# Frequency, Channels and Bands

So what is the difference between a Frequency, a Channel and a Band?

## Frequency

To quote a formal definition:

“frequency is the number of occurrences of a repeating event per unit time”.

In the realm of electronics we often refer to these ‘repeating events’ as oscillations. The unit of oscillations per second is called the ‘Hertz’. Kilo-Hertz (or kHz) means a thousand oscillations per second. Mega-Hertz (or MHz) means a million oscillations per second.

Frequencies can be any positive (even fractional) number.

• The highest frequency humans can hear is around 20,000 Hz (20 kHz)
• Commercial AM broadcast stations have frequencies like 972, 1233, 1512 kHz
• Commercial FM broadcast stations have frequencies like 88.1, 96.8, 102.1 MHz
• Moteinos can transmit on 433, 868 and 915 MHz
• Bluetooth and Wi-Fi have frequencies of 2.412, 2.442, 2.472 (all in GHz)
• Visible, red light has a frequency of around 450 THz (that’s TeraHertz)
Radio, like sound or visible light or x-rays, is part of the electro-magnetic spectrum, read more about it here

In reality it is very difficult to transmit on exactly one frequency. There are several reasons for this. Firstly, the hardware itself will be subject to small variations due to the manufacturing process. Secondly, external factors can be a big influence, especially the environmental temperature of the components involved in the oscillator circuit. Lastly, the modulation method (i.e. how we encode the information in the transmission) changes how much ‘around’ the frequency the signals will appear.

 Spectrum analyzer plot of transmitting on a ‘single’ frequency

## Channels

The concept of channels is made clearer with examples like CB (citizen band) Radios and Wireless Networks. By agreement through international regulatory bodies, fixed frequencies are used to make it as easy as possible for a user of these radios to get access to the various frequencies. In the case of early CB radios these frequencies are achieved by selecting different crystal oscillators with a simple turning knob.

 Spectrum analyzer plot of transmitting on a ‘single’ frequency

For a typical HF CB Radio there are about 40 channels between 27.950 MHz and 27.4050 MHz in approximately 25 kHz steps.

In Wi-Fi (wireless networks) there is also the concept of channels. In most cases however, the Wi-Fi device you use (e.g. mobile phone, home router or wireless access point) will automatically select the best (i.e. quietest) channel to work on when it powers up.

 802.11 x 2.4GHz Wi-Fi channels

The half-circles you see in the above image indicate that there will be some overlap between adjacent channels. Although this is as per the Wi-Fi specification, it may be of interest to move to a channel ‘further away’ from another nearby Wi-Fi network if you experience any interference from it.

## Bands

The concept of Bands is usually understood as a range of frequencies with a certain upper and lower limit. These limits can differ slightly from country to country but some are internationally agreed upon.

#### Sometimes a band means a general frequency range:

 Band upper/lower limit HF 3MHz – 30MHz VHF 30MHz – 300MHz UHF 300MHz – 3GHz SHF 3GHz – 30GHz

#### Sometimes the band designates a certain use:

 Band upper/lower limit Usage AM Broadcast 535 – 1700 kHz Commercial Radio Stations 40 meters around 7MHz Amateur Radio HF CB around 27MHz Citizen Band, walkie talkies 2 meters around 145MHz Amateur Radio 70 cm around 433Mhz Amateur Radio, and ISM UHF CB around 477MHz Citizen Band, walkie talkies

There are actually quite lot of bands that are allocated to all sorts of uses. To give you an idea, here is a map of the Canadian radio spectrum and band allocation form 3 kHz (top left) all the way up to 300 GHz (bottom right).

Every little block is an allocated band with specific rules and regulations to adhere to. As you now may appreciate, it is not all that straightforward to just start transmitting somewhere.