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By DARREN YATES An 80-metre AM/CW transmitter for amateurs You don’t need lots of money to get started on the 3.5MHz amateur band. This low-power transmitter puts out about 100mW PEP, is powered by a 6V battery & is ideal for use by novice & QRP operators. This little transmitter won’t set the world on fire with its performance but, on a dollar-for-dollar basis, you won’t find much better in terms of simplicity. And if you enjoy the chal­ lenge of operating QRP (ie, at low-power), then this unit is just the shot. Operating QRP is a real test of skill when it comes to chasing those distant DX contacts. In order to keep it as simple as possible, and in the interests of stability, 30  Silicon Chip the transmitter is crystal-locked to 3.579MHz. This puts in right slapbang in the middle of the novice band (3.525-3.625MHz), so it is an ideal way to get start­ed in amateur radio – there’s no need to lash out on expensive gear and you gain the experience of building and operating your own transmitter. Other features of the design include the ability to operate either CW (Morse) or AM (voice) at the flick of a switch, and the use of bog-standard components. Most transmitter designs require a swag of handwound coils which are used in the output-stage filtering and tuning stages. By contrast, this circuit uses standard pre-wound RF chokes which look just like 1W resistors (and, in fact, are installed in exactly the same manner). You can’t make things much easier than that! Another good feature of the design is that it’s portable, an important consideration if you want to “go bush”. The power comes from four 1.5V AA cells which should give about 20 hours continuous operation in most conditions. By the way, you must have an amateur radio licence before using this transmitter. If you don’t already have a licence but are interested in amateur radio, you can find out more by contacting the Wireless Institute of Australia (WIA) in your state. How it works Fig. 1 shows the circuit for the 80-Metre AM/CW Transmit­ter. As you can see, there isn’t much to it – just a handful of common transistors, a crystal, a couple of pre-wound RF chokes and a few passive components. Transistor Q1 is connected as a Colpitts oscilla­tor whose frequency is set by a 3.579MHz NTSC TV colour burst crystal (X1) This stage oscillates by virtue of the feedback path provided by the .001µF and 100pF capacitors and the crystal itself. The output appears at the emitter of Q1 and is buffered by emitter follower stage Q2. This is done to prevent loading of the oscillator output which would otherwise cause it to stop. After that, the signal is split along two paths and used to drive two different stages: (1) a voltage doubler/ diode pump stage based on D1 and D2; and (2) an output amplifier stage based on Q4. The voltage doubler/diode pump stage converts the 3.58MHz signal into a steady DC voltage. This DC voltage then switches on transistor Q5 which in turn lights LED 1 to indicate that the carrier signal is present. In addition, the output from D2 provides the bias All the parts, including the 4-way battery holder, fit neatly inside a small plastic case. Make sure that switch S1 is correctly oriented on the front panel & check that none of the parts short together when the lid is closed. for driver stage Q3, either via switch S1 (for AM operation) or via KEY 1 (for Morse code operation). This may seem a little unusual but it ensures that if, for some reason, the carrier signal fails to appear, the output stage isn’t wasting current trying to transmit something that doesn’t exist. The AM (amplitude modulated) signal is also fed in at this point. This can come from just about any source (eg, a microphone preamplifier) and is applied via a 0.1µF capacitor and a 10kΩ level pot (VR1). After that, the signal passes via a 4.7kΩ resis­tor and is mixed with the bias voltage before being applied to the base of Q3 via S1 (for AM mode). Switch S1 controls the mode of transmission. With S1 open, KEY 1 is called into play and the transmitter S2 +6V ANTENNA 100 16VW 330  6V A  LED1 Q1 BC548 C B 68k X1 3.579MHz Q2 BC548 10k B C Q5 BC548 10k S1 C E 1.5k 100pF 100pF B E E .001 K +6V 0.1 100pF D2 1N914 1k 10k 3.3k B 4.7k 470  .001 B E 0.1 C VIEWED FROM BELOW A K AM INPUT +6V C L1 2.2uH 680pF E 0.1 0.1 D1 1N914 KEY1 Q3 BC337 Q4 BC337 B 100pF 100pF 100pF L2 4.7uH L3 2.2uH C 180pF E 180pF VR1 10k 80M CW/AM TRANSMITTER Fig.1: the transmitter operates on 3.579MHz, as set by the Colpitts oscillator based on Q1 & crystal X1. June 1994  31 Fig.2: install the parts on the PC board & complete the wiring as shown in this diagram. Make sure that all parts are correctly oriented on the board & be careful not to confuse the transistor types. MORSE KEY LED1 S1 VR1 A K 6V