Let’s start by defining modulation. Modulation converts data into radio waves by adding information to a carrier signal. A carrier signal is a wave with a steady amplitude of frequency and information is added by changing these parameters.
A modulated signal can be thought of as a bus moving people. The people need to get to a different place and cannot get themselves to their destination without getting a ride from the more capable bus. The bus makes their trip significantly easier.
Once the modulated signal is sent out by an antenna, the receiving antenna picks up the signal and then the signal is demodulated so the listener can receive the information. Transmitters and receivers are responsible for modulating and demodulating signals.
Different Types of Modulation
The two most popular kinds of modulation are FM and AM, but there are several more kinds all serving different purposes. Phase modulation and single side band modulation are two more examples of different types of modulation used by HAMs. This concept might be hard to visualize, so I encourage you to take a look at some pictures while reading about the differences described below.
FM radio uses frequency modulation (FM). In frequency modulation when the information is added to the carrier signal, the frequency of the carrier signal adjusts to match the information being carried. FM radio falls in the VHF/UHF spectrum of radio which doesn’t travel much farther than the horizon and can also be blocked by tall buildings and hills.
AM radio uses amplitude modulation (AM). In amplitude modulation the information added to the carrier signal affects the carrier signal’s amplitude in order to convey the information being sent. A carrier signals height or intensity is affected with amplitude modulation.
AM radio is not generally used for hi fidelity music but more for talk radio because of the way it amplifies noise and electromagnetic interference as much as it amplifies the signal. AM broadcasts are used on several frequency bands.
Single side band (SSB) is used mostly in the HF ranges and was derived from amplitude modulation. SSB is used mostly for voice transmissions and has a lot of advantages over AM and is the most popular way to transmit voice in the HF ranges.
SSB works more efficiently than AM because of its ability to remove elements. It first removes the carrier wave which is reintroduced at the receiver and it also removes one of the sidebands. The sidebands are mirror images of each other and carry the same information.
SSB also has the ability to travel much farther than AM or FM because of its ability to travel up to the F layer of the ionosphere 75 to 250 miles above the Earth and be reflected back down to another location. SSB is by far the most popular way to communicate world wide.
Phase modulation is similar to frequency modulation. The phase of the carrier wave is changed to match the frequency of the information. Wave phase is the offset of a wave from a given point. This type of modulation is more commonly used as a digital form of modulation for PSK31.
Why use modulation?
So why do we even need modulation? Why can’t we just send the information out by itself?
The thing to remember is the longer the wavelength, the taller the antenna you’ll need.
Speech frequencies range from 20 Hz to 20 kHz. If you decide to transmit a radio frequency of 20 kHz, you will need an antenna that is approximately 15,000 meters tall which is nearly impossible.
Say you use a carrier frequency of 1000 Hz to carry your 20 kHz signal to its destination. That changes the necessary height of an antenna to around 300 meters which is a big difference and much more doable!
Using modulation also helps to improve the quality of the signal because when you use a carrier signal, it decreases the likeness of your signal getting mixed up with another signal that is being transmitted from someone else.
Modulation is taking a signal that you want to transmit and amplifying it so that it can travel farther. Modulation helps to reduce the size of the transmitting antenna and the receiving antenna by increasing the size of the wave.
There are many different ways to modulate a signal and all of the ways are used for different things. FM radio is used mostly for high fidelity voice over shorter distances. AM is used for low fidelity voice over longer distances.
SSB is the most common way to transmit voice over very long distances such as around the world. Phase modulation is used mainly for digital signals such as PSK31.
We definitely need modulation to beef up our signals and get them to where they need to go. Without modulation, our signals would be lower quality and barely intelligible. Modulation is the reason we can have reasonably tall antennas in our rig. Modulation is super cool!