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Teach-In 2026 by Mike Tooley World of Wireless – An Introduction to Radio and Wireless Technology Series 12, part 1: Welcome to the World of Wireless! S tep into the fascinating world of radio and wireless technology with our latest Teach-In series. It is designed to illuminate the science, innovation and electronic technology behind the devices and systems that connect our world. Over ten engaging parts, you will embark on a journey that traces the origins and evolution of wireless communication, explores the fundamental principles of radio frequency (RF) circuits and delves into the modern marvels of software-defined radio, Wi-Fi, Bluetooth and more. We will begin by setting the historical stage, highlighting major milestones and scientific breakthroughs from the earliest theories to the advanced systems of today. You’ll discover what radio truly is, learn how electromagnetic waves travel across the RF spectrum and understand key concepts such as frequency, wavelength and wave propagation in different bands. The series will then guide you through the essentials of RF circuits, the construction and operation of tuned circuits, oscillators and amplifiers. It will introduce you to the magic of crystal sets and phaselocked loops. With practical advice on building and using antennas, measuring RF signals and constructing your own simple devices, you’ll be well equipped to experiment and innovate. As we proceed, we’ll unravel the complexities of radio communication systems, exploring methods of modulation and demodulation. We’ll look at the art of transmitting information over long distances and the principles behind both analogue and digital transmissions, including 8 cutting-edge digital technologies like PSK and FT8 (see Fig.1.1). The series also opens the door to the dynamic world of modern wireless communications. Discover how Wi-Fi and Bluetooth work, how to extend wireless networks at home and how microcomputers like Arduino and Raspberry Pi are revolutionising connectivity. You’ll get experience with RF measurement tools, virtual network analysers and spectrum analysis using a software-defined radio (SDR), learning how to test and optimise performance (see Fig.1.2). For those drawn to the excitement of two-way communication, there will be dedicated sections on amateur radio, hand-held radio devices and aircraft radio systems. You’ll learn not just the technical essentials but also Fig.1.1: low-power intercontinental communication is made possible using modern data modes such as FT8 and PSK. This shows a link being established between amateur radio stations in the UK and New Zealand using PSK Reporter software. Practical Electronics | November | 2025 Fig.1.2: using a low-cost software-defined radio (SDR) to investigate the transmitted signal spectrum produced by a low-cost 144MHz ‘walkie talkie’. how to join the global community of radio enthusiasts, obtain a licence and participate in activities like radio ‘fox hunting’ and direction finding. Whether you’re a newcomer eager to grasp the basics, or a seasoned practitioner looking to deepen your understanding, this series provides a roadmap through the rich landscape of wireless technology. Prepare to tune in, reach out and discover the invisible forces that make modern communication a reality. Setting the scene Radio-frequency signals typically span a range from several tens of kilohertz (kHz, 103Hz) to several hundred gigahertz (GHz, 109Hz). The lower segment of this spectrum, particularly frequencies below 30kHz, is limited to narrow-band communication due to practical constraints. At such low frequencies, signal propagation primarily occurs as ground waves that follow the Earth’s curvature, enabling transmission over extensive distances. Conversely, the upper portion of the radio frequency range, extending beyond 30GHz into the microwave region, offers substantial bandwidth. It suits applications like point-topoint television transmission and advanced high-definition radar systems. At these higher frequencies, Electric field lines From source of radiated energy signal propagation generally requires line-of-sight (LOS) paths. Signals at certain frequencies, such as those within the range from 3-30MHz, can travel long distances via ionospheric reflection, enabling intercontinental communication. The radio spectrum is divided into bands for convenience, with each band’s use determined mainly by its propagation characteristics. Some considerations include the efficiency of aerial systems within the relevant range and the available bandwidth. Competition for frequencies is intense, and little space remains unallocated. Frequency allocations are made through international agreements, with each user group fiercely protecting its assigned