Home  |  Contact  |  Site Map  |  Reciprocal Links & Credits

Well radio waves have carried Terry Wogan, Tony Blackburn, Chris Tarrant and Coronation Street to us - riding on the wave, that's what!

Radio waves are part of the Electromagnetic Spectrum.  All waves of the electromagnetic spectrum travel at the same speed of 300,000 kms per second in space.  Note that sound waves are NOT part of the electromagnetic spectrum.

The electromagnetic spectrum stretches from radio waves at the lowest frequencies (longest wavelengths) at one end (the relatively harmless end) through Infra Red (heat) waves, visible light waves all the way up to Gamma Rays at the very shortest wavelengths (highest frequencies) - the harmful end of the spectrum.  Gamma rays are harmful to human tissue and are produced by events including the detonation of atomic bombs and from celestial objects such as quasars.  

THE ELECTROMAGNETIC SPECTRUM:

We will concentrate on the relatively harmless end of the electromagnetic spectrum - Radio Waves - which include Radio, Television and mobile phones.

THE RADIO PORTION OF THE ELECTROMAGNETIC SPECTRUM:

Classification ELF - VLF LF MF HF VHF UHF SHF - EHF Longest Waves Shortest Waves Wavelengths Very Long Waves Long Waves Medium Waves Short Waves Very High Frequency Ultra High Frequency Super to Extremely High Frequencies Examples Of Types Of Us Marine & Military AM Radio AM Radio SW Broadcasts FM Radio TV &   Mobile Phones Satellite Transmissions A Representation of the RADIO portionThe Electromagnetic Spectrum

The receiving device has to tune into the required transmission and be able to reject all other transmitters within range.  This is done with a Tuned Circuit.   The tuned circuit acts as a Filter, without one all available broadcasts would be received, which would be very confusing to the listener and quite useless.   The tuned circuit filters out the transmissions that are not required.

If we look at the humble crystal set it has a very simple tuned circuit consisting of a coil and a variable capacitor.  The size of the coil and the number of windings of wire on it and the value of the variable capacitor determine which part of the radio spectrum is received.  The variable tuning capacitor provides a convenient means of tuning to the required station.  The illustration below shows the circuit diagram and the typical components used.  Once the tuned circuit has selected the frequency of the required radio station it is passed on to the detector or diode that will provide audio to the headphones.

A READER'S QUESTION!

