Radio Communications Systems During Crisis Situations (Modes and Modulation)

What are the best modulation methods to use?

• Modulation is the process of attaching or sending the data to be communicated along with the carrier wave.  Properties of the carrier wave are manipulated by the modulation process so the carrier changes as the data changes.  The carrier wave is the transmitting frequency of the radio.  It’s created in the Variable Frequency Oscillator (VFO) and then modulated by the desired method and amplified in the final stages so there is enough power radiated off the antenna.
• The first modulation method, if you can call it that, was continuous wave or CW. The telegraph key literally turns the transmitter’s carrier frequency on and off. There really was no modulation other than turning on/off the carrier frequency.  By turning the carrier on and off using a prescribed code (Morse code, a combination of short and longer times the carrier is turned on spaced with the carrier off) the data could be sent out the transmitter.  Since the only energy being sent is radio frequencies and the human ear can’t hear them we need to figure out a way to make it audible to the human ear.  The receiver has built in another oscillator called a Beat Frequency Oscillator (BFO). The BFO usually outputs from 650 to 800 Hz.  When the receiver’s VFO frequency is tuned exactly to the transmitter’s VFO frequency (the carrier frequency) the receiver’s BFO frequency gets set to an audio amplifier and then to a speaker or headphones so we can hear it.  As the receiver tunes away from the transmitters carrier frequency the BFO frequency changes in pitch either up or down depending on which way the receiver is tuned, up or down from the transmitters frequency.  Using a wide CW filter one can hear many different signals at different BFO frequencies at once.  With a little practice and “brain tuning” (using your brain to concentrate on the one pitch and “tune” out the others this method works well.  By using a narrower filter one can physically tune out more of the unwanted signals making copying easier.  Because the transmitter “carrier” doesn’t actually carry any information that could get scrambled or not received in the noise and the data is based on only receiving or not receiving the carrier (the receiver actually makes the data intelligible) it is a very good method for low power or poor propagation conditions.  If the receiver can hear the transmitter carrier going on and off then it makes a sound to match in the speakers.
• Soon after CW was invented people wanted to try to actually modulate the carrier to see if it could carry audio created by a person’s voice. In the early days the easiest way to modulate the carrier was to change the amplitude or power of the carrier. This modulation technique is called Amplitude Modulation or AM.  When modulated with sounds of a human voice the power of the transmitter was changed which superimposed the sound onto the carrier.  Now we have a true modulation in the sense that there is actual data on the carrier instead of the carrier just being turned on and off to a predetermined code.  So now the receiver listens to the AM carrier and demodulates (the opposite of modulation) the audio from the carrier and sends the sound to the audio amplifier and speakers.  When transmitting using AM the carrier is on all the time or 100% duty cycle even if you are not talking.  Because of this the transmitter pulling its fully rated power from the batteries every time the transmit key is pressed. Also because it’s a 100% duty cycle mode the amplifier’s power has to be reduced (or built about 4 times more powerful to handle the load) so as not to overheat and burn up the amplifiers transistors.  What this means is modern transmitters rated at 100 watts are rated this high for a lower duty cycle mode like SSB and the power for AM has to be reduced to 25% or 25 watts.  So to transmit AM at 100 watts we’d need a 400 watt amplifier.
• After AM was invented, people look at ways to improve on AM so it wouldn’t waste as much energy when transmitting.  One way to improve AM was to not send the carrier signal unless sound was actually being modulated, or an even better way was to not send the carrier at all.  So if we don’t send the carrier at all how is there a signal to demodulate at the receiver?  When voice modulates the amplitude of the carrier it creates what are called sidebands where the actual data is stored.  Data is stored on a small section about 3.5kHz above and below the carrier frequency.  If we could suppress the carrier frequency and only send out the data in the side bands then we could double our power output.  This mode is called carrier suppressed double side band or DSB for short.  It maintains the full bodied audio of the AM signals but the carrier is suppressed at transmitted and only the frequencies of the side bands are transmitted.
• Now the magic of sideband is for our brains to understand the data we really only need to hear half of the DSB signal. So a way was invented to suppress one of the sidebands and only transmit the other. Now were up to a whopping four times power output over AM. This method of modulation is called carrier suppressed single sideband or SSB.  Since we can choose which side band to transmit when we transmit the sideband located above the carrier frequency it is called upper side band or USB and when we transmit the sideband located below the carrier frequency it is called lower side band or LSB.  Because we hear only ½ the original voice the quality is not as good as AM or DSB but it is still intelligible and can actually get through the noise about 50% better than AM because of the power increase.
• So now that the methods of improving modulating the amplitude of a carrier had been fine-tuned people began to look at other methods of modulation.  Another parameter of a carrier wave is its frequency.  If we change the frequency of the carrier with our data then we are modulating the frequency. This method is called frequency modulation or FM.  FM is a 100% duty cycle mode as we're not changing the output power of the transmitter as we speak it is constant. The amount the carrier changes frequency is called deviation as the frequency deviates from the carrier frequency as it is being modulated with the data.  Too much data or a loud voice will cause the frequency to over deviate and the receiver which has been designed to only look for a certain frequency fluctuation will not hear all the data the sound will be distorted.  This also causes interference to the adjacent frequencies since the over deviated signal will encroach into their space.  Because FM changes the carrier’s frequency the width of the signal is much wider than SSB. 15 kHz for most amateur radio applications and 75 kHz for professional FM radio stations.  Since the bandwidth requirements are more, FM is not allowed on any bands below 10 meters.  One nice feature with FM is called the capture affect. This allows the receiver to only tune in the stronger of the received signals at the same frequency and helps eliminate background noise. This causes a condition called full quieting since the capture affect locks on to the transmitter’s frequency and all the weaker signals (noise) are filtered out.  This is why this mode was used for FM broadcast stations.
• One other parameter of a carrier signal that can be changed is its phase.  This is mode works very similar to FM but instead of changing the frequency the phase of the carrier is shifted forward or backwards with the data.  This method is used mostly with UHF and above commercial applications like wireless WiFi routers and cell phones.

