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Amateur Radio Info & Exams - Modes

Modes, or Modulation Techniques

There are a range of methods of putting the "intelligence", or information onto the radio signal, often called the carrier.

From a circuitry point-of-view, CW, or Morse code is the simplest to generate. While technically CW is just a constant carrier signal, and the correct term is ICW - Interrupted Continuous Wave, CW is in common use for Morse code. There are several ways to make this, but the fairly modern method is to switch the output of a constantly running oscillator going to the power-amplifier stage, using a switch called a key. This generates sequences of short and long elements which represent letters, numbers, punctuation and procedural characters. The simple low power (or QRP) transmitter can be made by just switching power on and off an oscillator consisting of a transistor, a cheap 3.58 MHz NTSC "Colorburst" crystal, and a few other bits (note that at higher crystal frequencies this becomes "chirpy". Keying an audio oscillator feeding an AM, FM, or even digital transmitter is called MCW - modulated CW. It is used for repeater identification, some on-air Morse training and practice groups, and some aviation beacons. In days of old, this involved keying a large Tesla coil on and off, creating a high-voltage spark, a system which generates a raspy, broadband signal, as sparks contain wide-band radio-frequency energy, which is fed into an antenna. These are now illegal, as they obliterate all other communications. Beyond the exam, very low frequency signals, overlapping with high audio frequencies, could be generated using a large, multi-pole rotating generator, called an Alexanderson Alternator, rotating at much higher speeds that used for power generation. The Varberg station, operating at 17 kHz is an example.

Morse has a dah-di-dah-dit - dah-dah-di-dah sound (this being "CQ, the general call). The exam specifies the required bandwidth as 150 Hz. With a straight key maybe 20-odd words is the practical limit. Side-swipe keys / bugs / automatic keys, including by Vibroplex, which use pendulums to automatically generate dits, or more complex electronic keys, which generate dits and dots, can give better speeds, and are less tiring to operate. Morse can also be generated by PCs, micro-controllers, single-board computers, and dedicated Morse generating keyboards from the '80s. Computers can also be used to decode Morse, but are not perfect. Very slow Morse, called QRSs (from the request to reduce speed, QRS), is used in some beacons, as these can be received at very, very low levels by feeding the received audio into a PC, and using averaging techniques to extract the signal from the noise. Very low power transmitters may be called QRPp, from QRP = reduce power. In later sections you will read about 500 kHz filters being used to receive CW, hence one of the spoilers in a question on another mode.

AM, or amplitude modulation is generated by mixing an audio (voice, music, etc) signal with the carrier. High-level modulation involves amplifying the carrier to the desired power level, and using an audio amplifier of half this power, to vary the power going into the final amplifier stage, using a "modulation transformer".

If we observe an AM signal on a "spectrum analyser", we see that the signal consists of the carrier, with a "sideband" containing the voice below the carrier, and another above it, in frequency. If we were sending AM on 1.846 MHz, or 1846 kHz, we would see the carrier at 1846 kHz, and sidebands down to around 1842, and up to about 1850, varying with the level and frequency of the input. Thus AM requires about 8 kHz of bandwidth, demonstrated in the 10 kHz spacing of 27 MHz CB channels. While US airband uses 25 kHz spacing, Europe has roughly tripled the available channels, by going to 8.33 kHz spacing.

A spectrum analyser is a device which displays the level of signal across a range of frequencies, and displays this relationship on a cathode ray tube or LCD screen. This differs from a cathode ray oscilloscope (CRO), or LCD equivalent, which displays level vs time, thus showing the waveform of a signal, be this sine-wave, complex voice waveform, square-wave, video lines or frames, etc; or the "envelope" of an audio frequency signal modulated onto a radio frequency carrier.

Also in the AM family, SSB, or single-sideband involves filtering out the carrier, and one sideband, before feeding this low-level signal into a power amplifier stage, also termed a linear amplifier stage. Due to how this signal was generated in the past, there is a convention to use lower sideband below 10 MHz, and upper sideband above, so 160m, 80m and 40m use LSB, 20m, 17m, 15m, and so-on, into the VHF and UHF bands use USB. FOr VKs, this includes USB on 30m. An exception is 60m (5MHz), where USB is mandated, in accordance with modern military and commercial practice. This allows "government" (military) users to ask an Amateur station to cease operation on the channel, if required. Also, nets using old military and commercial gear may use USB on 80 metres. According to my Broadcasting course teacher, also a Ham, using the non-conventional sideband is an indication you don't wish to be interrupted. SSB requires about 3 kHz of bandwidth, usually filtered to cover 300 to 3400 Hz voice frequencies.

A related modulation method is double-sideband, with the two sidebands without the carrier, also fairly easy for a "home-brewer" to build. To receive this one sideband is selected on an SSB receiver.

Modern transceivers with an AM mode generate this at low-level, before feeding it into the power amplifier stage. AM can be described as "double-sideband, full carrier (DSB-FC).

