Chapter 1: Decibels, Can You Hear Me Now?
You have probably heard of decibels, in the context of loud noises, rock concerts and similar. Decibels are also used in a radio context, and have a very similar meaning. In this case, the frequencies used are far beyond what humans can perceive – but the concept is the same, it represents “how loud is the signal from the other end?” Much of the content later-on requires that you understand decibels, so this section explains them to you.
You have probably heard of decibels, in the context of loud noises, rock concerts and similar. It is a logarithmic scale, doubling every 3 decibels (abbreviated to “dB”) in audible volume. So a whisper at 1 dB is half the volume of a whisper at 4 dB.
Decibels are also used in a radio context, and have a very similar meaning. In this case, the frequencies used are far beyond what humans can perceive – but the concept is the same, it represents “how loud is the signal from the other end?”
In the case of radio, you are often dealing with negative numbers. For example, your receiving radio may have incoming signal strength of -55 dB. The sensitivity threshold at which your radio can hear is also negative; for the sake of example, -75 dB. So long as the signal is more (less negative) than the receiving signal, all is well. So, -55 dB is audible; -80 dB would not be.
Despite the numbers being negative, the same basic principle applies to doubling in power every 3 dB. -55 dB is twice as strong as -59 dB.
You will often encounter several terms that use decibels (dB) as a measurement. These include:
« Chapter 1, How Does Radio Work? Up To Contents Chapter 1: The Shanon-Hartley Theorem »
Received Signal Strength Indicator (RSSI): A measure of low loud the received signal is. For example, most Ubiquiti equipment functions best with an RSSI in the -50 to -59 dB range. RSSI is a little counter-intuitive in that it is negative – with numbers closer to zero being strongest. This is because RSSI is a measure of how much signal remains after all of the signal degrading factors (distance, atmosphere, noise) are factored in.
Noise Floor: A measure of how loud other signals than the one for which you are listening are. Even in a remote field with no other antennas within a hundred miles, you will always have some degree of noise – from the sun, natural background radiation, etc. Typically, you want your RSSI to be at least 20 dB better than your noise floor. For example, in a quiet area with a noise floor of -100 dB, a link with an RSSI of -80 dB may work. In a terribly noisy environment with a noise floor of -75 dB, even a -60 dB link is only 15 dB louder than the surrounding noise – and is unlikely to perform well.
Signal-to-noise ratio/Carrier Information Noise Ratio (SNR/CINR): This is the ratio of the two numbers defined above. A strong ratio is necessary for clear transmission.
If you like this book, please consider making a donation to the author, Herbert Wolverson. If there is sufficient interest
in this e-book, we'll post more chapters.