spectrum. Table 1.1 summarises radio spectrum allocations from 30kHz to 30GHz, covering bands from LF to SHF. Radio waves Like light, radio waves originate from a transmitter and are made up of electric (E) and magnetic (H) fields at right angles to each other. These interconnected fields form an electromagnetic wave that travels outward, with both E and H vectors orthogonal to the direction of propagation, as shown in Fig.1.3. Radio waves are said to be polarised in the plane of the electric (E) field. Thus, if the E-field is vertical, the signal is said to be vertically polarised whereas if the E-field is horizontal, the signal is said to be horizontally polarised. Fig.1.3: the two orthogonal components of a radio wave travelling away from a source of radiated energy. E-field Direction of propagation H-field Magnetic field lines Table 1.1 – The radio spectrum from 30kHz to 30GHz Velocity of propagation = 3 ´108 m/s Designation Range Common applications Low frequency (LF) 30-300kHz Submarine communication, time signals Medium frequency (MF) 300kHz to 3MHz Navigational beacons, AM broadcasting (530-1700kHz), amateur radio High frequency (HF) 3-30MHz Shortwave broadcasting (various bands), amateur radio, aviation, military uses Very high frequency (VHF) 30-300MHz FM broadcasting (88-108MHz), aviation, marine, DAB, military uses Ultra-high frequency (UHF) 300MHz to 3GHz TV broadcasting, mobile phones, Wi-Fi, Bluetooth, satellite navigation (eg, GPS) Super-high frequency (SHF) 3-30GHz Radar, satellite TV, point-to-point microwave links, Wi-Fi Practical Electronics | November | 2025 9 A Brief History of Radio James Clerk Maxwell (1831-1879) first suggested the existence of electromagnetic waves in 1864. Later, Heinrich Hertz (1857-1894) used an arrangement of rudimentary resonators to demonstrate the existence of electromagnetic waves. Hertz’s apparatus was extremely simple and comprised two resonant loops, one for transmitting and the other for receiving. Each loop acted both as a tuned circuit and as a resonant antenna (or ‘aerial’). Hertz’s transmitting loop was excited by an induction coil and battery. Some of the energy radiated by the transmitting loop was intercepted by the receiving loop, and the received energy was conveyed to a spark gap where it could be released as an arc. The energy radiated by the transmitting loop was in the form of an electromagnetic wave—a wave that has both electric and magnetic field components, and travels at the speed of light. In 1894, Guglielmo Marconi (1874-1937) demonstrated the commercial potential of the phenomenon that Maxwell predicted and Hertz used in his apparatus. It was also Marconi who made radio a reality by pioneering the development of telegraphy without wires (‘wireless’). Marconi was able to demonstrate very effectively that information could be exchanged between distant locations without the need for a landline. Marconi’s system of wireless telegraphy proved to be invaluable for maritime communications (ship-to-ship and ship-to-shore) and was instrumental in saving many lives, including when the Titanic sank in 1912. The military applications of radio were first exploited during the First World War (1914 to 1918). Radio broadcasting took shape early in the 1920s, with many listeners using crystal sets and simple one- or two-valve receivers. Broadcasting quickly took on a national dimension, with radio becoming the principal source of entertainment in most households. Significant technological advancements were made in the two decades leading up to the Second World War (1939-1945). Among the most notable were the introduction of single sideband (SSB) transmission for long-distance voice communication, frequency modulation (FM) for high-fidelity broadcasting and television transmission in the VHF band (30-300MHz). Following the Second World War, car and portable radios became common and more affordable with the advent of transistors in the 1950s. At the same time, stereophonic broadcasting began in the UK. Communication and navigation satellites were introduced and microwave point-to-point links were established. Colour television followed in the 1960s. From the 1980s onwards, radio started to embrace the digital revolution by providing a means of transmitting digital data without the need for a wire or cable connection. Satellite and terrestrial links extended the range, availability and bandwidth. The start of the 21st century saw the introduction and wide availability of low-cost wireless networks, including Wi-Fi (IEEE 802.11). Table 1.2 (opposite) shows some important milestones in the development of radio and wireless technology. Fig.1.4 shows the electric E-field lines in the space between a transmitter and a receiver. The transmitter antenna (a dipole) is fed with high frequency