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Background source: https://unsplash.com/photos/silhouette-of-trees-under-starry-night-O7FxxiZr-Hk By Andrew Woodfield, ZL2PD Micropower SSB Single Side BAND Transmitters This project is an attempt to see how small an HF SSB transmitter can be made with a minimal parts count. One of the three versions uses just three transistors while generating a surprisingly clean signal! D uring one of our local radio club meetings a little while ago, someone tossed a PTT microphone over to me. “Here, maybe you can use this!”, they joked. Lacking the usual curly cord and connector, it had clearly seen better days. Nevertheless, I took it home with me and overnight, an idea came to mind. The Pixie range of CW (continuous wave, ie, Morse Code) transceivers is very well known, as is Doug DeMaw’s March 1976 Tuna Tin Two CW 70 Silicon Chip transmitter and George Burt’s “OXO” three-transistor CW transmitter. Having more interest in QRP SSB transceivers (www.zl2pd.com), I wondered if an SSB transmitter could be shoehorned inside that microphone shell. Naturally, only a very low-power SSB transmitter would be possible, a design using an absolute minimum number of parts. As an extra challenge, I decided to avoid using SMD components or ICs. The result, for the cost of one more Australia's electronics magazine transistor than the famous OXO transmitter, is a small milliwatt-level LSB transmitter operating close to 3.7MHz on the 80m band. Named the “Mike-One”, it uses just four general-purpose NPN transistors and a set of low-cost crystals. In its present form, it will never achieve transoceanic communications. Instead, covering short distances across the shack or at the radio club, it’s intended to be a lighthearted example of minimalist analog design. It’s also quick and easy to build, so it can be used as a teaching aid to illustrate the generation of conventional SSB signals at a very low cost. Circuit description This transmitter is cut to the bone. As Fig.1 shows, it features a microphone amplifier (Q1), a carrier generator (Q2), a balanced modulator (diodes D1 and D2), a three-crystal ladder-type SSB filter, an unusual ‘autodyne’ oscillator-mixer (Q3) and a single radio frequency (RF) amplifier stage (Q4). All stages use the generic BC548 NPN small signal transistor or one of its equivalents. The relatively high level audio signal from the electret microphone allows a single transistor amplification stage (Q1) to generate sufficient audio from the microphone to directly drive the balanced modulator. The 18.432MHz carrier, set by a small 22pF series capacitor (Cx), is balanced out in the mixer using VR1, a 100W trimmer. This arrangement saved several bypass capacitors that are usually required in such stages. siliconchip.com.au The autodyne converter stage (Q3) allowed a further useful reduction in the parts count. This mixer is a 14.7456MHz Colpitts crystal oscillator, amplitude modulated by the 18.4320MHz SSB signal. The output includes the desired 3.7MHz lower sideband (LSB) output, the difference between these two frequencies. This type of mixer was very common in the first stage of cheap transistor AM broadcast receivers, and it was also briefly popular in a few early commercial and amateur radio VHF FM transceivers. While it saves a few parts, the output of this mixer demands a good bandpass filter (L1, L2 etc) to remove the other unwanted products, including the 14.7456MHz oscillator output. A single π filter at the transmitter output also contributes to the low spurious and harmonic products of the design. It serves a secondary purpose – the output load is unlikely to be a perfect 50W load. I’ve mostly demonstrated it with just a length of hookup wire, perhaps half a metre long. Of course, it will work perfectly into a good load, but the useful feature of a π filter is that it transforms the output impedance of awful loads, such as the very low impedance of my 50cm of hookup wire (less than 1W) or an off-resonant long wire (possibly a few thousand ohms) to an impedance of 25-200W at the collector of Q4. Q4 is most unlikely to suffer damage as a result. The bandpass filter (BPF) is designed to be as flexible as possible to allow for a variety of crystals, as the following Photo 1: MikeThree (40m band) is even less complex than the others, with just three transistors. sections will show. In the Mike-One, the BPF is arranged in a series-­parallel arrangement to reduce the loading on the autodyne mixer. The choice of carrier, filter and mixer crystals is dictated by the current selection of readily available crystals. If your parts bins are well-stocked, you may prefer to use other crystals. In the days of analog TV, 6.552MHz crystals were widely available, as were 10.245MHz crystals for converting 10.7MHz intermediate frequency (IF) signals to a second IF of 455kHz. This combination will also produce