BATTERY 0.1 10k X1 D1 operates in CW mode. Note that you need to remember to remove the AM input when operating CW. With S1 closed, KEY 1 is bypassed and AM signals are fed to the base of Q3 via a low-pass filter consisting of a 3.3kΩ resis­tor and a 100pF capacitor. This filter network attenuates any unwanted RF signals in this part of the circuit. Transistor Q3, a BC337 NPN type, is used as a driver for the main output stage, Q4 (another BC337). This stage is also driven by the carrier signal which appears at the emitter of Q2, as described earlier. A 100pF capacitor to ground from the base of Q4 provides some light filtering and improves the quality of the carrier signal. Basically, Q3 controls the bias applied to the base of Q4. In CW mode, 3.3k D2 0.1 Q2 10k 100pF S2 LED1 A K Q5 10k L3 L2 100pF 1k Z .001 Q1 1.5k +6V 68k 0.1 ANTENNA L1 4.7k 330  .001 100uF Q3 680pF 470  AM INPUT +6V 180pF Z GND 100pF 100pF Q4 180pF KEY 1 turns Q3 on and off and this, in turn, switches Q4 on and off. Thus, each time KEY 1 is pressed, Q4 is biased on and a burst of carrier signal is fed to the antenna circuit. When AM operation is selected, the signal on Q3’s emitter continuously varies the bias applied to Q4 and so Q4 amplitude modulates the carrier. Output stage Q4 operates as a common emitter amplifier with a parallel LC circuit making up a tuned collector load. This tuned circuit consists of a 2.2µH inductor and a 680pF capacitor and has a fre­quency of resonance which is close to the 3.58MHz carrier fre­quency. This not only ensures maximum gain at the desired frequen­cy but helps to remove unwanted harmonics as well. From here, the signal passes through an output filter stage consisting of inductors L2, L3 and their associated capacitors. L2, L3 and the 180pF capacitor to ground form a low-pass filter which rolls off the response below 4.5MHz, while L3 and its parallel 180pF capacitor form a notch filter which is centred on about 7.2MHz (the notch frequency is also set, to some extent, by the second 180pF capacitor). This notch filter is used to curtail the second harmonic, so that we are left with a carrier sinewave of quite good purity. The reason we are after a pure sinewave is to prevent interference to other frequencies in the RF spectrum. Finally, a 100pF ceramic capacitor decouples the antenna from the output stage. The antenna should be RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  3 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 32  Silicon Chip Value 68kΩ 10kΩ 4.7kΩ 3.3kΩ 1.5kΩ 1kΩ 470Ω 330Ω 4-Band Code (1%) blue grey orange brown brown black orange brown yellow violet red brown orange orange red brown brown green red brown brown black red brown yellow violet brown brown orange orange brown brown 5-Band Code (1%) blue grey black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown brown green black brown brown brown black black brown brown yellow violet black black brown orange orange black black brown This close-up view shows the completed PC board assembly. Use PC stakes at the external wiring points & keep all component leads as short as possible. a 10-metre length of hook-up wire and should be about 10 metres above ground if possi­ble. The ground connection can be taken from the board to a stake in the ground. Power for the transmitter is supplied by four “AA” alkaline cells. The transmitting current is approximately 70mA so alkaline cells should give about 20 hours of continuous use. Note that Q4 is always biased on by an AM modulated base current when operating in the AM mode. The result is that if you increase the supply voltage beyond 6V, the current consumption quickly rises to about 100mA (at approx. 8V). This, in turn, will lead to a rapid rise in Q4’s temperature and, eventually, it will self-destruct. Even with brand-new cells, the circuit is perfectly reli­ able and no problems should be found if you stick to the 6V supply specified. Construction Most of the parts for the 80-Metre CW/AM Transmitter are installed on a PC board coded 06106941 and measuring 101 x 39mm. Check your PC board against the published pattern (see Fig.4) before installing any of the parts, to make sure that the board has been etched correctly. It’s much easier to find and correct any prob­lems at this stage rath- er than later on when the parts have been mounted. Fig.2 shows the parts layout on the PC board. Begin the assembly by installing PC stakes at the external wiring points, then install the resistors, inductors and diodes. Note that, because sections of this circuit handle RF signals, it’s import­ant to keep all component leads as short as possible. Check that the diodes are correctly oriented, then complete the board assembly by installing the capacitors, PARTS LIST 1 PC board, code 06106941, 101 x 39mm 1 front panel label 1 zippy