alternating current. This gives rise to an alternating electric field between the ends of the antenna and an associated alternating magnetic field around and at right angles to it. The direction of the electric field lines alternates with each cycle of the signal as the wavefront propa- gates from the source. The receiving antenna detects the changing field, resulting in a similar voltage and current being induced in it, but at a lower amplitude. In Fig.1.4, where the transmitter and receiver are close (in the near field), the field spreads out spherically. However, at greater distances (the far field), the wavefront becomes planar and the angular distribution of the field no longer depends on the distance between the antennas. Frequency and wavelength Radio waves propagate in air (or space) at the speed of light (approximately 300 million metres per second). The velocity of propagation, v, wavelength, λ and frequency, f, of a radio wave are related by the equation v = (f × λ)m/s. This equation can be arranged to make f or λ the subject, ie, f = (v ÷ λ)Hz or λ = (v ÷ f)m. v is the velocity of propagation (~3 × 108m/s in air or space). di Ra ed at Ionospheric path ld fie Ere he sp Sc att RX er wave TX Sky wave Space o Ion re he sp po Tro Tropospheric path Ground wave Transmitting antenna Fig.1.4: the electric field pattern in the near-field region between a transmitter and a receiver. 10 Receiving antenna Fig.1.5: various modes of radio wave propagation through the atmosphere; note how signals can take more than one path between two points. Practical Electronics | November | 2025 Table 1.2 – The history of radio and wireless technology Year Event 1864 Maxwell predicts the existence of radio waves and lays the theoretical foundation for radio and wireless communication 1888 Hertz generates and successfully detects electromagnetic waves in a Berlin laboratory 1895 Marconi demonstrates wireless telegraphy over a few miles and gains commercial interest in its exploitation 1901 Marconi sends the letter “S” in Morse code from Cornwall, England to Newfoundland, Canada 1906 Fessenden transmits voice and music from Brant Rock, Massachusetts, USA 1920 The world’s first commercial broadcast station (KDKA) opens in Pittsburgh, Pennsylvania, USA 1921 Radio amateurs demonstrate the possibility of transatlantic communication via radio 1922 The BBC establishes the first national public radio network in the UK 1926 The first major radio network standardises nationwide programming in the USA 1933 Armstrong patents FM, which would eventually replace AM, reducing noise and improving audio quality 1939 W47NV in Nashville, TN, USA, begins FM broadcast programming for the public 1945 After WW2, radio amateurs experiment with SSB, which becomes the standard for efficient voice transmission 1953 Radio amateurs Ross Bateman (W4AO) & Bill Smith (W3GKP) record echoes of their transmissions reflected from the moon 1954 The Regency TR-1 allows truly portable listening, fuelling radio’s golden era in popular culture 1960 First two-way communication via moon bounce between radio amateurs 1961 The first amateur radio satellite, built by volunteers, transmits radio telemetry from Earth orbit 1962 Telstar 1 is the first satellite to relay across the Atlantic, paving the way for global broadcasting 1969 A network is developed that allows digital devices to communicate wirelessly 1973 GPS satellites are launched in the USA as a joint military and civil project 1978 Digital data transmission is demonstrated over radio waves, laying the groundwork for wireless networking 1981 Digital audio broadcasting is tested, promising more channels and (arguably) better sound quality 1983 The first mobile phone is made available to the public 1994 The forerunner of today’s touch screen and email smartphones is created 1995 The full constellation of 24 GPS satellites becomes operational 1997 The 2.4GHz 2Mbps wireless standard heralds the start of Wi-Fi (IEEE 802.11) 1999 The first laptop computer with integrated Wi-Fi goes on sale 2000 Civilians worldwide gain access to GPS satellite signals 2011 Digital radio gains mainstream adoption, diversifying the broadcast landscape 2015 DAB is fitted as standard in many cars and domestic radios 2017 FT8, a weak-signal digital model, revolutionises low-power communication under poor propagation conditions 2020 Satellite radio reaches a global audience of 33 million subscribers As an example, a signal at a frequency of 1MHz will have a wavelength of 300m, whereas a signal at a frequency of 100MHz will have a wavelength of 3m. Note that when a radio wave travels in a cable or waveguide (rather than in air or free space), it usually travels at 60-80% of the speed of light. We will be returning to this later in the series. Radio wave propagation Depending