Radio waves are simply electromagnetic radiation, the same as light or radiated heat. They consist of an electric and magnetic field that oscillates from one polarity to the other, and their frequency is the number of times that oscillation occurs per second. For example in the UK, the BBC Radio 5 Live AM transmitter on 909 kHz oscillates between polarities 909 000 times per second.
The simplest radio transmitter produces this oscillation as an electrical waveform, and induces the radiation by passing that waveform to an antenna.
The radiation travels at the speed of light, so the distance between the start of each oscillation and its end, for a given frequency, will always be the same. This is called the wavelength, and most antenna designs achieve maximum efficiency by matching their dimensions to a fraction of the wavelength at their chosen frequency.
So, if you build an oscillator and connect it to an antenna, you have created a radio transmitter. Were you to listen to it though, you would hear only silence. It has no speech, music, or anything else applied to it. To attach some information to your transmission, you must modulate the information upon it, and there are many different ways in which this can be done depending upon the information in hand.
For analogue broadcast radio, you will probably recognise the two different modulation schemes as AM, for Amplitude Modulation, and FM, for Frequency Modulation. In an AM transmitter, the speech or music is mixed with the radio frequency signal to produce a result with varying amplitude, while in an FM transmitter the frequency of the radio signal changes with the speech or music waveform.
Completing the circuit
Both forms of transmission can be received with relatively simple circuitry, but an AM transmitter can be constructed with the barest minimum of components. A radio receiver captures the radio waves from electromagnetic radiation through air and space into an electrical signal which it then amplifies and demodulates to retrieve information such as speech or music. It must possess selectivity, or the ability to narrow down to a single signal on one frequency, among the many others that will be pulled in by its antenna, and in all but the very simplest of receivers it must also have sufficient gain to amplify any signals such that they can be demodulated.
With well over a hundred years of development behind them, both transmitter and receiver designs can be anything from the very simple to the extremely complex, and leave many opportunities for the experimenter and home constructor. Surprisingly this does not mean only the oldest of designs – the latest software-defined radios have provided a new vista for anyone who wishes to tinker with radio, through free software such as GNU Radio. It’s worth looking at some of the simplest radios to give a basic introduction to radio technology. We’ll show you the simplest of AM receivers, a simple regenerative FM receiver, and – even though it is illegal to operate without a licence in many territories – we will also show you a transmitter to help illustrate some of the technology.
The earliest radio systems were developed long before transistors or even valves or tubes were developed. Their owners didn’t have the luxury of amplification, and thus their receivers had to work using only the components available to them. The crystal set, so named because its earliest versions used a rough germanium crystal, is the simplest possible radio receiver and receives AM broadcasts. It uses a minimum of parts and has the handy bonus of not requiring any batteries, but with the penalty of working best with only the strongest of stations. It consists of only four components: a coil of wire and a tuning capacitor that together make a tuned circuit to select a particular frequency, a germanium diode to recover the speech or music signal from the radio signal, and a crystal earpiece to play the sounds to the listener.
A crystal set may be extremely simple but, given a long piece of wire as an antenna, it can deliver surprisingly good results for very little outlay. With a bit of searching, the parts can be scavenged from scrap electronic devices, making it an extremely cheap first radio project.
If AM and a crystal set is a little tame for you, simple radios don’t stop there. The regenerative receiver was one of the earliest improved radio designs, and uses a single transistor or tube as an amplifier adjusted to the point at which it is almost oscillating. At that point its selectivity is hugely enhanced, making it much more sensitive on the particular frequency it is tuned to. Regenerative radios can be made to work at many frequencies – including the FM broadcast band, where they are probably the simplest way possible to make a receiver.
This is London calling##
Probably the simplest transmitter possible for most readers will come from a surprising source. The Raspberry Pi contains an oscillator designed to provide a clock signal to peripherals, which some clever hardware hackers realised could be repurposed into a low-power transmitter. The PiFM package can be readily downloaded and, as its name suggests, it turns the Pi into an FM broadcast transmitter. By connecting a piece of wire to a GPIO pin and running it, you can transmit for a short distance, perhaps throughout your house.
As we mentioned earlier, though, running your own transmitter without a licence is illegal in most countries. This is partly for bureaucratic reasons, but also it is this way because part of the licence conditions mandate that any transmitters have good spectral purity (which is to say that they must transmit on only one frequency). If your transmitter also interferes with your local emergency services or air traffic control, then obviously it shouldn’t be allowed to continue, and that is why government agencies, such as OFCOM in the UK or the FCC in the USA, have teams enforcing the technical side of the licence conditions. With a tiny transmitter such as the Raspberry Pi you might get away with it, but to ensure that it or any other transmitter does not emit on other frequencies, you will also require a filter on its output.
A low-pass filter (a network of capacitors and inductors calculated to let through only frequencies below a certain point) removes anything above the desired frequency.
Calculating component values for such a filter is beyond the scope of this article, but fortunately you can locate plenty of freely available software such as the Qucs simulation suite (hsmag.cc/nuYhkD), which can be a great help if you want to design one for yourself.