I have just spent the last three hours reading and re reading your website on radio, it is a fantastic documentation and thank you for taking the time to put together such an informative read. My reason for emailing you is quite simple I am currently writing an essay on radio, including both technological and sociological issues both past and present. I have read your site several times and the main thing that confuses me is information on broadcasting frequencies/distances and formats. I don't understand how they all relate to each other, for example 'Radio Four lost 206m (1457kHz) and 261m (1151 kHz) 'how do they relate? is the meteredge, the distance they can transmit or the wavelength or is this the same thing, or is 206M the distance and 1457kHZ the wave length, if so how are these related. Also I presumed LW could be transmitted further, but its seemed a lot of 'National BBC' programmes were transmitted on MW, how is this possible. Also how does the power of a transmitter/radio mast come into the equation, does it transmit to smaller masts which amplify the signal to get the coverage on MW and is the higher the power the further it can transmit. You also commented on early stereo transmission how was it possible as both local and national masts where used for each channel how can you tune to two stations at once. How does FM come in all this as this seems to be the standard and highest quality radio achievable now as early BBC broadcasts did not use this method.   Thank you for your time, I do hope you can help me out on these questions, maybe im being stupid and the answers are just staring me in the face. Kind Regards Lee Slater I will do my best to answer, Lee: FREQUENCIES and WAVELENGTHS A radio station is transmitted on a specific frequency within a given defined 'wave-band'.  For the general ANALOGUE broadcasting that we a talking about here, there are three domestic wavebands: 1/ Long Wave  2/ Medium Wave  3/ VHF. In Europe the authorities (co-ordinated by the the International Telecommunications Union [ITU]) have allocated the domestic broadcast radio stations space within the limited radio frequencies portion of the electromagnetic spectrum as follows: Long Wave - occupies the part of the electromagnetic spectrum stretching from a frequency of 148.5 Kilohertz to 283.5 kilohertz (kHz) [Note: 1 kilohertz is one thousand cycles of a radio wave per second - referred to as kilocycles per second from the 1920s to the 1970s when the term kilohertz was more widely adopted.  kilocycles per second is abbreviated to kc/s : Rememer that a radio wave is an alternating electrical current constantly cycling from positive to negative and back again, and so on.  An oscillation is simply the time it takes for the voltage of the transmitter to go from + ve to - ve and back again and is expressed as the frequency.] [Martin Watkins adds:- A transmitter is really just a battery being whirled round and round between its terminals very fast!), and that this can (and was) in the early days achieved by speeded up generators] Medium Wave -  occupies the part of the electromagnetic spectrum stretching from a frequency of 526.5 kilohertz (kHz) to 1606.5 kilohertz VHF (Very High Frequency) (Band II) Broadcast Band stretching from 87.5 megahertz (MHz) to 108.0 MHz [Note: 1 Megahertz is one million cycles of a radio wave per second, previously referred to as megacycles per second. abbr. mc/s ] DAB in the UK (Digital Audio Broadcasting - or plain simple 'Digital Radio') - A small part of radio spectrum streching from around 209 MHz to 216 MHz.  Digital radio stations are broadcast is blocks or 'ensembles' called "Multiplexes".  Each single multiplex carries not just one radio station, but a number of different stations depending on the level of digital compression used.  The higher the compression (i.e lower bit-rates) the lower the quality of the audio will be, but the larger the number of stations that can be accommodated into the multiplex.  Many areas in the UK can receive DAB radio and can typically obtain three multiplexes:  BBC National (all the BBCs radio national stations - including Radio One, Two, Three, Four, FiveLive, 6-Music, BBC 7, 1-Xtra etc),  Commercial national Digital One (Classic, Virgin, Talk Sport,  OneWord, PrimeTime etc etc), Local (BBC Local Radio and various commercial local stations).  Some areas, especially around London, can receive more local multiplexes or a regional multiplex, whereas other areas receive fewer multiplexes (e.g. Northern Ireland due to lack of frequencies available). MORE TELEVISION in the UK - Currently both analogue 625 line colour broadcasts and digital terrestrial television (DTT), in the form of "Freeview" and "Top-Up-TV", use frequencies in the UHF (Ultra High Frequency) part of the radio spectrum between  470MHz and 860MHz (known as Band IV and Band V).  