More Wireless Basics
This article explains the types of wireless signal. What makes these signals different? What is the difference between a transmitter and a receiver? What do you call a device that both transmits and receives? Make sure you can name two types of antennas and the reason for choosing each type.
Types of Wireless Signals
There are many, many types of wireless technologies. You may be familiar with AM and FM radio, Television, Cellular phones, Wi-Fi, Satellite signals such as GPS and television, two-way radio, and Bluetooth. These are some of the most common signals, but what makes them different?
Frequency
First of all, wireless signals occupy a spectrum, or wide range, of frequencies: the rate at which a signal vibrates. If the signal vibrates very slowly, it has a low frequency. If the signal vibrates very quickly, it has a high frequency. Frequency is measured in Hertz, which is the count of how quickly a signal changes every second. As an example, FM radio signals vibrate around 100 million times every second! Since communications signals are often very high in frequency, we abbreviate the measurements for the frequencies - millions of vibrations a second is Megahertz (MHz), and billions of vibrations a second is Gigahertz (GHz). One thousand Megahertz is one Gigahertz.
| Example Frequency Ranges
Below we can see the span of frequencies that are commonly used in communications. Broadcast transmitters for AM, FM and Television use frequencies below 1000 MHz, Wi-Fi uses two bands at higher frequencies - 2.4 and 5GHz. Cellular phones use many different frequencies.
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Modulation
In addition to having different frequencies, wireless signals can be different in the way they convey information. A wireless signal needs to be modulated--or changed--to send information. There are many types of modulation, and different technologies can use one or more types to send and receive information. In the two examples below - AM and FM radio - the M stands for modulation. The type of modulation is what makes them different.
Example one: AM radio. The A in AM comes from Amplitude - the energy or strength of the signal, operating at a single frequency. An un-modulated AM wave might look like:
And a modulated AM radio wave has higher and lower energy (amplitude) waves indicating higher and lower audio frequencies in the signal:
From left to right, we have the normal, un-modulated wave, then the lower amplitude wave (representing low points in audio waves), then the higher amplitude wave (representing crests or high points in audio waves).
A more detailed version of an AM signal is below:
The audio signal is the wave on the top, with the corresponding Amplitude Modulated wave below it.
Example two: FM radio. The F in FM comes from Frequency - defined by how quickly the wave vibrates every second. An un-modulated FM wave might look like:
And a modulated FM radio wave has higher and lower frequencies indicating higher and lower audio frequencies in the signal:
From left to right, we have the normal, un-modulated wave, then the lower frequency wave (representing lower audio amplitudes), then the higher frequency wave (representing higher audio amplitudes).
The type of modulation various technologies use to communicate can be very different, and are often not compatible. Satellite equipment cannot speak directly to your laptop or smartphone, which uses Wi-Fi to send and receive information. This is because the radios in different devices can listen only to certain types of modulations and frequencies.
As an example, some broadcast radio receivers have a switch to select between AM and FM signals, for two reasons: they use different frequencies to transmit, and they use different modulation types. If you try and listen to an AM signal with a radio in FM mode, it won't work. The opposite is also true - in AM mode, an FM signal doesn't make sense to the receiver. It is important that transmitters and receivers use the same frequencies and modulation types to communicate.
Devices in your daily life use many types of wireless signals. Look at the table below to see the various frequencies and types of modulation each uses:
| Technology or device | Type of wireless signal |
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Nearly every device or technology uses a different wireless frequency and modulation. This means most devices can only understand a very specific kind of wireless signal.