Amateur hour
If you are new to radio, then maybe the information here has given you something of a taster for what can be an extremely interesting and multifaceted field of electronics. You may be happy to play with a simple regenerative receiver or a cheap RTL software-defined receiver but, if your interest goes further, the field of amateur radio is a logical next step. As part of the global regulation of the radio spectrum, there are multiple internationally agreed frequency bands upon which private experimenters can get a licence to operate using any transmitter that meets the regulations, including ones they have made themselves. The result opens up a hugely varied array of different radio technologies, from satellites through computerised data modes to atmospheric propagation research, television, and much more.
Different countries have their own routes to getting an amateur radio licence, but in most cases one can be yours for passing a technical examination.
In the UK you can find more information from the RSGB (rsgb.org), and in the USA from the ARRL (arrl.org), but then all other countries will in turn have their own similar organisations.
However you experiment with radio, though, have fun and let us know what you build. Tweet us at @HackSpaceMag or email at hackspace@raspberrypi.org.
Building a crystal set
The simplest possible radio is the crystal set, a tuned circuit coupled to a high-impedance earpiece through a germanium or Schottky signal diode. A quick web search will reveal multiple crystal radio kits, but this is a very easy circuit to build from parts you have found for yourself or retrieved from an older scrap AM radio.
Looking at each part individually, if you can’t find them in a scrap AM radio, you can easily buy both the tuning capacitor and ferrite rod for the antenna from multiple suppliers including Rapid or Bitsbox. Your scrap radio may provide you with a ready-wound coil, but if not then you should wind 50 turns of enamelled copper wire on the ferrite rod and secure it with tape.
The diode should be a germanium point-contact type, which was once ubiquitous but is now obsolete. You will find them in older scrap radios and TV sets, but Bitsbox carries the 1N34A type, should you need to buy one.
A high-impedance earpiece such as older telephone units, army surplus headphones, or a crystal earpiece completes the component list. The crystal earpiece should be available from multiple suppliers.
FM regenerative receiver
For the more advanced or confident constructor, it is possible to make a simple FM receiver with only two transistors. This is a regenerative receiver using a J310 field-effect transistor and a 2N3904 bipolar transistor as an audio amplifier, that can be built on a piece of prototyping board, as long as it is made with care to keep all component leads as short as possible.
All the parts should be available from most component suppliers – try either Bitsbox or Rapid if you draw a blank. The coil is seven turns of stiff enamelled copper wire wound on a 5 mm former, which is removed to leave a free-standing air-cored coil. You will need to scrape a little of the enamel off at about one-and-a-half turns, to solder on a 1 m antenna wire. If you don’t have any enamelled copper wire, follow the example in our prototype and scavenge some from a toroidal mains choke found in a dead PC ATX power supply.
In use, this radio is a little more tricky than those you may be used to because, in addition to the trimmer capacitor which is the tuning control, it has the 1 kΩ variable resistor which is a regeneration control. You should adjust this to the point at which you hear the noise in your headphones, and tune the radio to a station. The regeneration may need adjustment for each station and, if you can find a plastic screwdriver for the tuning, you will find that it does not cut out while you are adjusting it. With some trimmers, you can fashion a tuning tool from a matchstick.
Our prototype was able to receive several strong local FM stations in this way. It may not perform as well as a commercial radio, but for its simplicity it does quite an impressive job.
Harmonics and low pass filters
A transmitter that is capable of producing a pure sine wave should in theory only emit one single frequency. Unfortunately it is almost impossible to produce such a perfect transmitter, and inevitably any real-world device will produce an element of distortion. This distortion appears as spurious frequencies at multiples of the original, referred to as harmonics. A transmitter producing square waves, such as those from the Raspberry Pi’s clock generator, will contain a significant proportion of these harmonics, enough to cause interference to radio users on other frequencies.
All transmitters will therefore contain some form of low-pass filter designed to only let through frequencies below a certain point. The low-pass filter is a network of inductors and capacitors calculated for a particular cut-off frequency. The figure shows a design calculated by the Qucs circuit simulation package for a filter with a 120 MHz cut-off. The component values are calculated by the software; a real-world version of this filter would use the closest available off-the-shelf values.
Software defined radio on the cheap
The radios described so far owe their roots to a much earlier age in radio experimentation. The cutting edge of the radio engineer’s art lies in software-defined radio, or SDR, in which the hardware simply digitises a piece of radio spectrum into a computer, and all the signal processing work is performed in software.
Unusually for a cutting-edge technology, SDRs are extremely affordable thanks to a happy accident when a commodity USB TV receiver chipset was found to have an undocumented mode allowing it to be used as an SDR. The Realtek RTL2832-based USB sticks can be bought for under £10, and readily form a software-defined receiver that can have a bandwidth from 30 MHz to 2 GHz. They work with the free GQRX, SDRSharp, and GNU Radio software, delivering a lot of scope for radio experimentation at pocket-money prices.
