .001µF ceramic 1 470pF ceramic 3 330pF polystyrene 3 220pF ceramic 4 3 2 1 2 4 2 4 2 2 1 7 2 150pF ceramic 100pF ceramic 68pF ceramic 68pF polystyrene 56pF ceramic 47pF ceramic 33pF ceramic 15pF ceramic 10pF ceramic 5.6pF ceramic 2.7pF ceramic 5-40pF plastic trimmer (VC1-VC4, VC7-VC9) 5-60pF plastic trimmer (VC5-VC6) Resistors (0.25W, 1% or 5%) 3 10MΩ 8 1kΩ 1 1MΩ 6 470Ω 2 100kΩ 3 150Ω 1 47kΩ 2 100Ω 9 10kΩ 2 10Ω 2 4.7kΩ Miscellaneous Screws, nuts, spacers, hook-up wire, 0.4mm & 0.2mm enamelled copper wire, aluminium sheet, white cardboard. output to drive headphones or an external speaker. Construction Start construction by checking that the components with larger pins fit the holes in the PC board. This is especially true for the oscillator coil L9. You may also need to enlarge the holes for the trimmer capacitors and PC pins as well. There is one wire link on the board and this should be installed first. Follow this with the resistors, trimpot and the RF chokes. If you are using one percent resistors, double check the value before you solder them in as it is quite easy to read the wrong value. The 150Ω resistors associated with the main Tune control are actually soldered on the gangs and not on the PC board. The capacitors can be fitted next. Take particular care with the polarity of the electrolytics and the values of the capacitors associated with the bandpass filters. The filters will not work properly if wrong values are used. Note that VC5 and VC6 are 60pF trimmer capacitors while the rest are 40pF. The board has been designed to accommodate common 3 and 2-pin trimmer capacitors. Solder in the PC pins next. These make wiring easier and fault finding simpler, if needed. Install the semiconductors and crystals next, starting with the diodes. Note that IC3 and IC4 are installed upside down with respect to the rest of the IC’s. Coil winding At this stage we need to wind the two coils. Fig.3 gives the details. T1 is wound on a large two hole balun former using 0.4mm wire. The primary winding consists of 3 turns. A turn consists of passing the wire up through one hole and back down the other hole. The secondary winding consists of 23 turns and is wound over the top of the primary winding. The four ends of the windings will be at the same side of the former. You might label the windings so that you do not get the primary and secondary mixed up when you solder them in the PC board. The oscillator coil L9 is wound on a 5mm former which attaches to a 6-pin base and is enclosed in a metal can. The inductance of the coil is varied by an adjustable ferrite slug in the former. Start the coil by gluing the former into the base with a drop of Super glue. The coil requires 80 turns of 0.2mm wire and this needs to be wound in two layers of 40 turns each. Solder one end of the wire onto the start pin as shown in Fig.3 and starting at the base of the former, carefully wind on 40 turns side-by-side, ensuring that the turns are kept firmly in place. When the 40th turn is finished place a tiny drop of Super glue on it and hold the wire until the glue dries. Wind on the next 40 turns proceeding back down the former and solder the end of the wire to the end pin. Put a couple of drops of glue on the coil to keep the winding from moving. When the glue is dry, place the base into the PCB and screw the slug into the former leaving about half the slug outside the former. Place the can over the assembly, passing the slug through the hole in the can. This ensures the former is centrally positioned within RESISTOR COLOUR CODES    No.   Value ❏ 3 10MΩ ❏ 1 1MΩ ❏ 2 100kΩ ❏ 2 47kΩ ❏ 9 10kΩ ❏ 2 4.7kΩ ❏ 8 1kΩ ❏ 6 470Ω ❏ 3 150Ω ❏ 2 100Ω ❏ 2 10Ω 4-Band Code (1%) Brown Black Blue Brown Brown Black Green Brown Brown Black Yellow Brown Yellow Violet Orange Brown Brown Black Orange Brown Yellow Violet Red Brown Brown Black Red Brown Yellow Violet Brown Brown Brown Green Brown Brown Brown Black Black Brown Brown Black Black Brown 5-Band Code (1%) Brown Black Black Green Brown Brown Black Black Yellow Brown Brown Black Black Orange Brown Yellow Violet Brown Red Brown Brown Black Black Red Brown Yellow Violet Brown Brown Brown Brown Black Black Brown Brown Yellow Violet Black Black Brown Brown Green Black Black Brown Brown Black Black Black Brown Brown Black Black Gold Brown CAPACITOR MARKING CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 17 1 1 2 1 1 2 1 2 3 Value     IEC Code    EIA Code 0.1µF 100n 104 .047µF 47n 473 .015µF 15n 153 .01µF 10n 103 .0047µF 4n7 472 .0033µF 3n3 332 .001µF 1n 102 470pF 470p 471 330pF 330p 331 220pF 220p 221 ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 4 3 3 2 4 2 4 2 2 1 Value     IEC Code   EIA Code 150pF 150p 151 100pF 100p 101 68pF 68p 68 56pF 56p 56 47pF 47p 47 33pF 33p 33 15pF 15p 15 10pF 10p 10 5.6pF 5p6 5.6 2.7pF 2p7 2.7 September 1996  35 Once you have the PC board finished and the front panel and case drilled, final assembly is quite straightforward. Note the two resistors soldered directly to the Tune potentiometer, VR2. 