Frequency Bands

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How can your cell phone and your television work at the same time? Both use antennas to receive information from electromagnetic waves, so why isn't there a problem?

The answer goes back to the fundamental secret of the universe. No matter what information you want to send, that waveform can be represented as the sum of a range of frequencies. By the use of modulation (which in a nutshell shifts the frequency range of the waveform to be sent to a higher frequency band), the waveforms can be relocated to separate frequency bands.

As an example, cell phones that use the PCS (Personal Communications Service) band have their signals shifted to 1850-1900 MHz. Television is broadcast primarily at 54-216 MHz. FM radio operates between 87.5-108 MHz.

The set of all frequencies is referred to as "the spectrum". Cell phone companies have to pay big money to get access to part of the spectrum. For instance, AT&T has to bid on a slice of the spectrum with the FCC, for the "right" to transmit information within that band. The transmission of EM energy is greatly regulated. When AT&T is sold a slice of the spectrum, they can not transmit energy at any other band (technically, the amount transmitted must be below some threshold in adjacent bands).

The Bandwidth of a signal is the difference between the signals high and low frequencies. For instance, a signal transmitting between 40 and 50 MHz has a bandwidth of 10 MHz. This means that the energy of the signal is contained between 40 and 50 MHz (and the energy in any other frequency range is negligible).

We'll wrap up with a table of frequency bands along with the corresponding wavelengths. From the table, we see that VHF is in the range 30-300 MHz (30 Million-300 Million cycles per second). At the very least then, if someone says they need a "VHF antenna", you should now understand that the antenna should transmit or receive electromagnetic waves that have a frequency of 30-300 MHz.

Frequency Band Name Frequency Range Wavelength (Meters) Application
Extremely Low Frequency (ELF)
3-30 Hz
10,000-100,000 km
Underwater Communication
Super Low Frequency (SLF)
30-300 Hz
1,000-10,000 km
AC Power (though not a transmitted wave)
Ultra Low Frequency (ULF)
300-3000 Hz
100-1,000 km
Very Low Frequency (VLF)
3-30 kHz
10-100 km
Navigational Beacons
Low Frequency (LF)
30-300 kHz
1-10 km
AM Radio
Medium Frequency (MF)
300-3000 kHz
100-1,000 m
Aviation and AM Radio
High Frequency (HF)
3-30 MHz
10-100 m
Shortwave Radio
Very High Frequency (VHF)
30-300 MHz
1-10 m
FM Radio
Ultra High Frequency (UHF)
300-3000 MHz
10-100 cm
Television, Mobile Phones, GPS
Super High Frequency (SHF)
3-30 GHz
1-10 cm
Satellite Links, Wireless Communication
Extremely High Frequency (EHF)
30-300 GHz
1-10 mm
Astronomy, Remote Sensing
Visible Spectrum
400-790 THz (4*10^14-7.9*10^14)
380-750 nm (nanometers)
Human Eye

Table 1. Chart of Common Frequency Bands

Basically the frequency bands each range over from the lowest frequency to 10 times the lowest frequency. Antenna engineers further divide the bands into things like "X-band" and "Ku-band". That is the basics of frequency. To understand at a more advanced level move on to the next topic.

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