on several complex factors, radio waves propagate through the atmosphere in various ways, as shown in Fig.1.5. These include: • ground waves • ionospheric waves • space waves • tropospheric waves As the name suggests, ground waves (or surface waves) travel close to the surface of the Earth and propagate for relatively short distances at HF and VHF, but for much greater distances at MF and LF. For example, at 100kHz, the range of a ground wave might be more than 500km, whilst at 1MHz (using the same radiated power), the range might be no more than 150km, and at 10MHz, no more than about 15km. Ground waves have two basic components: a direct wave and a Practical Electronics | November | 2025 ground reflected wave (as shown in Fig.1.6). The direct path exists on a line-of-sight (LOS) basis between the transmitter and receiver. Examples of the use of direct paths are terrestrial microwave repeater stations, which are typically in towers spaced 20-30km apart so they have a line of sight. The direct path is also used for satellite TV reception. To receive signals from the satellite, the receiving antenna must be able to ‘see’ the satellite. In this case, since the wave travels directly through the atmosphere, the direct wave is often referred to as a space wave. Such waves travel over LOS paths at VHF, UHF and beyond. Fig.1.6 shows that signals can arrive at a receiving antenna via both the direct path and by reflection from the ground. Ground reflection depends very much on the quality of the ground, with sandy soils beTransmitting antenna ing a poor reflector of radio signals, and flat, marshy ground being an excellent reflecting surface. Note that a proportion of the incident radio signal is absorbed into the ground; not all of it is usefully reflected. An example of the use of a mixture of direct path and ground (or building) reflected radio signals is the reception of FM broadcast signals in a car. It is also worth mentioning that, in many cases, the reflected signals can be stronger than the direct path (or the direct path may not exist at all if a car happens to be in a heavily built-up area). Ionospheric waves (or sky waves) can travel for long distances at MF, HF and exceptionally also at VHF under certain conditions. Such waves are predominant at frequencies below VHF, and we shall examine this phenomenon in greater detail a little later. Direct path Receiving antenna Reflected path Fig.1.6: the constituents of a ground wave. Signal absorbed into the ground 11 Fig.1.7: the effects of different ionospheric layers. ave ew Spac m 0k wa v y Sk VHF (30 10 M LF (30 to 500 kHz) z) H (0 .5 m ) to F to 3 lay er lay Hz 0M o3 (3 M k 75 F1 – t (10 F H m 5k 12 HF m 0k 20 e Hz) F2 – to 300 M 30 E–l er aye D– r lay er M Hz ) nd wave Grou Ground Still, before we do, it is worth describing what can happen when waves meet certain types of discontinuity in the atmosphere or when they encounter a physical obstruction. In both cases, the direction of travel can be significantly affected according to the nature and size of the obstruction or discontinuity. Four different effects can occur: • reflection • refraction • diffraction • scattering Reflection occurs when a plane wave meets a plane object that is large relative to the wavelength of the signal. In such cases, the wave is returned with minimal distortion and without any change in velocity. The effect is like the reflection of a beam of light off a mirror. Refraction occurs when a wave moves from one medium into another in which it travels at a different speed. For example, when moving from a denser to a less dense medium, the wave is bent away from the normal (an imaginary line at right angles to the boundary). Conversely, when moving from a less dense to a denser medium, a wave will bend towards the normal. The effect is like that experienced by a beam of light when it encounters a glass prism or lens. Diffraction occurs when a wave 12 meets an edge (a sudden impenetrable surface discontinuity) that has dimensions that are large relative to the wavelength of the signal. In such cases, the wave is bent so that it follows the profile of the discontinuity. Diffraction occurs more readily at lower frequencies (typically VHF and below). An example of diffraction is the bending experienced by VHF broadcast signals when they encounter a sharply defined mountain ridge. Such signals can be received at some distance beyond the ‘knife edge’, even though they are well beyond the normal LOS range. Scattering occurs when a wave encounters one or more objects in its path with a size that is a fraction of the wavelength of the signal. When a wave encounters an obstruction of this type, it will become fragmented and re-radiated over a wide angle. Scattering occurs more readily at higher frequencies (typically VHF