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.