The Basics of Radio Reception
Written by Bryce Ringwood   
Wednesday, 20 October 2010 22:59

 

Generating and receiving a signal

A radio transmitter generates rapidly varying electric voltages and currents in a conductor. If these are connected to a suitable radio antenna, then an electromagnetic wave is produced. According to the manual for one of my receivers, electromagnetic waves have frequencies of 20000 Hz and above, but I think they are merely referring to what is practice for most communications.

 (The unit of Frequency was at one time measured in ‘cycles per second’ which was abbreviated to ‘c/s’. Out of respect to one of the Marconi Quenched Spark Setfathers of radio the unit was renamed to the Hertz (Hz)). Atmospheric electromagnetic waves can have frequencies well below that which we humans normally use to communicate. The electromagnetic spectrum covers microwaves used in cookers and radar, and as the waves become shorter and shorter they pass through the visible spectrum. The shortest electromagnetic waves we detect are considered to be gamma rays.

The earliest transmitters were spark-gap transmitters. These consisted of an induction coil used to produce a high voltage across a spark-gap. The spark gap had a tuned circuit connected in parallel. One end of the tuned circuit was connected to earth, and the other to an antenna. A telegraph key was used to switch the primary of the induction coil on and off. These transmitters were used from about 1887 to 1916. The illustration is of a Marconi quenched spark transmitter.

The disadvantage of spark-gap transmitters is that it is not easy to modulate or impress speech on the transmitted waveform. To overcome this problem, a special type of alternator (AC generator) capable of generating a high enough frequency to produce radio waves was developed by Alexanderson. The first Sunday in July is Alexanderson day, and is celebrated by the Swedish government operating a 17.2 kHz Alexanderson alternator at Grimetown.

Radio valves (triode valves) were patented in 1907 by Lee De Forest and valves came into general use from about 1915 onwards.

A radio receiver is connected to an antenna that, in turn converts incoming electromagnetic waves into small voltages. It is the job of a radio receiver to select these small voltages according to their frequency and convert them into usable output. There are many good books available on the subject of aerials, and magazines also publish interesting ideas.

The radio transmitter has to impress some form of intelligence on to the transmitted wave. This process is referred to as modulating the wave. The simplest form of modulation is to switch the wave on and off according to an agreed sequence. The sequence most often used is the ‘Morse code’. The receiver must be able to make this sequence audible to a receiver operator. This form of transmission is referred to as 'CW’ or ‘Carrier Wave’. The other forms of transmission familiar to most of us are amplitude modulation (AM), in which the signal intensity is varied in proportion to an applied sound wave. Some receiver manuals refer to AM as ‘Radio Telephony’ or ‘R./T’. A microphone and amplifier are used to convert sound to an elModulating a Carrierectrical signal that can be impressed on the output. This form of transmission is used in AM transmitters. These usually operate in the frequency band 500 kHz to 1600 kHz on medium wave. The majority of broadcast short wave transmitters also use this form of modulation.

In ‘FM’ or frequency modulated transmitters, the signal frequency is varied about a central point in sympathy with the audio from the microphone. Local radio stations in the range 88-108 MHz use FM transmissions. At one time broadcasts used to be in the long wave band. This seems to have varied from country to country, but some car radios still have a long wave 150 - 300kHz and on a recent visit to the United Kingdom, I was surprised to pick up many long wave broadcasts from the BBC and other European transmitters. There is also a form of modulation referred to as ‘Phase Modulation’ which requires similar demodulators to FM.

In the 1950’s many amateur radio stations began to use a form of modulation known as single sideband (SSB). It turns out that when transmitting AM, there is no need to transmit the central (or ‘carrier wave’) frequency, since it conveys no information. The theory of sidebands propounded by Carson states that an AM signal can be resolved into a central carrier wave and an upper and lower sideband. Since the sidebands are symmetrical about the carrier, there is no need to transmit both in order to retain the intelligence of the transmission. It is customary to transmit the lower sideband only on frequencies below 10 MHz, and the upper sideband on frequencies above. Double sideband with a suppressed carrier (DSB) is occasionally used by commercial transmissions.

Modern satellite and digital receivers (including cellular telephones) convert the signal to a series of digital codes. There are a few portable receivers that can receive these signals directly from orbiting satellites and decode them with compact-disc (CD) quality sound, although satellite radio in South Africa has received a setback with the demise of “Worldspace”.Alexanderson Alternator

Digital and ‘Pulse Code Modulated’ (PCM) transmissions might be thought to be outside the scope of this document, but equipment dating back 50 years or more may use this or other pulse transmission modes, so collectors should be aware of their existence. In Europe and the UK there is a concerted move toward terrestrial digital transmissions for television and sound broadcast. With increasing Internet bandwidth, we are beginning to see “Internet Radios” and “internet TVs”.

Detecting and amplifying a Signal

From about 1906 to 1927, crystal sets were used for reception. It is instructive to look at the circuit for a crystal set.