and above) and regularly occurs in the troposphere at UHF and EHF. The troposphere extends from the Earth’s surface up to 18km in the tropics, reducing to 6km in the polar regions. Radio signals can also be directed upwards (by suitable choice of antenna) so that signals enter the troposphere or ionosphere (48965km altitude, ionised by solar radiation). In the former case, signals can become scattered (partially dispersed) in the troposphere so that a small proportion arrives back at the ground. Tropospheric scattering requires high-power transmitting equipment and high-gain antennas, but is regularly used for transmission beyond the horizon, particularly where conditions in the troposphere (rapid changes of temperature and humidity with height) can support this mode of communication. Tropospheric scattering of radio waves is analogous to the scattering of a light beam shone into a heavy fog or mist. There is also tropospheric ducting, in which radio signals can become trapped because of the change of refractive index at a boundary between air masses with different temperature and humidity. Ducting usually occurs when a large mass of cold air is overrun by warm air (a ‘temperature inversion’). Although this condition may occur frequently in certain parts of the world, this mode of propagation is not very predictable and is therefore not used for any practical applications. The ionosphere In 1924, Sir Edward Appleton (1892-1965) was one of the first to demonstrate the existence of a reflecting layer at a height of about 100km (now called the E-layer). This was soon followed by the discovery of another layer at around 250km (now called the F-layer). Practical Electronics | November | 2025 Table 1.3 – Ionised layers of the atmosphere and their effects Layer Altitude Characteristics Effect on radio waves D 50-95km Develops shortly after sunrise and disappears shortly after sunset. Reaches maximum ionisation when the sun is at its highest point in the sky. Responsible for the absorption of radio waves at lower frequencies (below about 4MHz) during daylight hours. E 95-150km Develops shortly after sunrise and disappears a few hours after sunset. The maximum ionisation of this layer occurs at around midday. An intense region of ionisation that sometimes appears in the summer months (peaking in June/July in the Northern Hemisphere). Usually lasts for only a few hours (often in the late morning and recurring in the early evening of the same day). Appears a few hours after sunset, when the F1- and F2-layers merge to form a single layer. Reflects waves below 5MHz but tends to absorb radio signals above this frequency. Highly reflective at frequencies above 30MHz, up to 300MHz on some occasions. Of no practical use other than as a means of long-distance VHF communication for radio amateurs. Reflects radio waves up to 20MHz and occasionally up to 25MHz. ES 80-120km F 250-450km F1 150-200km Occurs during daylight hours with maximum ionisation reached at around midday. The F1-layer merges with the F2-layer shortly after sunset. Reflects radio waves in the low HF spectrum, up to about 10MHz. 250-450km Develops just before sunrise as the F-layer begins to divide. Maximum ionisation of the F2-layer is usually reached one hour after sunrise, and it typically remains at this level until shortly after sunset. The intensity of ionisation varies greatly according to the time of day and season, and is greatly affected by solar activity. Capable of reflecting radio waves in the upper HF spectrum with frequencies of up to 30MHz and beyond during periods of intense solar activity (eg, at the peak of each 11-year sunspot cycle). F2 This was achieved by broadcasting a continuous signal from one site and receiving the signal at a second site several miles away. By measuring the time difference between the signal received along the ground and the signal reflected from the atmosphere (and knowing the velocity at which the radio wave propagates), it was possible to calculate the height of the atmospheric reflecting layer. Today, the standard technique for detecting the presence of ionised layers (and determining their height above the surface of the earth) is to transmit a very short pulse directed upwards into space and accurately measure the amplitude and time delay before the arrival of the reflected pulses. This ionospheric sounding is carried out over a range of frequencies. The different layers within the ionosphere (Fig.1.7) provide us with a reasonably predictable means of communicating over long distances using MF and HF radio signals. Much short and longdistance communication below 30MHz depends on the bending or refraction of the transmitted wave in the earth’s ionosphere. The useful regions of ionisation are the E-layer (maximum ionisation at about 70mi/100km) and