case, 130 x 68 x 41mm 1 black 4mm socket 1 black 4mm plug 1 red 4mm socket 1 red 4mm plug 1 3.579MHz colour burst crystal 1 SPDT toggle switch 2 3.5mm socket 1 5mm LED bezel 1 knob 6 PC stakes 1 AA x 4-cell long battery holder 1 9V battery snap connector 1 10kΩ log pot 1 10-metre length of hook-up wire (antenna) Semiconductors 3 BC548 NPN transistors (Q1,Q2,Q5) 2 BC337 NPN transistors (Q3,Q4) 1 5mm red LED (LED 1) 2 1N914 diodes (D1,D2) Capacitors 1 100µF 16VW electrolytic 3 0.1µF 63VW MKT polyester 2 0.001µF ceramic 1 680pF ceramic 2 180pF ceramic 4 100pF ceramic Inductors 1 4.7µH RF inductor (L2) 2 2.2µH RF inductors (L1,L3) Resistors (0.25W, 1%) 1 68kΩ 1 1.5kΩ 3 10kΩ 1 1kΩ 1 4.7kΩ 1 470Ω 1 3.3kΩ 1 330Ω Miscellaneous Screws, nuts, washers, solder, hook-up wire. June 1994  33 80-METRE AM/CW TRANSMITTER AM AUDIO LEVEL CW TRANSMISSION KEY IN 3.58MHz CARRIER Fig.3: this full-size artwork can be used as a drilling template for the front panel. Use a small pilot drill to drill the holes initially, then carefully ream them to size using a tapered reamer. transistors and the 3.579MHz crystal. Be sure to install the correct transistor type in each location – Q1, Q2 and Q5 are BC548s, while Q3 and Q4 are BC337s. The crystal can go in either way around, since it is not polarity conscious. Final assembly The circuit board is designed to fit inside a standard plastic box measuring 130 x 67 x 42mm. As shown in the accompany­ing photo, the board sits in the bottom of the case, with the battery holder mounted down one side. Before mounting the board, attach the front-panel label to the lid of the case and use this as a template for drilling the holes for the front panel hardware. Note that these holes are best drilled using a small pilot drill and then enlarged as necessary using a tapered reamer. Once these holes have been drilled, mount the various items in position, then drill holes in the box for the power switch and audio input socket at one end and the antenna and ground sockets at the other. The PC board can now be positioned in the case alongside the battery holder and used as a template for marking out its mounting holes. Drill these holes to size, then secure the board using machine screws and nuts. The assembly can now be completed by installing the remain­ing items of hardware and running the necessary wiring connec­tions. Testing Check your wiring and the PC board assembly carefully before applying power. In particular, check that all components are in their correct locations and that the front panel wiring is correct. When you’re satisfied that everything is correct, connect your multi­meter (set to the 400mA range) in series with one of the power supply Fig.4: check your PC board against this full-size etching pattern before installing any of the parts. The board measures 101 x 39mm & is coded 06106941. 34  Silicon Chip leads, set S1 to CW (ie, S1 should be open) and apply power. You should find that the carrier LED is now lit and that the quiescent current is about 5-10mA. If the cur­ rent drain is much higher than this or if the LED doesn’t light, switch off and check the circuitry around Q1, Q2, D1-D2 and Q5. If everything appears to be OK, you can now check that the transmitter actually operates. To do this, you will (obviously) need a shortwave receiver or, more specifically, a receiver that will tune the relevant frequency (ie, 3.579MHz). If you don’t already have a receiver, then there are a number of low-cost units to choose from at your local electronics retailer. The final test simply involves making a transmission (note: you must have an amateur radio licence). To do this, connect an audio source to the AM input, switch S1 to AM and then tune to 3.579MHz on your receiver. Even with just a short length of antenna lead, you should have no problems picking up the signal on the radio. Note that, in this mode, the meter should register a cur­rent drain of 7080mA. If it’s more than this, switch off imme­diately and check for assembly errors. CW transmissions Most low-cost commercial receivers cannot receive CW trans­ missions, since they don’t include a BFO (or beat frequency oscillator). However, there is a way around this problem. If you do wish to transmit Morse code to one of these receivers, all you have to do is feed a 1kHz sine or square wave signal into the AM input and switch S1 to CW. That way, each time the Morse key is pressed, brief bursts of amplitude modulated signal are radiated by the antenna. Of course, if your receiver does have a BFO, you can remove the AM input and transmit straight CW only. Finally, note that the better the antenna used, the better the results from this little transmitter. Our tests were performed using a simple 10-metre long-wire antenna but more elaborate antennas should give better perforSC mance.