G7TXU

Knowing which mode to use is essential to being able to hear your signal correctly. You need to have your receiver set to the same mode as the signal is transmitted in. By convention, certain bands will be associated with certain modes and becoming familiar with the wavebands means that you will learn which mode will be found where.

If you try to listen to an AM transmission when in FM receive mode, you will hear what appears to be a silent carrier. If you monitor in AM what is an FM transmission, you will just hear it but the transmission will be very quiet. If you listen to a NFM signal in WFM, it will also appear extremely quiet. If you listen to an SSB signal in AM, it will sound distorted and a bit like Donald Duck

• AM - Amplitude Modulation
• FM - Frequency Modulation
• SSB - Single Sideband
• CW - Continuous Wave

I suppose it's helpful to look at this from a historical perspective. So we'll start with a history lesson.

When radio was discovered, the pioneers started by generating sparks and noted that energy could be transmitted a short distance to a coil of wire which sparked in sympathy. Later, it became possible to send speech over the air. The discovery which allowed them to do this was the AM detector. At that time, the crystal set was the cutting edge in technology and it was the crystal which was the early form of detector. What it did was to take in the AM radio signal and turn it into an audio frequency which could be heard by the human ear.

We're all familiar with the term AM but few people understand what it means. AM or Amplitude Modulation is not a waveband as commonly but incorrectly described, it is in fact a form of modulation. It is the way in which a radio signal or RF carrier is modulated so that it can convey information such as the human voice.

If you listen to an AM transmission, in which no one is talking and it is silent, the waveform looks like this

This is familiar to some of us as a pure sine wave. The carrier goes up and down in a sinusoidal wave but does not vary in amplitude.

Amplitude is shown here represented as the height of each peak and trough and as you can see, they are all the same. The carrier is unmodulated and therefore silent.

So what would the obvious thing to do be to make this RF carrier convey human speech? You might have guessed. It is to make the amplitude of the signal pump up and down in sympathy with the human voice.

A typical amplitude modulated voice signal looks like this

So you can see the height between peak to peak varies in sympathy with the speech waveform. Remember that speech is a much lower frequency than radio and this diagram actually represents two whole cycles of audio and around 11 cycles of RF carrier.

The frequency of the carrier remains constant, it is only its strength which varies.

So AM was the first and most simple form of modulation. Speech could be transmitted very simply by wobulating the power of the transmitter in time with the voice, and detected by a simple crystal detector.

AM is still in common use today. As we know, it is used for broadcasting in the short wave, long wave and medium wave bands. But it is also used for aircraft in both the civil and military wavebands.

Airbands - Civil VHF & NATO UHF
BAND	Bottom (MHz)	Top (MHz)
Civil Airband	108	136.975
NATO Military Airband	230	400

AM used to be used for PMR quite commonly with VHF but gradually it went from High Band and then from Low band and now it is all but gone from commercial usage.

Some fire and rescue services still use AM though for their 70MHz radios but they are eventually all going over to TETRA.

Radio Amateurs use all sorts of weird and whacky modes and AM is one of them. In fact, there has been a small resurgence of interest in AM on the 4 metre, 6 metre and 2 metre bands.

AM is also widely used in simple wireless devices such as car alarm key fobs.

So AM was crude but effective but it had a few drawbacks. The most significant one of these was interference. If you are representing your wanted signal by the strength of the radio signal you receive, you are also prone to demodulating stuff which you never intender to demodulate. For instance, in a petrol vehicle, the ignition coil generates sparks and going right back to the pioneering days of radio, we can see that you are being followed around by a spark transmitter everywhere you drive. This manifests itself as pulse noise on the AM radio. But not only cars cause this problem, radio is in fact a very natural phenomenon and a lot of interference comes from way out in space. In the modern world, there are so many electronic devices, many of which contribute to us living in a very electrically noisy environment.

It was an American by the name of Edwin Armstrong who, back in the mid 1930's published his paper entitled "A Method of Reducing Disturbances in Radio Signalling by a System of Frequency Modulation" It wasn't until quite a few years later that his method saw wide scale use. It began in Europe in Germany after WWII and in 1948, a European agreement on what we still have today as the FM broadcast band was reached in Copenhagen. During the fifties, FM broadcasting began to take off.

So, how is FM different from AM? Well, AM conveys its audio signal by varying the transmitter power, and FM does it by varying the carrier frequency. So, instead of the carrier signal wobulating up and down in strength, it wobbulates up and down in frequency. Now, because of this, any interference such as pulse noise is not demodulated and does not come out of the speaker.