the F-layer (lying at about 175mi/250km at night). During the daylight hours, the F-layer splits into two distinguishable parts: F1, at a height of about 140mi/225km, and F2, at about 200mi/320km. After sunset, the F1 and F2 layers recombine into a single F-layer. Practical Electronics | November | 2025 During daylight, a lower layer of ionisation known as the D-layer exists in proportion to the sun’s height, peaking at local noon and largely dissipating after sunset. This lower layer primarily acts to absorb energy at the low end of the high-frequency (HF) band. The F-layer ionisation regions are primarily responsible for longdistance communication using sky waves at distances of up to several thousand km (much more than can be achieved using VHF direct wave communication). The characteristics of the various ionised layers are summarised in Table 1.3. Ionograms Ionograms, like the one shown in Fig.1.8, provide us with a means of visualising prevailing ionospheric conditions at a given location and time. They are plotted from data returned from an ionospheric probe, a frequency-agile transmitting station that directs its output in a series of bursts aimed vertically into the atmosphere. Following each burst, the signal reflected from one or more of the ionospheric layers is detected by a co-sited radio receiver. The time difference between the transmitted and received bursts is used to calculate the altitude of the reflecting layer. Ionograms are useful for assessing and predicting radio propagation, as they show ionisation regions, their Fig.1.8: an ionogram from the UK’s Chilton ionospheric station at 14:30 on the 10th of May, 2025. It clearly shows the presence of both the F1 and F2 layers. 13 Maximum usable frequency (MHz) 30 25 20 15 10 5 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (UTC) Fig.1.9: the variation of maximum usable frequency (MUF) over a 24-hour period for the radio path between London and New York. altitude and the critical frequency—the highest vertically radiated signal reflected by a layer. Signals above this frequency pass through the ionosphere into space. Highest & lowest usable frequencies The maximum usable frequency (MUF) is the highest frequency that will allow communication over a given terrestrial path at a particular time and on a particular date. The MUF varies considerably with the amount of solar activity and is basically a function of the height and intensity of the F-layer. During a period of intense solar activity, the MUF can exceed 30MHz during daylight hours, but is often around 16-20MHz by day, and around 8-10MHz by night. The variation of the MUF over a 24-hour period for the London to New York path is shown in Fig.1.9. A similar plot for the summer months would be flatter, with a more gradual increase at dawn and a more gradual decline at dusk. The reason for the significant variation of the MUF over any 24hour period is that the intensity of ionisation in the upper atmosphere is significantly reduced at night. Consequently, lower frequencies have to be used to produce the same amount of refractive bending and to give the same critical angle and skip distance as by day. Fortunately, the attenuation experienced by lower frequencies travelling in the ionosphere is much reduced at night. This makes it possible to use the lower frequencies required for effective communication. The important fact to remember from this is simply that, for a given path, the frequency used at night is about half that used for daytime communication. The lowest usable frequency 14 (LUF) is the lowest frequency that will support communication over a given path at a particular time and on a particular date. The LUF depends on the amount of absorption experienced by a radio wave. This absorption is worst when the D-layer is most intense (during daylight). Hence, as with the MUF, the LUF rises during the day and falls during the night. A typical value for the LUF is 4-6MHz during the day, falling rapidly at sunset to 2MHz. The frequency chosen for HF communication must therefore be somewhere above the LUF and below the MUF for a given path, day and time. A typical example might be a working frequency of 5MHz at a time when the MUF is 10MHz and the LUF is 2MHz. The silent zone is the region that exists between the extent of the coverage of the ground wave signal and the point at which the sky wave first returns to earth. Note also that, depending on local topography and soil characteristics, when a signal returns to the Earth from the ionosphere, it is sometimes possible for it to experience a reflection from the ground. The onward reflected signal will suffer attenuation but, in some circumstances, it may be sufficient to provide a further hop and an approximate doubling of the working range. This is known as