As all energy is concentrated into a single, narrow sideband, and not wasted on the carrier, the mode is more efficient than either AM, or FM. Thus it is used for DX (long distance non-repeater contacts) in VHF and UHF, as well as being the standard mode for HF.

Note that here "linear" is a technical term, relating to the output being proportional to the input. Amplifiers for CW and FM need not be linear. While external amplifiers are illegal for CB, they are fine on Amateur bands, provided power limits are complied with. The term is generically applied to any external power amplifier, even if non-linear.

FM is a little more complex, and is sometimes terms angular modulation. Pure FM directly varies the frequency of the oscillator, and amplifies this. However, many commercial FM transceivers use "phase modulation", where the phase of a lower frequency RF signal is varied by the audio signal. This is then doubled or tripled in frequency by several stages, with 12 or 16 times being normal. If we want to put an old radio, with a ÷12 specification onto 146.520 MHz, we need to have a 12.21 MHz crystal made, complying with the specs for that radio. FM signals are at full power. If we look on a spectrum analyser, we will see the signal varying is width, depending on the level of the input signal. Modern FM is 16kHz, or 10.1 kHz wide, with channel spacings such as 30kHz in the US, and 25 kHz or 12.5 kHz in Europe. Note that wide and narrow are relative terms, so while my Chinese HT allows Narrow and Wide settings on the transmitter, these being 10K1 and 16K0, my Yaesu has receive modes of NFM, meaning ~16 kHz, and FM, meaning wide FM (aka WFM) of around 150 kHz, suitable for broadcast FM, and the slightly narrower TV audio sub-carrier). In the UK 70 MHz (4m) 10k1 NFM is recommended, as this allows more users in this narrow band.

Reception of AM can be achieved using simple receivers, with SSB and CW needing a more complex receiver, with good frequency stability, as the carrier is not there to act as a reference. For SSB, using the receiver in a ham transceiver, select the appropriate sideband for the band, and tune until you find a voice, tuning until it sounds natural. If you hear CW, and want to listen, this works in SSB, but selecting CW (or CW-R) gives you narrower filtering. If using a shortwave receiver with a BFO (beat-frequency oscillator), and you have an advertised net frequency, tune to this, then adjust the BFO to get speech. If you want to listed to a true CW Morse training or beacon channel, likewise tune to this, and manipulate the BFO for a good tone. FM receivers are perhaps in between in internal complexity, but are easy to use, just select the required channel.

A wide range of data modes are also permitted, including older modes, such as "RTTY", standing for Radio-teletype, and the more recent "packet radio", where data is divided into "packets", using the AX-25 protocol. This allows messages and bulletins to be forwarded like emails, and is also used in the APRS, where usually GPS positions are transmitted and forwarded. More modern modes are able to operate using weak received signals, and in very narrow bandwidth.

Simple data modes, such as radio-teletype involve rapidly switching between two fairly closely spaced radio frequencies, one a logic 0, the other a logic 1, (Frequency Shift Keying, FSK) although in practice this is generated by feeding two audio tones into an SSB transmitter. Packet radio involves feeding audio tones into an FM transceiver (Audio Frequency Shift Keying, AFSK). Some modes involve phase shift keying (PSK), and/or multiple audio carriers, and/or multiple levels.

Digital voice, or DVrequires complex modulation, and are usually generated in a proprietary IC, due to commercial licensing, and typically involve taking thousands of samples per second of the voice signal, and converting this into a stream of data. This is then compressed to form a stream which can be transmitted in a narrow RF channel. These have been cracked, allowing generation and reception using basic PCs or single-board computers, such as the Raspberry Pi. There are also amateur developed modes, such as "CODEC2", developed in Australia, where the data stream generated using a PC can be fed into a SSB transceiver.

Examples of these modes are P25, named for being APCO Project 25, developed for law-enforcement use, with most gear being auctioned off or donated Motorola gear; D-Star, a nominally open Japanese Amateur system, but sold by Icom and Kenwood; Yaesu's Fusion (C4FM); a rare Alinco proprietary system; and the much more open DMR, or Digital Mobile Radio, with both commercial grade, and affordable Chinese made HTs - just made sure they are "Tier-II" or better, and also not "dPMR". There are no restriction on US hams using these mode at VHF and UHF. Backward rules down-under prevent Foundation licence holders from using these modes, due to a petty ruling by the dinosaurs of the WIA, that the identification information in the datastream is a "data transmission", so verbotten; this influencing ACMA to ban them. DMR may be the mode which replaces FM as the standard Ham chat and emergency service mode on VHF and UHF. It does support IP (Internet Protocol) based linking of repeaters, both regionally, and globally.

By the way. "weak signal" refers to the idea that the received signal may be low in level, not that the transmitter power is low, it may in fact be quite high, including using linear amplifiers to operate at the maximum legal power.