Basic Crystal Radio - Incorrect Circuit

In this simple circuit:-

  • The antenna converts the electromagnetic wave to small voltages

  • The tuning coil/capacitor combination resonates at the desired frequency

  • The diode detector (optionally “cat’s whisker” as described later) demodulates the received signal.

  • The headphones allow you to hear the signal.

Instructions are given elsewhere on the site for making crystal sets.

Receiver Circuits

The simple crystal receiver suffers from some faults. The principal ones are:

  1. It requires headphones.

  2. It can not separate one station from another when the two are broadcasting on nearby frequencies.

  3. It is not sensitive to the reception of weak signals.

  4. Although frequently published, it doesn't work very well. This is because there is a capacitance of about 470pF between antenna and ground. This acts in parallel with the tuning capacitor, making the circuit all but useless for anything except possibly long-wave reception. The crystal set projectsHertzite Advertisement from Old Radio Publication overcome these deficiencies.

The crystal in a crystal set radio was usually Galena (lead sulphide). The cat's whisker was a piece of phosphor bronze. Nowadays we use a germanium diode (starting to be hard to get). Some Schottky diodes show promise, but silicon diodes simply don't work at all. Then as now there were charlatan's waiting to take your money for nothing for a "fancy crystal".

Tuned Radio Frequency (TRF) Sets

Overcoming the first problem is easy - attach an audio amplifier to the output of a crystal set. In order to overcome the second and third problem, a radio frequency amplifier may be placed between the aerial and crystal detector. These arrangements of ‘RF-Detector-AF’ stages work well and are simple but do not always address the second problem of preventing adjacent stations from interfering with one another.

TRF receivers are still used today for the reception of very low frequencies (VLF). VLF is used for certain forms of marine communications and for monitoring solar activity.

Regenerative and Super-Regenerative Receivers

In 1912, Edwin H Armstrong invented the regenerative or feedback circuit. In regenerative receivers, the detector stage may take a portion of its output and feed it back to the input until just below the point where oscillation occurs. This point being found by the user through a ‘reaction control’. The circuit provides improved selectivity and higher overall receiver gain, at some expense of usability as far as the modern user is concerned. Once the point of oscillation is achieved - the set becomes a transmitter.

What if we could proceed beyond the point of radio frequency oscillation and still receive signals? This is possible in the super-regenerative receiver. In this set another valve or transistor is used to prevent oscillation of the receive frequency by means of a ‘quench’ oscillator that operates above audible frequency. Adjustment of the ‘quench’ and ‘reaction’ controls to achieve good results with these sets to provide good audible output required considerable skill and patience on the part of the user compared to modern receivers. It is not easy to make a super-regenerative receiver for the medium wave band, because the quench frequency is audible to many people.

These super-regenerative circuits were used for ultra high frequency receivers some years after the superheterodyne circuit was in general use. The ‘Wireless Set No 19’ used in tanks during World War 2 had such an internal set operating at 235 MHz using an E1148 triode valve. It was referred to in the instructions as the ‘B’ set. I think the same valve was also the transmitter.

Please refer to the projects menu for details on making your own one valve regenerative and super-regenerative receivers.

The Superheterodyne Receiver

In 1918 the same Armstrong invented the superheterodyne receiver. The superheterodyne uses the principle of mixing two different frequencies to produce two new ones. A heterodyne, is simply a ‘whistling noise’ that is usually caused when two adjacent stations interfere with each other. A superheterodyne is a whistle above the normal audible range. In a superheterodyne (or ‘superhet’) receiver a heterodyne is produced at the sum and at the difference of the receiving frequency. One of these is chosen as the so-called ‘intermediate frequency’ (I.F.) and is amplified before detection. The device used to mix the two frequencies is often called the ‘First detector’ and the device used to convert the I.F. to audio is then called the ‘Second detector’. For example, suppose we choose a common I.F. of 455 kHz as used in a great deal of American equipment to this day. If we wish to receive a station broadcasting on 702 kHz (Now sadly gone to the FM band), then we will need to produce a frequency of either 1107 kHz (1107 - 702 = 455) or 247kHz(702 - 247 = 455). The stage in a receiver used to produce the mixing frequency is called the ‘local oscillator’ and the stage that mixes the two together is referred to as the ‘mixer’ or ‘first detector’. There are circuits that combine all these functions using only one valve or one transistor called ‘reflex’ receivers.

In order for a superhet to work properly, all the circuits have to be adjusted to operate at the correct frequency. The process of adjustment known as alignment is that of carrying out the tuning of each circuit in turn using a signal generator tuned to the correct frequency of each stage. The manuals for vintage sets are readily available on the Internet for most radio receivers, and these contain details on how to align the sets. Never deviate from these instructions.

As a matter of interest, I have only had to align a superhet as a result of someone else's unsatisfactory attempts. Even brutally savaged receivers have come back to life in perfect alignment, once the missing parts have been installed.