multi-hop propagation. The skip distance is simply the distance between the point at which the sky wave is radiated and the point at which it returns to Earth. Where signals are received simultaneously by ground wave and sky wave paths, the signals will combine both constructively and destructively due to the different path lengths. This, in turn, will produce an effect known as fading. This effect can often be heard during the early evening on medium-­ wave (AM) radio signals as the D-layer begins to fade. You should be able to hear this effect when you complete this month’s Hands-On Project. Hands-On: A modern ‘crystal set’ Each of our Teach-In instalments will feature a simple project designed to enhance learning and encourage further experimentation (see Fig.1.10). This month, we start with a modern ‘crystal set’ radio, which swaps the original galena crystal for an integrated circuit and two small-signal transistors— Fig.1.10: our Hands-On modules use readily available, low-cost parts and can be built on a small piece of stripboard. Practical Electronics | November | 2025 Fig.1.11: the full circuit of our modern ‘crystal set’. S1 On/off R1 100kΩ P1-1 L1 200μH (see text) VC1 270pF R3 1kΩ 2 IC1 TA7642 P1-2 C1 10nF 1 R4 100kΩ R4 C4 1kΩ 2.2μF + 3 TR2 BC548 R6 5.1kΩ P2-3 c + b TR1 BC548 c C5 100μF B1 1.5V e b + P2-1 C2 100nF R5 100kΩ e RV1 1kΩ + J1 Headphones C3 100μF P2-2 components that were unavailable a century ago. Our Hands-On ‘crystal set’ receives local medium wave (MW) transmissions using headphones and is powered from a single AA 1.5V cell. The complete circuit diagram is shown in Fig.1.11. The receiver makes use of a handwound coil on a ferrite rod. Tuning is managed by L1, which also serves as the antenna, together with a small solid dielectric variable capacitor, VC1. The precise value for VC1 is not critical, but a maximum value of between 270pF and 350pF will prove ideal. Such capacitors are often supplied with multiple sections (and some also with smaller value trimmers), in which case you can safely ignore them. The dimensions of the ferrite rod are also uncritical. The component used for the prototype receiver had a measured inductance of 200µH and was 200mm in length and 10mm in diameter. If you cannot locate a part with these dimensions, there is plenty of scope for experimentation, but a shorter rod will usually require a few more turns of wire to obtain the same inductance value. IC1 is a TA7642 single-chip radio receiver housed in a TO-92 package. R3 12KΩ 2 T1 C1 12pF T2 1 25 × 64mm piece of stripboard (9 × 24 holes) 1 200mm-long, 10mm diameter ferrite rod (L1) 1 1kΩ miniature horizontal trimpot (RV1) 1 ABS enclosure (optional; see text) 1 3.5mm miniature jack socket (J1) 1 SPST miniature toggle switch (S1) 1 single AA cell holder 4 brass or nylon M3 × 10mm hex stand-off spacers 4 short M3 panhead machine screws 1 2-pin male 0.1in/2.54mm header (P1) 1 3-pin male 0.1in/2.54mm header (P2) Semiconductors 1 TA7642 single-chip radio receiver (IC1) 2 BC548 small-signal NPN transistors (TR1, TR2) Capacitors 2 100µF 16V radial electrolytic (C3, C5) 1 2.2µF 16V radial electrolytic (C4) 1 100nF 50V ceramic (C2) 1 10nF 50V ceramic (C1) 1 270-350pF miniature solid-dielectric variable (VC1) Resistors (all ¼W axial, 5% or better) 3 100kΩ (R1, R4, R5) 1 5.1kΩ (R6) Qty 3 1 2 Value 4-band code 5-band code 100kW 5.1kW 1.0kW 2 1kΩ (R2, R3) Additional components for the optional RF gain control 1 250kΩ linear carbon track potentiometer (VR1) 1 100kΩ ¼W 5% resistor (R7) The internal schematic of the TA7642 is shown in Fig.1.12. The chip operates from a 1.5V supply and consumes less than 1mA of current. Its silicon substrate contains ten transistors that provide 3 R5 12KΩ R6 12KΩ R9 12KΩ R11 12KΩ R13 12KΩ C2 12pF R4 12KΩ R2 3.3KΩ Parts List – A modern ‘crystal’ set R1 5.6KΩ T5 T3 R10 12KΩ R7 12KΩ R8 12KΩ T4 T6 R14 74.6Ω R12 12KΩ T7 R15 C4 12KΩ 23pF T8 T9 Fig.1.12: the internal schematic of the TA7642 single-chip radio receiver T10 (from its data sheet) shows it includes 10 transistors, 15 resistors and four capacitors. 1 Practical Electronics | November | 2025 15 1 3 TA7642 BC548 1 GND collector 2 IN base 3 OUT emitter 2 Fig.1.15: the pinouts of the semiconductor devices used in the ‘crystal set’. Table 1.4 – Expected voltages Fig.1.13: the component layout (top view) of our ‘crystal set’ on stripboard. Device IC1 TR1 TR2 Fig.1.14: the underside of the stripboard, showing the track cuts and soldered leads. sufficient gain to amplify low-level incoming signals of a few millivolts or less. Two external transistor stages contribute additional voltage gain and impedance matching for headphones with a nominal impedance of 32Ω. The project is constructed on a small piece of perforated copper stripboard measuring 25 × 64mm, arranged as nine strips