an 80m LSB signal close to 3.685MHz. In this case, the carrier frequency sits at the lower corner of the crystal filter passband. This means that capacitor Cx in Fig.1 is replaced by a 15μH RF choke (RFC1 in Fig.2). This lowers the carrier crystal frequency to 6.5500MHz. I’ve named this version the Mike-Two. The different array of outputs generated by the autodyne mixer requires a slightly different BPF, but the output LPF remains unchanged. The Mike-Three By this stage, I could see a way to further reduce the number of parts used in these first two versions. Mike-Three is an example of an ‘on-frequency’ SSB transmitter that avoids the need for the mixer stage. This time, it produces a 7.2MHz LSB signal on the 40m amateur band (Fig.3) with just three transistors. These crystals are also very widely available. Mike-Three uses the same PCB as the others, but with fewer components, as shown in Photo 1. Construction All three versions can be built on the same small single-sided PCB, which was designed to fit into the prototype push-to-talk (PTT) microphone case (see Photo 2). This style of PTT microphone has been made in very large numbers by many manufacturers over Photo 2: an otherwise useless PTT microphone lacking a cord was the inspiration for this tiny SSB transmitter. Fig.1: the Mike-One circuit features a basic three-crystal SSB filter, along with an unusual autodyne oscillator-mixer stage to minimise the parts count. siliconchip.com.au Australia's electronics magazine June 2026  71 the years. It’s fairly likely you can lay a hand on a suitable microphone without much difficulty. In case you can’t, I’ve created STL files so you can 3D-print one! To fit everything in the limited space, almost all resistors are mounted on-end. The PCB is coded 06103261 and measures 44.5 × 76.5mm. Refer to the overlay matching whichever version you are building – Fig.4 for MikeOne, Fig.5 for Mike-Two or Fig.6 for Mike-Three. Start by fitting all the resistors and capacitors, then proceed to fit the parts to complete each stage, one by one, testing each completed stage as you proceed. T1 is made by twisting three 120mm lengths of 0.25mm enamelled copper wire (ECW) together. Two or three twists per centimetre is all that is 72 Silicon Chip required. Wind eight turns of this ‘trifilar’ triple wire arrangement onto an FT37-43 toroidal core. The toroid may be replaced by a similar-sized toroid recycled from an old fluorescent lamp. I found a less expensive approach to winding T1: wind four trifilar turns of 0.25mm ECW on a low-cost ferrite bead. Avoid ferrite beads with a small 1.5mm hole. They can be used, but it’s quite difficult to get all that wire through the small centre hole. I had some ferrite beads with a 2mm hole, which allowed for the required turns to be achieved far more easily. The material used to make these ferrite beads can vary enormously, so this may not work for you with your parts and your carrier frequency. Using the FT37-43 toroidal core is the most reliable option. When the toroid or ferrite bead has Australia's electronics magazine been wound, identify the start and end of each winding. These are numbered on the circuit diagrams and PCB overlays to help with construction. Solder one set of three wires into the holes marked 1, 3 and 5 in any order. Now, using a continuity tester or ohmmeter with an audible continuity (‘buzzer’) function, identify each of the matching ends for each wire, one by one, and solder them into the correct matching holes, marked 2, 4 and 6. L1 and L2 are inexpensive 7×7mm unshielded variable inductors. These have 26 turns and a range of 3-6μH. The similar-looking inductors with only about 12 turns (0.6-1.7μH) cannot be used here. However, if you buy them by accident, just rewind them with the required number of turns. They will work just fine. The BPF has been designed to allow siliconchip.com.au Fig.2: Mike-Two uses a 6.552MHz carrier and filter crystals, along with a 10.245MHz mixer crystal to give SSB on 80m. Besides the crystal changes, some capacitor values have been altered, and the BPF has been reconfigured. Fig.3: the circuit of the ultra-simple 40m Mike-Three SSB transmitter. The crystals change again, plus some capacitor and inductor values. In addition, the autodyne mixer and its associated band-pass filter have been removed and bypassed. either a series-parallel BPF (Mike-One) or a coupled BPF (Mike-Two). The relevant PCB overlays show the location of the wire added to configure these correctly. Other arrangements are possible with this PCB layout for those wanting to experiment further. L3 is made by winding 27 turns (Mike-One or Mike-Three) or 15 turns (Mike-Two) of 0.375mm enameled copper wire onto a T37-6 core. However, a less expensive solution is to use a 2.2μH RF choke (Mike-One or Mike-Three) or an 820nH RF choke (Mike-Two). Both methods gave similar results for me. In the case of