Here's how the FM waveform looks. The top waveform is the audio signal, in this case a sinewave. The bottom one represents the RF carrier after it has been modulated. You can see the when the voltage if the sinewave is high, the corresponding part of the carrier is 'bunched up', in other words it has shifted up in frequency. When the audio voltage is low, the carrier is 'stretched out' and therefore a slightly lower frequency than normal.

The two extremes of frequency do in fact cancel each other out and the average frequency of the carrier is the same as the carrier's centre frequency.

The carrier is constantly at its centre frequency when there is no audio present. Like the AM example above, it is just a regular sine wave when it is not modulated. The louder the signal is that modulates it, the wider it deviates from the centre frequency.

There is also another quality about FM which sets it apart from AM. When you have two AM stations transmitting on the same frequency, you can hear both of them at the same time, usually accompanied by a whistle known as a heterodyne. Not so with FM as the stronger signal wins and effectively blots out the weaker one. With aircraft, for reasons of safety, it is beneficial to hear if another station is transmitting an urgent message under another which is one of the main reasons they have stayed with AM.   

Wide FM and Narrow FM

You may have both wide and narrow FM modes on your scanner, so what's the difference? Well, broadcast FM, as I've said varies in frequency. That means that the carrier moves up and down the waveband when the music is playing. It is because of this that an FM transmission occupies a certain chunk of the radio spectrum. The louder the music, the more the carrier wobbles up and down in frequency and the more space or bandwidth the station takes up. Now obviously there has to be a limit to this or else we would just have one very loud FM station taking up the whole of the waveband

The more bandwidth is used, the wider the range of audio frequency is that can be transmitted. With a music broadcast, you need to cover the whole range of frequencies that the human ear can hear and that's about 40Hz for your deep sub bass right up to around 16.5kHz or 16,500Hz for the top reaches of treble. After this, you get into the sort of frequencies that only cats and dogs can hear.

In order to transmit this entire audio bandwidth, you need around 200kHz of radio bandwidth. You will find that broadcast stations are set around 200kHz apart in order to give each one room to operate in without running into each other.

Now the radio spectrum is a finite resource and there's only so much space to go around. With communications radio, you don't actually need bangin' bass and crisp cymbal sounds in order to pass a message.  You will be familiar with the way in which someone sounds over the telephone, it has that 'telephone' quality to it. This is what is known to communications engineers as the commercial speech bandwidth. Instead of conveying the full audio range, we concentrate instead in portraying a limited 300Hz to 3300Hz. It is because of this that the resulting radio bandwidth is correspondingly lower. After all, if all comms users needed to transmit in BBC quality, there wouldn't be enough spectrum for the taxis, let alone anyone else.

You can see from this table how relatively spectrum hungry WFM is. For every broadcast station, you can fit twenty or so simplex comms users in the same piece of spectrum.

Relative required FM Bandwidth (approx)
	Bottom Freq (Hz)	Top Freq (Hz)	Audio Bandwidth Required ((Hz)	RF Bandwidth required (kHz)
Broadcast	40	16500	16460	270
Communications	300	3300	3000	12.5

So, when and where to we use WFM when scanning? In most of the world, the FM broadcast band extends from 87.5-108MHz. There are a few quirks such as Japan which uses 76-90 MHz. The former In the former Soviet republics, and some Eastern Bloc nations use 65.9 - 74 MHz in addition to the international band. These stations can sometimes be heard in Western Europe under the right conditions.

Analogue television uses WFM for its sound channels. These are what is known as a sound subcarrier as it is a component of the overall sound and vision carrier. Nevertheless, if you have WFM on your scanner and it covers the TV bands, then you can listen to television sound. You may need a TV licence to do so :-) You may ask why you would want to do such a thing but if you want to give it a go, check the TV frequencies. Remember, analogue television only has a few more years to run so you won't be able to do this for ever.

If you were surprised by how much bandwidth an FM broadcast takes up, take a look at the TV frequencies table too. Television broadcasting actually takes up more spectrum than any other sector!

We're not done yet with WFM, there are a few other places where high quality analogue audio is used and this too needs wide FM. There is a sector known as SAB (Services Auxiliary to Broadcasting) or PMSE (Program Making and Special Events). Ofcom don't administer these directly but hand them over to a body called the JMFG (Joint Frequency Management Group) and they administer frequencies to end users such as broadcasters and program makers. Full frequencies are given on their sites but audio transmissions under a JFMG licence generally fall into two categories. Links and talkback. Links such as OB links and cordless microphones are part of the broadcast chain and so will need to be WFM. They also allocate talkback frequencies to send cues to cameramen and presenters etc. These will be narrow FM as they do not need to be such high quality.