each of 24 rows. The component layout (viewed from the top) is shown in Fig.1.13, while the corresponding track layout (viewed from below) is given in Fig.1.14. The required 18 track breaks can be made using a spot face cutter or small drill bit, and the five links on the upper side of the board can be made using short lengths of tinned copper wire. The pin connections for the semiconductor devices are shown in Fig.1.15. Fig.1.16 shows the finished stripboard module, together with the external tuning components, L1 and VC1. L1 comprises 54 turns of 0.55mm enamelled copper wire close wound over a 70mm length of heatshrink sleeving, into which the ferrite rod is slid. The ends of the winding are secured by PVC tape. The resulting winding will have an inductance of around 200µH, and with the 270pF variable capacitor, will provide a tuning range from around 700kHz to 1.6MHz. When completed, the tuning components, switch, headphone connector and battery holder can be mounted in an ABS enclosure of Pin Voltage 1 0V 2 1.26V 3 1.05V C 0.85V B 0.74V E 0.15V C 1.54V B 0.85V E 0.12V your choice, as shown in Fig.1.17 (we’ve used a larger enclosure than strictly necessary, as our next Hands-On project can fit in the same enclosure). Connections to the off-board components (L1, VC1, J1 and S1) are made using the two 0.1” male header connectors (P1 and P2) and short lengths of hook-up wire. Testing Fig.1.16: the finished circuit board and tuning components, L1 and VC1. As always, it’s well worth carefully checking the stripboard and internal wiring before applying power. When these checks are complete, connect the headphones and insert a 1.5V battery into the holder. You should immediately hear some noise from the headphones, and a quick sweep of VC1 should reveal several strong broadcast signals. If that’s not the case, remove the battery and re-check the board and wiring. It is also worth checking the headphone connector. For comparison and to assist fault-finding, Table 1.4 provides the test voltages obtained from our prototype. 16 Practical Electronics | November | 2025 Fig.1.19: the additional wiring required for the RF gain control. In use Despite its simplicity, the receiver is very sensitive and therefore susceptible to local noise sources, such as computers, TV receivers and many other electronic devices. For best results, keep the ferrite antenna well away from other devices. The antenna is directional; maximum signal will be obtained when the antenna is side-on to the bearing of the station being received. These directional properties can be advantageous, allowing you to null or minimise the effects of local noise and interference. Simply rotate the antenna for the least interference and best signal! During the day, you will normally be able to receive several mediumwave broadcast signals at good strength. More signals will be present during the hours of darkness, and you may also notice some of the ionospheric effects we discussed earlier. In the UK, BBC Radio 5 Live broadcasts on 693kHz and 909kHz nationally, and should be received at good strength, as will Lyca Radio, which broadcasts on several frequencies from London and Manchester. TalkSPORT is available on 1053kHz, 1071kHz, 1089kHz and 1107kHz, and provides live coverage of many sporting events, plus news and discussions. With the receiver in a favourable position, all these stations should be received easily. Adding an RF gain control When very strong local signals are present, due to the very high internal gain present in IC1, you may sometimes find that our modern crystal set becomes severely overloaded. This can lead to significant audio distortion, but the effect can often be reduced by placing the receiver in a less favourable position or by changing the orientation of the ferrite antenna. However, if you find this to be a persistent problem, it can easily be resolved by fitting an RF gain control. Fortunately, only two extra components are required for this modification – see Fig.1.18. The control potentiometer, VR1, should be fitted close to the stripboard (see Fig.1.19). The RF gain can then be adjusted as necessary, but a mid-position setting should prove satisfactory for a wide range of signal levels. Coming up! Part 2 of Teach-In 12 will cover RF circuits, components and their underpinning principles. We delve into the constructional techniques required for successfully building radio and wireless equipment. Our Hands-On Project will be an upgraded portable MW AM receiver PE with a loudspeaker output. P1-2 VC1 270p L1 200u P1-1 IC1 pin-2 R1 100k C1 10n R7 100k VR1 250k Gain P2-2 Fig.1.17: internal assembly and wiring of the ‘crystal set’ in our plastic box. Practical Electronics | November | 2025 Fig.1.18: an RF gain control can easily be added to the set if it’s required. 17