36  Silicon Chip the can. Hold the can against the PCB and solder the can pins and then the former pins. Final construction The front panel layout can be seen in the photographs. If you are not building the receiver from a kit with a pre-punched front panel, use the front panel drawing to locate the holes for the front panel controls and drill to suit the potentiometers. The switch requires a rectangular hole and is easily made by drilling a couple of holes first and then filing to shape with a small flat file. The case needs to be drilled to mount the antenna socket on the left hand side and the binding posts and speaker socket on the right hand side. Place the PC board in the bottom of the case to mark the position of the four mounting holes and drill them with a 4mm drill. Mount the controls and switch on the front panel and the binding post and sockets on the case. Place a solder tag under one screw of the antenna connector for the earth connection point. If you are using potentiometers with long shafts, they will need to be cut to length with a hacksaw so that the knobs fit closely to the front panel. (This should be done before they are soldered or mounted). Mount the PC board in the bottom of the case with 3mm screws and nuts and 6mm spacers. All the wiring between the board and controls and sockets is done with hook-up wire. Leave just enough wire between the board and the front panel so that it can be lifted off and turned over to allow access – about 100mm should be enough. The front panel needs to be earthed to avoid hum getting into the Tune control wiring. The best way to do this is to solder short lengths of tinned copper wire (cutoff resistor pigtails are ideal) from the earth lugs of the RF attenuator and volume controls onto their respective metal cases. You will need a good, hot iron to solder to the pot cases and may need to slightly scratch the surface first to ensure the solder "takes". Providing a frequency readout on a receiver is never easy. The modern approach is to use a digital frequency display but these are complex, power hungry, expensive and can cause interference in the receiver sections. This receiver does not have The front panel and dial scale are reproduced actual size, so you can photocopy them and use them as templates for marking your front panel if not working from a kit. September 1996  37 one for all these reasons, although if the receiver is to be used permanently on a desk then a remote digital frequency meter could be attached to the VFO OUT point. This scheme would not give a direct frequency readout, however it would be accurate and the actual frequency could be easily deduced. To keep costs down and make the unit portable, the receiver has an analog dial attached to the main Tune control. It is expected that kit suppliers will provide screened dials but if you are building this receiver from scratch you will need to make your own. There are several ways to do this but the easiest way is to cut an 80mm diameter circle from aluminium, and glue a photocopy of the dial drawing to this. Drill a hole in the centre of the dial large enough to clear the threaded shank of the Tune control (about 12mm). Glue the large tuning knob onto the centre of the dial with suitable adhesive: silicone adhesive proved successful. It is obvious that a little care is needed here so that the knob is centred, otherwise the dial will rotate off centre. When the dial is complete it can be placed on the main Tune control. The marker at the top of the front panel above the dial provides a reference point to read the frequency. Initial testing The front panel should be left unscrewed from the case until all the testing and alignment is finished. Before we apply power, double check the wiring one more time. A minute here could save hours later on, not to mention dollars. Connect a 12V power supply to the binding posts with a multimeter set to measure mA in the positive lead. Plug a speaker into the speaker socket and turn the volume control fully anticlockwise. Turn on the power supply and note the current drawn. The prototypes drew around 40mA with no signal. Obviously no