In other parts of Europe and the world, Band I (approx 46 MHz to 67 MHz) and Band III (approx 175 to 210 MHz) are still used for television broadcasting. Beyond The Broadcast Bands Outside of these defined 'broadcast bands', the frequencies in between are used for all manner of other communications - e.g. police, fire, ambulance, boating and shipping, private 2 way radio, amateur radio etc. etc. etc. The methods of conveying the sound produced by the radio station vary according to the band in question: For the Medium Wave and Long Wave bands,  it was decided from the outset of radio broadcasting in the 1920s that the method of modulating the radio wave  would be AMPLITUDE MODULATION (AM). When the VHF band II was released for broadcasting in the 1950's AMPLITUDE MODULATION was experimented with, but it was eventually decided to use FREQUENCY MODULATION (FM) as FM was found to be somewhat less susseptible to some forms of interference - too complex to go into here. So you see 'FM' is not a 'band' and niether can 'AM' be described as a 'band' - they are just  methods of conveying the radio station output.  FM could be used to broadcast on Medium Wave or Long Wave (not that anyone would be able to hear it on an existing Medium Wave AM radio!) just as AM could have been used to broadcast on the VHF broadcast band. Now we come to the WAVELENGTH question - A radio wave is an oscillating wave in the radio frequency portion of the electromagnetic spectrum.  The electromagnetic spectrum stretches from the lowest frequencies at the 'radio' end of the spectrum through the higher frequencies of 'sub-millimeter' ; 'Infra Red' (i.e. Heat!); Visible Light; Ultra Violet; X-Rays (as used in hospitals) and harmful 'Gamma Rays' (as emitted by fissile material and atomic bombs) All these waves have one thing in common, they all travel at 300,000 kilometers per second (in free space) - commonly called the speed of light. As all electromagnetic waves oscillate at a certain frequency to form a wave of a certain length, the Wavelength and the Frequency relationship is inextricable linked.  Since the speed at which the wave travels is fixed, at 300,000 km per second, if the frequency of the wave is varied the only other variable that can change is the wavelength.  (Refer to Mr A Einstein for more detail) The mathematical relationship between wavelength and frequency is expressed in this fundamental and very important formula: Wavelength (in meters) = 300,000 / Frequency (in kHz) and also to put it another way: Frequency (in kHz) = 300,000 / Wavelength (in meters) SO: As the frequency of the wave is increased the wave length decreases . If we look again at the broadcast bands we can see the relationship between frequency and wavelength: Long Wave: 148.5 kHz to 283.5 kHz or expressed in wavelengths as 2020 meters to 1058 meters. Medium Wave: Frequencies from 526.5 kHz to 1606.5 kHz  or expressed in wavelengths as 570 meters to 187 meters. Wavelengths in Meters were always quoted by radio stations and marked on radio dials until the 1980s when frequencies were more generally adopted as the method of expressing where the particular station appeared on the radio dial. VHF radio has always used mc/s or MHz to express the position on the dial, but just for clarification here are the meter values too! VHF:  Frequencies from 87.5 MHz to 108.0 MHz  could be expressed as 3.428 meters to 2.778 meters  (but in reality the expression of meters is never used on air). Now look at a practical example: Many years ago Radio Four lost the use of 1457 kHz frequency.   But when that happened radios were marked in Meters, so what was the wavelength of that transmission? Take 300,000 and divide it by 1457 and you will get an answer of 205.90 Meters.  This is rounded up for simplicity when being quoted on the air by presenters as  206m .  Simple! DISTANCE TRAVELLED The fact that a radio station, such as BBC Radio Four from the 198 kHz Droitwich transmitter has such a very long wavelength means that for each cycle of oscillation of the radio wave it must travel 1515 Meters. Compare that to Capital Gold Radio on 1548 kHz medium wave this has a much shorter wavelength, and the wave only travels 194 meters per cycle. Very simply put, for a given and equal transmitter power, for each cycle of the radio wave the BBC Radio Four 1515 m transmission covers a much bigger area than the shorter wavelengths used by Capital Gold Radio on 194 m.   Shorter wavelengths are absorbed and attenuated (reduced in intensity) more quickly and therefore cannot cover such a large area for a given transmitter power. This effect explains why BBC Radio Four only requires 3 main long wave transmitters on 198 kHz to cover virtually the whole of the UK, while BBC Radio Five Live needs 10 main medium wave transmitters to obtain near national coverage.*  The longer waves of BBC Radio Four at 1515 meters are absorbed and attenuated to a much lesser degree than those of BBC Radio Five Live using medium wavelengths, which are absorbed to a greater degree which causes the signal to fall off more rapidly with distance from the transmitter. [Martin Watkins adds: It's not an exact analogy, but the bass drum of a brass band playing in the distance will be heard more clearly than the piccolos or fluegel horn.  