Mike-Three, no mixer or bandpass filter components are fitted or required. This time, a short jumper wire connects between two empty capacitor pads, as shown in Fig.6. However, the output pi filter siliconchip.com.au Figs.4-6: the PCB overlay diagram for ▶ the Mike-One (top), -Two (middle) and -Three (bottom) variants; use the component values and locations shown here to build each version. There are some small component differences between each diagram such as the crystals and lack of mixer circuitry. should still be fitted to ensure any spurious and harmonic products are minimised, and to deliver some useful impedance matching. The PCB mounts component-side down in the case, with the electret microphone (MIC1) soldered on the solder side of the board. I used several drops of hot-melt glue at the top edge and the PCB corners to hold it in place. 3D-printed microphone shell While the microphone shell shown is readily available, it’s likely some readers will still find it difficult to locate or expensive. For that reason, I’ve also designed a lowcost 3D-printed version, shown in Figs.7(a)-(d). In this version, the electret mic capsule mounts on the same side of the PCB as the other components. You can download the STL files from siliconchip.au/Shop/11/3582 The case is in four parts: lower, middle and upper sections, plus the ‘pressel’ lever. The lever’s hinge fits into the mating slot in the middle section. I used a 10mm-long scrap of copper wire to hold the lever in place on one version, and a cut-down 1.6mm panel pin (ie, a small nail) on another. This assembly is then placed on the back shell of the microphone. Three or four drops of hot glue will hold these together. Avoid getting any glue near the pressel. Australia's electronics magazine June 2026  73 The transmitter PCB can then be inserted into place – component-side up this time – and a drop of hot glue applied at the top edge to hold it in place. The battery, LED and related wiring can now be added, and the length of wire to be used for the antenna also connected to CON2 and fed through the antenna hole in the shell. The upper shell of the microphone may then be placed on top of the assembly. Three 20mm-long self-­ tapping screws hold the microphone case together. Alignment Depending on the crystals you use and the version of the transmitter you are building, you will need to mount either a small capacitor, Cx/Ca, or an RF choke, RFC1, in the top-right corner of the PCB. Nominal values for these parts have been shown in Figs.1-6. This allows the carrier crystal frequency to be at the upper or lower corner of the SSB crystal filter respectively. The values shown (22pF, 15μH or 18pF) were found to be best for the prototypes, and are likely to suit most applications, but your crystals may require slight changes. Values are likely to be in the range from 4.722μH for RFC1, and 10-33pF for Cx/ Ca. You can listen to your signals on an SSB receiver to confirm the audio quality is reasonable and the opposite sideband is nearly inaudible. L1 and L2 should be adjusted for maximum transmitter output. These have a reasonably broad tuning response. Of course, this step is not required for the Mike-Three. VR1 in the balanced modulator should be adjusted to give minimal carrier output in the absence of modulation. This setting is very sharp and will be close to the midpoint of the adjustment range of the trimmer. I built all the prototypes using a variety of crystals, which delivered about 0dBm into 50W with a carrier suppres- Photo 3: this version uses Cx to set the correct carrier frequency (PCB upper right), a ferrite bead for the balanced modulator (PCB centre), and an RF choke for the LPF (PCB lower edge). Photo 4: Mike-One uses more crystals than transistors! The narrow bandpass filter required for SSB demands at least three crystals, while the carrier oscillator and mixer add two more. sion of 40dB or better, and 30-40dB of unwanted sideband rejection. This latter value depends on the audio modulation frequency. All spurious and harmonics were attenuated by 50dB, and many by as much as 60dB. The second method I tested used a small A23 or A27 12V alkaline battery. An A27 battery has a diameter of just 8mm and a length of 28mm. The capacity of an alkaline A27 battery is about 30mAh. While modest, it proved ideal for the original microphone shell. I also tried fitting a slightly larger A23 battery (the holder is visible in Photo 3). The higher A23 or A27 battery voltage of 12V is perfectly OK for the transmitter. Usefully, it also allows a blue or red LED to be fitted in series with the supply wiring to CON1. The supply voltage at the transmitter is dropped by almost 3V by a blue LED and by about 2V by a red LED. The LED is lit during transmit mode and its brightness gives an approximate indication of the battery level. Power supply options If you are building the Mike-One in a small box rather than in a microphone, you can use a standard PP9 type 9V battery. The transmitter only draws 15mA, so the battery will last for quite a long time. Fitting a battery in the limited space available inside the microphone shell presented a challenge. One approach tested used a small recycled 3.7V 70mAh Li-Po cell and a tiny boost converter module. This gave a very reliable 9V supply. Figs.7(a)-(d): this 3D-printed microphone shell has been designed for those unable to locate a suitable microphone shell. 