Other Types

In recent years, there has been some interest around "homodyne" receivers. These can be thought of as a form of superhet with an intermediate frequency of zero. To do this, the local oscillator must operate at exactly the receive frequency. In 1928 when experimenters tried this principle, it was quite difficult to get a satisfactory receiver operating. Nowadays, particularly with digital transmissions used in mobile telephones, the idea has had a resurgence. There has always been a core of keen hobbyists who have experimented with this system.

Another type of radio is the "Fremodyne" - a term coined by Hazeltine for its 1-Valve FM tuner. This is a superhet with a superregenerative IF stage. See the projects section, but be aware that it is not a good beginners project, even though it uses a printed circuit board.

The Audio Stage

The audio amplifier in a radio receiver is used to take the small voltage at the detector and convert it to a level suitable to drive a loudspeaker. Without this stage, the radio would only be suitable for headphone listening. In the majority of radios, it consists of a pentode or tetrode valve driven by a triode preamplifier (which may also be the detector).

High-quality radio receivers, and a few Eddystone communications receivers use two valves in a so-called push-pull arrangement. Effectively one valve conducts, whilst the other is cut-off. This results in higher efficiency and greater power output, as well as greater fidelity.  

As always, Philips have been creative and sometimes use two output valves in series in a "Totem-Pole" configuration. This eliminates the need for an output transformer (but does need a high-resistance speaker). I thought the sound quality to be impressive.

Loudspeakers

The first loudspeakers were influenced by the gramophones of the day. These used brass and wooden horns to acoustically amplify the sound from the gramophone pick-up. Very often wireless loudspeakers,  were no more than a balanced armature movement attached to a paper cone, which was then placed at the entrance to the horn. Whilst this works, the laws of physics dictate that to get anything like a decent bass response , the horn has to be very big. In cinemas, where the loudspeaker could be almost as big as the living room, this worked admirably. Unfortunately, the small table-top horns did not produce such good results. In the 1930's,horn's had given way to the loudspeaker as we know it-the moving-coil loudspeaker. The early loudspeakers were often DC energised from the HT supply (see next) or used a permanent magnet.  Nowadays, super-strong permanent magnets mean that loudspeakers are really inexpensive and for the most part have very good quality.Lissenola horn speaker

For a loudspeaker to produce its best sound, it must be placed in a suitable enclosure. Very few radios indeed seem to do this. At best, you may find that the speaker is attached to a sturdy piece of plywood called a "baffle". Speaker fabric is stretched over the baffle and speaker opening, and the whole assembly is then nailed, screwed or glued to the cabinet. The only exception to this scheme that comes to mind is the modern "Tivoli Audio" range of products. They seem to have designed the speaker cabinet and put a radio into it. Most modern radios have a very inexpensive speaker mounted directly onto a plastic case. There is almost no bass - they sound tinny as a result.

Power Supplies

In the early days of radio, you needed three power supplies. One was needed for the valve heaters, another to provide grid bias - small negative supply, and finally an HT supply. Radio valves were designed around the use of "Accumulators" - the name for lead-acid batteries. These would be 2-Volt batteries that you would take to the garage at weekends for a charge. Early radio valves were designed with filament voltages of 2 or 4 volts - to work from accumulators. Later, 6.3 volts became the standard, with a few 12.6 volt variants. These were intended to be compatible with motor-vehicle battery supplies.

Most early sets also required a "Grid-Bias" battery. As late as the 1950's, grid bias batteries were available from "Woolies' - that's the original FW Woolworth high street store, that sold everything from glue to paint and confectionery. These batteries had a number of tappings and were great for young experimenters.

The high-tension or HT supply for the valve anodes was also available from Woolies as a 90-volt battery. 

It must not be forgotten that in the early days of radio, there was either no electricity supply, or if there were, it was often DC. Thus was borne the deadly "Universal" AC/DC mains radio. This is an arrangement where the valve heaters are arranged in series and the HT voltage is derived directly from the mains. Usually a selenium rectifier is used, followed by a smoothing capacitor. The valves had specially designed high-voltage heaters. In some cases, a high resistance mains-cord was used. (Watch out for that one! - and don't cut it short or replace it.)

Obviously,these sets pre-date "elf and safety". Never operate them directly from the mains - always use an isolation transformer.

High-quality radio receivers used a power supply (or battery eliminator). This would be a mains transformer with tappings for the valve heaters (low tension supply) and high voltage tappings for the HT supply. The high voltage would be converteted to DC with a valve rectifier, and the resulting waveform would be smoothed by means of smoothing capacitors ("condensers") and a choke. In some sets, the supply would also be fed to the energising coils of the loudspeaker.  

I have encountered quite a few South-African radios with a dual supply. They were capable of working from mains, or from a 12-volt car battery. Perhaps this is because many farms did not have a mains supply. The 12-volt supply is identical to that used in car radios (and never works). 

 


[1]The ARRL Antenna Book” The American Radio Relay League, Inc, Newington, Connecticut.


 

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Last Updated on Wednesday, 06 November 2013 10:29
 
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