As I say it's not an exact analogy with LW/MW attenuation, but HF sounds are screened by obstacles more than LF, explained at least in part by their wavelength, and by absorption in the atmosphere.] [ * BBC Radio Four also uses 9 filler transmitters on medium wave, while BBC Radio Five Live also uses an additional 13 medium wave filler transmitters] TRANSMITTER POWER Certainly the relationship between power and distance is important.  The more energy that is put into the radio wave the further away it will provide an effective an listenable signal.  This in exactly the same way as using a higher power light bulb will be more more effective and be seen futher away compared to a lower wattage bulb. (Of course light is part of the electromagnectic spectrum to which radio waves belong).  A 150 Watt light bulb will, for example, be much brighter and therefore be seen further away than a 25 Watt bulb. It is also interesting to note that doubling the transmitter power does not double the distance that the transmitter will cover.  To double the signal strength at the lisener's radio receiver aerial a power increase of 4 would be needed! AERIALS For a radio aerial (either trasnmitting or receiving) to have maximum effect it must be of a resonant length compared to the radio frequency that it is transmitting (or receiving, of course).  For a long wave station a very tall aerial is required to be able to resonate effectively at those wavelenths.  Long Wave = long wavelength = long or tall aerial. For medium wave the wavelengths are obviously shorter than long wave so a shorter aerial is required. Many medium and long wave stations simply use a tall metal mast as the aerial, the height of which will vary depending upon the wavelength of the service being transmitted, but will be many tens of meters in height. For VHF (or if you must call it FM) broadcasting the aerials are going to be much smaller, only a few meters across, and these will be attached to the top a tall masts or towers to gain height advantage. STEREO   In the early days the BBC experimented with Stereo broadcasts.  Stereo requires two AUDIO channels; (simply put) one to carry the sounds from the right hand microphone channel and the other transmitter to carry the sounds from the left hand microphone channel. The only way to do this in the 1920's was to use two radio transmitters on separate frequencies.  In this early stereophonic experiment the right hand audio channel was transmitted over the BBC longwave transmitter, which covered most of the country, while the left hand channel was simultaneously transmitted over the BBC's numerous local medium wave transmitters. For listeners lucky enough to have two radios, they could tune one into long wave and place it on the right of the room while the other was tuned to medium wave and placed on the left hand side of the room.  By sitting in between the two radios the listener could get a sence of the space and ambience and stereo soundstage that we all take for granted today! In the 1960's stereophonic transmissions were introduced to the VHF (FM) radio services of the BBC.  This time, however a new system had been developed, called the Zenith GE Pilot Tone system (developed by General Electric of America) that did not require a separate transmitter and radio for each audio channel.  The new system used the existing VHF frequency of the BBC station and added (Coded) in the extra components in the form of a 'multiplex' on top of the existing radio signal.  The idea was that the system would be compatible with existing mono radio sets, which would continue to receive the programmes unaffected and as normal in mono of course, but new radio receivers would be available that were fitted with the special Stereo Decoders required to extract the multiplexed stereo components and reproduce a stereo signal that would be fed to twin speakers via a stereo hi-fi amplifier.  The Zenith GE Pilot Tone System is still used today on the VHF/FM band.  Incidentally the Pilot Tone is an almost inaudible audio tone of 19kHz that is transmitted along with the sound of the radio station, when a suitably equiped radio tuner 'hears' this tone, it switches on the stereo decoder circuit so that the stereo programme can be resolved and heard.  (Incidentally, the 19kHz audio tone should then be stripped from the recovered audio by a special and very steep audio filter before it reaches the audio amplifier stage, however some poorly designed (read cheap) radio tuners do not do this effectively and people with very acute hearing can sometimes still hear the high pitched whistle!) Hope this answers your question in some small way. MORE QUESTIONS... I'm a faculty member in the MIS department at Temple University (Philadelphia, Pa.).  One of the courses I teach is the introduction to networking.  You're page on radio info is great!  I may direct some of my students to this page to get a better understanding of the various uses of each range of frequencies in the RF spectrum. Here is one additional question for you.  