74 Silicon Chip Australia's electronics magazine siliconchip.com.au A23/A27 battery holders Battery holders for these tiny A23 and A27 batteries are not always readily available. Faced with this, I designed and printed a simple 3D-printed holder for each type, shown in Fig.8. The battery contacts were fabricated from a pair of M2.2 solder tags. I filled the hole normally used for a bolt with a film of solder. These were pressed through the 3D-printed battery holder from the inside and held in place by the battery and the slight tension of the holder. I placed some clear adhesive tape around the battery before inserting it to make it easier to replace. Parts List – Micropower SSB Transmitter Fig.8: the 3D-printed 12V battery holders provide a low cost solution to fitting a small battery inside the microphone shell. 1 single-sided PCB coded 06103261, 44.5 × 76.5mm 1 electret microphone (MIC1) [Altronics C0170, Jaycar AM4011] 1 PTT microphone shell, salvaged or 3D-printed 3 M3 × 20mm self-tapping screws (for 3D-printed case) 1 A23 or A27 12V battery 1 3D-printed battery holder 2 2.2mm solder lugs (for 3D-printed battery holder) 1 2-pin header (CON1; optional) 1 PCB-mounting right-angle tactile pushbutton, 6×6mm, 6mm-long actuator (S1) [Jaycar SP0607 or AliExpress 1005007559876628] 1 FT37-43 toroidal core (T1) [www.minikits.com.au/FT37-43, AliExpress 1005009245292057] OR 1 4mm OD, 2mm ID, 5mm-long ferrite bead (T1) [Altronics L5250A] 1 360mm length of 0.25mm diameter enamelled copper wire (T1) 1 100W top-adjust trimpot (VR1) [Altronics R2605] 3 BC548 30V 100mA 300MHz NPN transistors (Q1, Q2, Q4) 1 red or blue 3mm LED (LED3) 2 1N4148 75V 200mA signal diodes (D1, D2) various lengths of light/medium-duty hookup wire Capacitors (all 50V radial ceramic) 6 100nF 2 10nF 3 1nF 2 100pF 2 33pF Resistors (all ¼W axial ±5% or better) 1 1MW 1 2.2kW 1 220kW 1 1kW 3 10kW 1 470W 1 3.3kW 1 47W Extra parts for both Mike-One & Mike-Two 2 3-6μH 5-pin variable inductors on 7×7mm formers (L1, L2) [AliExpress 1005008114591102] 1 2.2μH axial RF choke (L3) [Jaycar LF1514, Altronics L7014] OR 1 T37-6 toroidal core (L3) [www.minikits.com.au/T37-6, AliExpress 1005005686909567] AND 1 400mm length of 0.375mm diameter enamelled copper wire (L3) 1 BC548 30V 100mA 300MHz NPN transistor (Q3) 1 100nF 50V radial ceramic capacitor 1 100pF 50V radial ceramic capacitor 2 56pF 50V radial ceramic capacitors 1 22kW ¼W axial resistor (±5% or better) 1 6.8kW ¼W axial resistor (±5% or better) 1 4.7kW ¼W axial resistor (±5% or better) Extra parts for Mike-One only 4 18.432MHz HC-49 crystals (X1-X4) 1 14.7456MHz HC-49 crystal (X5) 2 330pF 50V radial ceramic capacitors 2 47pF 50V radial ceramic capacitors 1 10-33pF 50V radial ceramic capacitor (Cx, nominally 22pF; see text) Extra parts for Mike-Two only 4 6.552MHz HC-49 crystals (X1-X4) 1 10.245MHz HC-49 crystal (X5) 1 4.7-22μH axial RF choke (RFC1, nominally 15μH; see text) 2 680pF 50V radial ceramic capacitors 1 330pF 50V radial ceramic capacitor 1 150pF 50V radial ceramic capacitor 1 22pF 50V radial ceramic capacitor Extra parts for Mike-Three only 4 7.2000MHz HC-49 crystals (X1-X4) 1 820nH axial RF choke (L3) 2 47pF 50V radial ceramic capacitors 1 10-33pF 50V radial ceramic capacitor (Ca, nominally 18pF; see text) siliconchip.com.au Australia's electronics magazine Wiring The wiring is straightforward; it’s shown clearly in all three circuit diagrams, Figs.1-3. In brief, run a red wire from the battery + to the LED anode (longer lead), an orange wire from the LED cathode (shorter) lead to the + terminal on CON1 and a black wire from the – terminal on CON1 to the battery – terminal. Also refer to Photo 3. Operation This is not a complicated transmitter to use! Just press the PTT button and talk. Your LSB signal will appear very close to 3.7MHz or 7.2MHz, depending on the version you’ve built. While the range is not massive when using a short length of hookup wire for the antenna, the signal is quite audible in nearby receivers. Usefully, the design is such that many popular data modes can also be tested with the transmitter, and further amplifier stages can be added if desired. In short, Mike-One (or Two or Three) will allow you to quickly, easily and inexpensively enjoy a short yet rewarding voyage on the QQRP ultralow-power HF seas. I hope you enjoy making and using one (or more!) of SC these little SSB transmitters. June 2026  75