One of the spoilers is "DRM", which is a semi-experimental digital broadcasting mode, used in the long-wave, medium-wave, and short-wave bands. It stands for Digital Radio Mondiale (Mondiale being French and Italian for Worldwide).

SSTV, or Slow-scan TV, being a range of narrow-band image transfer modes; and fax modes are permitted in most bands.

Fast-scan TV, termed Amateur TV, or ATV from 70 cm up. North American TV, with only 525 lines, and using the NTSC colour system, has a bandwidth of 6 MHz. NTSC stands for National Television System Committee. 625 line PAL is used in countries which used this mode for broadcasting, with some groups implementing digital amateur TV - DATV. Standard NTSC or PAL TV uses VSB - vestigial sideband, a form of AM where most of the typically lower sideband is filtered out, leaving the low frequency content as full DSB, but the higher frequency content as SSB, preserving the phase information on the important low frequency syncing and related signals, while saving bandwidth compared to full AM.

With the exception of DMR, which can use timeslots (time division multiple access, TDMA) to allow two conversations at one on a repeater, a bit like GSM 'phones, all the above modes require each user to use a different frequency (channel) if they want to have a separate conversation; which can be called "Frequency Division Multiple Access", FDMA. A third system can allow multiple transmissions to occur across a large band of frequencies, each transmitting packets of data from the digital voice data stream, or other data in an apparently random sequence of frequencies. This is called "spread spectrum", and was originally used by the military, as it is hard to jam several, or multiple MegaHertz of spectrum. CDMA, or Code Division Multiple Access was/is used in 2.5G and some 3G telephone systems.

Various data modes can be used for telemetry and telecommand, reading measurements from a distance, and controlling equipment remotely.

Relevant Questions

These are actual exam questions, from the published NCVEC Technician pool.

Which of the following is a form of amplitude modulation?
A. Spread-spectrum
B. Packet radio
C. Single sideband
D. Phase shift keying

Single sideband is the only amplitude modulation example in the list, answer C.

What type of modulation is most commonly used for VHF packet radio transmissions?
D. Spread Spectrum

Packet equipment is usually connected to an FM transceiver, answer A.

Which type of voice mode is most often used for long-distance (weak signal) contacts on the VHF and UHF bands?

Direct, station to station contacts are made over hundreds of kilometres using SSB, answer C. Directional antennas mounted on towers are also usually needed.

Which type of modulation is most commonly used for VHF and UHF voice repeaters?

FM, answer D is the only repeater mode listed, and will remain the most common repeater mode (more so than all digital modes combined), until at least mid 2018. Some repeaters are dual-mode, such as Motorola Quantars, which are FM in - FM out, or P25 in - P25 out; and Yaesu's FM / C4FM "Fusion" jobbies. The very, very first repeaters were AM. Now forget that, the correct answer is FM.

Which of the following types of emission has the narrowest bandwidth?
A. FM voice
B. SSB voice
D. Slow-scan TV

Answer C, CW has the narrowest bandwidth in the list, and is probably only beaten by a few row-rate weak-signal data modes.

Which sideband is normally used for 10 meter HF, VHF and UHF single-sideband communications?
A. Upper sideband
B. Lower sideband
C. Suppressed sideband
D. Inverted sideband

All these are above 10 MHz, so the convention is Upper sideband, answer A.

What is the primary advantage of single sideband over FM for voice transmissions?
A. SSB signals are easier to tune
B. SSB signals are less susceptible to interference
C. SSB signals have narrower bandwidth
D. All of these choices are correct

SSB signals have narrower bandwidth, so the signal can be concentrated into this space; answer C, and thus narrower filters can be used. SSB requires accurate tuning to avoid having the voice sound high or low pitched, so, while not that hard to tune, is not easier than FM. A strong FM signal is effective in masking interference.

What is the approximate bandwidth of a single sideband voice signal?
A. 1 kHz
B. 3 kHz
C. 6 kHz
D. 15 kHz

An SSB signal is the width of most intelligence in the human speaking voice, about 3 kHz (3K00), answer B.

What is the approximate bandwidth of a VHF repeater FM phone signal?
A. Less than 500 Hz
B. About 150 kHz
C. Between 10 and 15 kHz
D. Between 50 and 125 kHz

Two official bandwidths are 10K1 and 16K0, so C, 10 to 15 kHz. This relates to communications FM. A full FM broadcast signal can be 150 kHz wide, and TV audio in the range of the last spoiler.

What is the typical bandwidth of analog fast-scan TV transmissions on the 70 cm band?
A. More than 10 MHz
B. About 6 MHz
C. About 3 MHz
D. About 1 MHz

A little narrower than the discontinued European and Australian analogue transmissions, American NTSC TV is 6 MHz, answer B.

What is the approximate maximum bandwidth required to transmit a CW signal?
A. 2.4 kHz
B. 150 Hz
C. 1000 Hz
D. 15 kHz

For CW, think narrow, about 150 Hz, answer B.

On to: Safety

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, October 2017.

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