Do you know where I can find information regarding the RF spectrum that explains other things about the various bands such as the type of range you can expect from each band, the data rates you might expect, the things that cause the most interference, etc.? Thanks, Mart Doyle Hi Mart, Thank you for your-mail and your kind comments. I don't pretend to be a radio engineer, or particular expert, but I have presented the information on my website as useful material that I have previously researched and used in my own hobby of radio. I hoped that others would find this useful, as I have done, and I am glad that you have too, As for your other questions, I don't immediately have a reference that may answer them. However I do have a number of friends 'in the know, so I will ask them for you. In simple terms, for local ground wave contacts, the longer the wavelength (the lower the frequency) the further the signal will travel - for an antenna of given efficiency. In Europe we still use Long Waves (150 kHz to 300 kHz) for the transmission of AM radio stations. This means that during daylight in ground-wave conditions (night-time is different due to sky-wave reflections) long wave stations in France and Germany are quite audible here in the UK. Also the 198 kHz transmissions from the BBC are very popular across north west Europe. This means that the whole of the UK can be covered with just three long wave transmitters using a single frequency (albeit that a few additional medium wave transmitters a required to fill in the 'mush zones' where the co-channel transmissions overlap and interact). Contrast this with the national medium wave (medium frequency, AM, 500kHz to1700 kHz) network that requires perhaps a couple of dozen transmitter to cover the UK and the or more, and the VHF (very high frequency 'FM', 88 MHz to 108 MHz) network that requires hundreds of transmitters to cover the UK (this is because VHF and UHF radio waves are very much line-of-sight. Medium wave and long wave signals tend to hug the earth more - following its curvature.) If we have a look a high frequencies (HF, short waves) then these are locally quite short range, useful for short range local contacts like CB radio, so it may seem odd that short waves are so popular for international 'world band' broadcasting. However short waves are readily reflected off the ionosphere in the earth's upper atmosphere. In fact short wave can travel all the way around the earth by multiple bounces off the ionosphere and the earth's surface. Medium wave radio waves are also subject to a similar effect during the hours of darkness when the ionosphere is able to reflect them, therefore allowing MW signals to travel a greater distance. VHF and UHF can also be reflected great distances but under a different and unpredictable effect known as "Sporadic E" - reflections off the E layer of the atmosphere. A consideration  for general domestic use is aerial efficiency. The longer the wavelength the longer the aerial required to work efficiently. As a rule of thumb a 1/4 wave aerial is a fair compromise for a reasonable efficient aerial system. This means that for a VHF (FM) transmitter using, say, 98 MHz a 1/4 wave ground-plane aerial would be about 0.76 meters long. That's quite easy to accommodate. If you decided to use, say, 1000 kilohertz medium wave, then your 1/4 wave monopole radiator would be 75 meters high. If you decided to use a long wave transmission on 200 kilohertz, then your 1/4 wave monopole radiator would be 375 meters high! (In practice professional broadcast transmission engineers use clever techniques such as loading a shorter aerial so that it appears electrically longer to the transmitter. One must also bear in mind that even though a VHF aerial, used for FM radio, is quite small, it must be mounted very high up so that it has a good 'view' of the horizon. So the masts used for both medium wave and VHF broadcasting might very end up being of  very similar height anyway.) Aerial efficiency can play an important part in local communications. Here in  Europe we can use (as I do myself) Citizens Band (CB) Radio at 27 MHz (high frequency, short waves), or PMR 446 (446 MHz, UHF) walkie talkies for short range two way communications. A CB radio uses a 4 watt transmitter and a PMR 446 radio uses a 0.5 watt transmitter. Given an efficient matched aerial of correct length (about 3 meters long for CB and 16 centimetres long for PMR446) CB Radio can cover about 5 miles and PMR446 about 1 or 1.5 miles. However when using hand held walkie-talkies with a need to use small easy to handle aerials it may be found that PMR446 has as good, if not better range than a CB Radio. This is because the aerial on a PMR446  radio is very short anyway and thus easy to handle and will be the correct length for good efficiency. However an aerial on a CB radio would need to be a couple of meters long for proper efficiency - not conducive to easy handling (!) - so often a short "Rubber Duck" aerial might be used that