current indicates an open circuit and a much larger current indicates a problem. This could be a wire in the wrong place, a component in the wrong way or a solder bridge on the PC board. If everything appears correct, measure the voltage at the outputs of REG1 and REG2. These should be close to 5.6V and 5V respectively. Check that 38  Silicon Chip The coil (L9) & transformer (T1) are quite simple to make but take care with the start and end of the windings. “ENCU” means enamelled copper wire – small rolls are available from most component suppliers. the voltage between pin 7 of IC5 and the negative supply rail is between +5 and +6V. All the stages of IC5 are direct coupled and any problems with this circuit will probably show up with this check. Turn the volume control to mid position and listen to the speaker. You should be able to hear some hiss, indicating that at least the final audio amplifier is working. At this stage, it may be possible to receive some signals with a suitable antenna but don’t expect too much until alignment is completed. Alignment equipment To properly set up the receiver two pieces of test equipment are required which may not be a part of the average constructor’s workbench . . . yet! You will need a frequency counter capable of reading to 12MHz and an RF signal generator with an output to 15MHz. In addition, a digital multimeter is needed but even novice constructors should have one of these! If you don’t own a digital frequency counter or RF signal generator, think about likely people who could help you out. Most schools would have such equipment in their science or technics areas. Perhaps a local amateur operator could help you out (they’re usually delighted to help beginners get “hooked” on amateur radio!). Look for antennas or towers in local backyards and don’t be afraid to knock on the front door and explain your problem. Take this article with you so the amateur knows what is required. A last resort could be a local technician or service shop. But be warned, these people are trying to earn a living out of electronics and may want to charge you a fee. Frequency setting Ensure that the receiver is powered up for at least 10 minutes before doing this section. This allows the oscillators to stabilise, especially the VFO. Switch the band switch to the 20m position and connect a frequency counter to pin 6 of IC4c. Adjust VC7 until the display reads exactly 12MHz. Switch to 40m and adjust VC8 for exactly 5MHz, and finally switch to 80m and adjust VC9 to show exactly 6MHz. Adjust the Fine Tune control VR3 and trimpot VR4 to halfway. The Fine Tune potentiometer may need rotating so that the pointer on its knob is vertical with the wiper at halfway. Connect the frequency meter to the VFO OUT point and rotate the Tune control almost fully anticlockwise. At the very end of the rotation there is a dead spot and it is not until a few degrees from the end that the potentiometer works properly. Adjust the core of L9 until the frequency counter reads 2MHz. The core of L9 is quite brittle. To avoid damage, use a good quality alignment tool – don't use a screwdriver! Rotate the Tune control almost fully clockwise, again noting the dead spot at the very end of the travel and adjust VR4 until the counter reads 2.5MHz. Go back and forth a couple of times till you are satisfied with the range, as there will be a some interaction between the adjustments. If you use the pre-printed dial or a screened dial from a kit supplier, you should be able to adjust the dial position so that it lines up with the frequencies being received. If not, you will need to mark your own dial - in any case, the following can be done to check the dial positions. Return the Tune control to the 2MHz point and make a mark with a pencil on the dial opposite the line on the front panel. This mark represents the 4MHz point for 80m, the 7MHz point for 40m and the 14MHz point for 20m. Slowly rotate the dial clockwise until the frequency is 10kHz higher and make another mark on the dial. Continue this process until 2.5MHz is reached. This mark represents 3.5MHz, 7.5MHz and 14.5MHz. With an ink pen or rub on lettering go over the marks to make them neat and permanent and at the 100kHz points mark a longer line. The 100kHz points should then be labelled for each band; eg, 4.0, 3.9, 3.8, 3.7, etc. Move the Fine Tune control from end to end and check the frequency shift. If the range is about 5kHz either way no changes need to be made. If you feel the range needs changing refer to the circuit operation section about altering the 150Ω resistors. Filter