What are the best modes (voice, digital, Morse code) of communication?

• Like frequencies, different modes work better than others at different times.  Though when propagation is at its best all modes work equally, so I guess it should be said during times of poor propagation some modes work better than others.
• The difference between modulation and modes:
• Modes of communications are not the same as modulation and can be confusing to the novice.  Modulation is how the carrier frequency is changed to carry the data being sent, whether it’s voice, Morse code, or a digital signal.  The communications mode is how the data is formed before it is modulated onto the carrier frequency.  The mode that is often confused with modulation is Morse code.  Morse code is just that, a predetermined code that humans devised to present the letters of the alphabet along with punctuation and other radio specific terms so that is can be sent via the carrier. CW is the modulation method.  The carrier in CW is modulated by turning the carrier on and then off in short and shorter increments.  The receiver then emits an audible tone every time it receives the carrier frequency being turned on and off at the transmitter.  Since Morse code and CW are used extensively together the terms CW and Morse code are used interactively. 
• CW and Morse code:
• With CW no data is modulated on the carrier other than the sequence that the carrier is turned on and off therefore the data cannot be interfered with during propagation. The receiver generates the audible signal when it hears the carrier so propagation just has to be good enough for the carrier to be heard in the noise.  A good ear can easily pick out the tones and with the advent of digital signal processing a digital filter can be implemented to amplify the tone while reducing the noise.  Because of how CW works it makes for a fine “mode” of communications. :)
• Single Sideband (SSB):
• Single sideband or SSB is a modulation method that was first used to modulate a person’s voice onto the carrier.  So SSB is the modulation and voice is the mode.  SSB voice needs better propagation than CW to be “copied” as the voice (data) can be interfered with during propagation and the ear can’t interpret the sounds as easily in the noise.  However during times of good propagation voice is easily recognized so untrained operators can easily copy it. Also no accessory equipment is needed to copy it.
• Computers and digital modes: Text messaging of the radio world.
• Ever since the invention of the Personal Computer, Amateur Radio enthusiasts have invented ways the two can be used together.  Everything from software to log radio contacts, designing radios and antennas to controlling and modulating the transmitter have been devised.  Earlier modes such as RTTY used a Terminal Node Controller or TNC which is a modem very similar to a telephone modem used for dialup internet access. The TNC sat between the computer and the radio and converted signals (text typed on the keyboard) from the computer to sounds that modulated the carrier. The TNC at the receiver converted those sounds back into signals the computer would recognize and display as text on the screen.  Very early RTTY stations used a teletype which was a keyboard and a print mechanism that printed the received code onto a strip of paper.
• When soundcards came onto the scene methods were devised where the computer encoded the data as sound and then that sound was hardwired to the transmitter microphone input and modulated on the carrier via SSB.  Again the opposite occurred at the receiver and text was displayed on the screen.
• The popular soundcard modes include RTTY, BPSK31, BPSK63, BPSK125 and BPSK250.  QPSK modes are also available.  Some PSK modes include error detection/correction methods.
• The PSK modes increase in bandwidth as the number goes up thus decreasing transmission time (more characters per second can be sent) but are affected more by poor propagation.  BPSK31 with a bandwidth of 62.5 hertz is a (very) narrow bandwidth mode and can transmit at about 50 words per minute, as fast as a human can type. In a well-disciplined environment 32 BPSK31 conversations could happen in the same space required for one 2.4kHz SSB voice conversation. Realistically about 20 BPSK signals are possible due to the need for space between the signals because of over modulation and frequency drift/creep.
• BPSK31 is a very viable low bandwidth mode and I have had many contacts on 20 meters to USA and Europe during solar minimum on less than 50 watts and at times 25 watts with “a not so optimal” antenna setup.  With a decent antenna setup and good propagation 5 watts can work the world.  With a 100aH battery you could transmit all day at 5 watts.
• Other soundcard modes:
• Other soundcard modes that work well during periods of poor propagation are Olivia, MT63, MFSK8 and MFSK16.
• HF/VHF packet is used to send messages by a store and forward method.  Automatic Position Reporting System (APRS) uses packet to report the GPS position of the transmitter and to send any messages the operator types in.  Any signals received by an APRS repeater are sent back out.  If these signals are again received by another repeater then they are sent out also.  This receive/re-transmit scheme sends the packets of data over a wide area. Internet gateways are also incorporated so data can be received worldwide.  APRS can be used without GPS for sending messages and of course one can set up a receive only station also.