is only a foot or so long. This reduces transmit and receive efficiency considerably, so much so that it may be that the 0.5 watt transmitter used by the PMR446 radio into an aerial of the correct length will be much more effective than a CB radio using 4 watts into an antenna of the incorrect length. As for frequency, it is my understanding that the higher the frequency, i.e .increased cycles per second, the rate at which data packets can be transmitted can be increased. I imagine that this is why ultra high frequencies of 2400 and 5000 MHz are used for wireless routers and access points used in Wi-Fi networks. I hope this is of interest. I will try to come back to you with more as I have it. Regards, Mike Dear Mike I have been reading your wonderfully informative web site about radio. To the layman like me you have really opened the subject up and educated me- thats got to be an achievement.   Anyway, I am wondering if you can help me with a query. I am a falconer in the UK and I use radio tracking equipment to recover my hawk if ever it is temporarily lost from view. The telemetry systems available in the UK operate on either 173.225 mhz, 216 mhz or 433mhz. As far as I am aware the only legal telemetry frequencies for this kind of use in the UK operate on the 173.225 and 433 mhz frequencies which means that 216 mhz is illegal. However, because telemetry in falconry is a fairly recent innovation the first equipment available was on 173.225. This had a drawback because the antennas on the transmitters were a bit too long and when attached to the hawk would on occasion snag on wire fences or other obstacles pulling feathers out or worse still electrocuting the hawk if it landed on a power pole. When equipment on 216 became available shortly afterwards (late 1980's) many falconers switched to because the antennas were shorter and to protect their valuable hawks. Most did this oblivious (or dismissive) of the law. Since then manufacturers have begun offering equipment on 433 mhz but it has been slow to catch on particularly as so many had already got receivers and transmitters on the two other frequencies.  A situation has now developed with the potential advancement of digital radio and there is a rumour in the falconry world that DAB may soon begin transmitting on 216mhz. Falconers with 216 equipment are worried understandably because they may have to fork out for new receivers and transmitters - the former are about £400 each and the transmitters about £100. Firstly do you know if DAB will be transmitted on 216mhz and if so will it affect the falconers analogue receivers? The obvious answer is that falconers on 216 should switch to legal frequencies I know but for now any help you could offer would be really helpful.   Darren Chadwick Hi Darren, Thank you for your e-mail and your kind comments. You raise some interesting issues. The reason that the use of 216 MHz (or any other un-authorised frequency) is illegal is that the particular frequency in question will have bee marked by the government and Ofcom for a specific use, either now or in the future. The illegal use of such  frequency by low powered transponders may not currently cause undue interference to legitimate and licenced users, especially if the frequency allocation is used by only a small number of legal transmitters. However 216 MHz has always fallen inside the legitimate 174 to 230 MHz 'sub-band' allocated by Ofcom to DAB radio. Ofcom are now in the process of expanding this band further, and as part of that process a new national network of about 170 high power DAB transmitters is due to be installed from next year (2008) onwards. This will use Block 11A - i.e. 216.978 MHz - in the DAB sub-band within VHF Band III. Although I don't think that there is an allocation as yet, there is the possibility that Block 10D may also be used at some time, this is at a frequency of 215.072 MHz - just on the other side of 216 MHz. I cannot say for certain if these DAB allocations will cause the users of illegal transponders problems, but I would think that there may be a chance that their reception would be 'wiped out'. Digital transmissions would appear simply sound like the "Hiss" of a de-tuned radio, so it's difficult to tell if you are receiving anything on an analogue radio. However these DAB signals will be quite strong and may be enough to make the very weak signals from the transponders un-readable. All I can advise is be prepared for the worse. See what happens. At least the legal 433 MHz equipment will use even smaller aerials! I hope that helps. Regards, Mike Hi Mike, Your response has been the most detailed and factual yet. This information is exactly the news I was searching for. I am so very grateful. The only thing is, now that I am equipped with this knowledge I now have more work to do! Once again so many thanks. PS. Still not had a reply from Ofcom on any of these topics. Darren Chadwick

See Also: RADIO DXing DAB Radio LOWE HF Series Receivers Aerials & ATUs Crystal Sets Back To:  Radio, Stations and Memorabilia