alignment Connect an RF signal generator to the antenna socket set to 3.6MHz. Switch the receiver band switch to 80m. Connect an oscilloscope or a digital multimeter set to a low AC volts range across the volume control. Move the Tune control until a beat note of around 1kHz is heard in the speaker. Adjust the volume control for a comfortable level. If the receiver is overloaded, giving a distorted tone in the speaker, decrease the output of the signal generator or adjust the RF attenuator until the tone sounds undistorted. Note that the RF attenuator will not completely cut off the input signal due to stray RF coupling around the control. Adjust VC5 and VC6 until a peak is observed in the level of the tone. Select the 40m position and change the generator to 7.1MHz. Move the tune control to give a 1kHz beat note and adjust VC3 and VC4 for maximum audio output. Now switch to 20m and set the generator to 14.2MHz. Move the Tune control to give a 1kHz beat and adjust VC1 and VC2 for maximum audio level. This process gives maximum sensitivity in the middle of the band and should provide a reasonably flat response across the whole range. If instruments are not available, a less precise method is to tune to a station in the middle of each band and adjust the relevant trimmer capacitors for maximum audio from the speaker. Remove all the instruments and screw the front panel to the case. The unit is now ready for use. Connect power, a speaker (or headphones) and SC an antenna and start listening! What about an antenna? For general shortwave listening, the basic rule for antennas ever since the days of Mr Marconi and friends seems to have been “as long and as high as possible”. While technically not quite right, a long, high antenna has been a reasonable choice given the fact that most short wave listeners want to cover frequencies from the broadcast band (around 1MHz) all the way up to 30MHz, and most communications receivers can handle high impedance antennas (which a long wire is). Add to that the fact that most people live in cities or towns and are constrained by their own back yards. For amateur radio it’s a bit more exact, or theoretically should be. To really pull in amateur DX signals, the antenna should be made to suit the band being used - that is, separate antennas for 80, 40 and 20 metres cut so they resonate at the centre of the respective band (or if you are interested in a particular part of the band, at that frequency). You will normally get acceptable performance over the rest of the band. With many variations, there are two basic types of antenna - horizontal and vertical. The horizontal antenna can be a dipole - that is, signal taken from the middle, or it can be a long-wire, with signal taken from the end. Talking generally, a dipole antenna cut to half the wavelength of the frequency of interest will be the better performer, giving good results for signals perpendicular to it - that is, a dipole mounted north/south will have its best reception east/west. Now, what length? The formula for working out the half wavelength (l/2)=150/f, where f is the frequency of interest in MHz. For several reasons which we won’t go into here, the dipoles are cut slightly shorter: dipole length (m) =71.25/f. Therefore a half wave dipole for 3.5MHz (80 metre band) would be 40.7 metres long, with each dipole 20.35 metres. That’s quite a length of antenna, given that the average suburban block is only 45 metres deep! Antennas for the 40 and 20 metre bands are much more manageable. And if you erect an antenna designed for 40m, you can expect at least reasonable performance on 80 and 20m. A dipole can be erected horizontally (supported high at each end), inverted (supported high at the middle with each end supported slightly off the ground), or even sloping (high support one end, low support the other). The last mentioned is often used in suburbia with the antenna supported at one end by a mast on the house and by the back fence at the other! Of course, you could mount a dipole vertically but where are you going to find a forty metre high non-metal pole? (The metal would interfere with the antenna). Strictly speaking, you should use a balun to match the 75Ω impedance of the dipole to the 50Ω impedance of the feedline and receiver. The truth is, especially for receiving, you can usually ignore the mismatch. If you wish to erect a long-wire antenna, theory says that an antenna tuner will be needed for optimum receiver performance. But if you don't have one? Give it a go anyway.You can't do any damage! September 1996  39