Not all scanners have SSB, it's relatively hard to operate with but is widely used on HF by the military, boats, planes and utilities as well as radio amateurs and CBers.

SSB is frequently referred to as a frequency or band but it is in fact a modulation mode. It is possible to transmit SSB on any part of the spectrum but in practice it is usually used at frequencies of under 30MHz.

SSB is in fact a rarefied version of the AM transmission mode. AM, when modulated consists of three parts. The carrier conveys no information at all, yet it uses two quarters of the power. When someone speaks, they generate an upper and a lower sideband either side of the carrier which is what contains the voice information. So, a while back, someone in the radio amateur fraternity had the clever idea of doing away with the carrier and concentrating power into where the information is being sent, the sidebands.

This is known as DSBSC (dual sideband, suppressed carrier) As things progressed, it became possible to strip out either the upper or lower sideband selectively. Both sidebands contained the same voice information so it made sense to do away with one of them to prevent redundancy. This gave us SSB (Single Sideband) and you could chose to transmit USB (upper sideband) or LSB (lower sideband) using around a quarter of the power the equivalent AM transmission would have used. You can also say that using the same overall power level or size of battery, you would be able to put four times as much power as before.

A receiver that can receive either of the SSB modes needs to re-create the missing carrier in order to receive correctly. Radios previously were often equipped with a  BFO (Beat Frequency Oscillator) even before SSB came along. This was originally used to hear CW (Morse code). It worked for both USB and LSB but it was not ideal as it was fussy to set up. Later SSB receivers came along which used true carrier re-insertion to 'put back together' the original signal so the BFO became history.

If you hear an SSB signal and you have a signal meter on your radio, you will notice that the signal strength varies entirely with the level of the audio signal. When there is no speech, there is no carrier and therefore no RF signal level will be shown.

This contrasts with basic AM where the signal strength will vary only very slightly with the audio level.

As a rule of thumb, SSB will only be found below 30MHz. Most broadcasters below this frequency will transmit in AM and utilities (i.e. non broadcast) will be in SSB.

Generally, all users below 30MHz use upper sideband as a convention. Radio amateurs use LSB below 10MHz and USB above.
Radio amateurs also use SSB at frequencies above 30MHz, they are the only user group that do. Outside the amateur bands, SSB is not used above 30MHz.

If you want to get some grounding in SSB reception, try tuning to some of the VolMets. They broadcast almost continually and can be received anywhere in the country with a modest antenna and receiver. When receiving SSB transmissions with a scanner, you may have to hunt around a little bit in order to clarify the signal. You need to set a very small step size and fine tune until the voice sounds natural. At first, you may not be able to tell if the speaker you hear is male or female or what language they are speaking. Once you resolve the audio and re-insert the carrier at exactly the right frequency, it should sound much like an ordinary AM transmission. This bit takes a bit of practice but you do get an ear for it. Better HF receivers are so accurate in frequency that you only need to go hunting about if the transmission is off frequency.

For a bit of practice, you can also tune into a regular AM broadcast station in SSB mode. When you have truly re-inserted your carrier, it should sound exactly like it would if you were receiving it in AM. You'll notice a whistle (otherwise known as a heterodyne) which will disappears when you are tuned correctly.

Several frequencies are used as with HF, conditions vary with the time of day. Lower frequencies are used at night, higher ones in the daytime.

HF VOLMETS
VOLMET	Frequency (MHz)	Frequency (kHz)	Notes
Shannon	3.413	3413	On the hour and half hour
	5.505	5505	On the hour and half hour
	8.957	8957	On the hour and half hour
	11.306	11306	On the hour and half hour
RAF	4.742	4742
	5.450	5450
New York / Gander	3.485	3485
	6.604	6604

This will be known to most people as Morse code. Generally this is transmitted is a simple carrier which is keyed on and off. In effect it is the oldest digital mode. It was used quite extensively for HF maritime mobile but now is only used by a few radio amateurs. Since CW was dropped as a requirement for the amateur licence, it looks set to die out all together. The attraction to many of CW was that it is very economical on bandwidth. It's a carrier with no sidebands and no deviation and is therefore very narrow.

Morse code can be sent over say FM by sending an interrupted or keyed tone but this is not true CW. You may hear this on amateur repeaters as they give out their idents.

You may hear these terms occasionally used but essentially they are similar to FM.

We're getting a bit silly here, this isn't used for analogue voice comms, its used with WiFi and the like. I just included it because I think it sounds cool.

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