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DA25 • 6
What Are We Driving At?
Time to pull some of that theory together and get some answers:
Since: λ = v x T
And T = 1 / f
We can substitute and get:
λ = v / f
Since the velocity equals “c” we wind up with:
λ = c / f
The wavelength of the radio wave equals the speed of light divided by the
frequency.
Lets plug some numbers into our equation and work out a few wavelengths. We
should notice some other properties of electromagnetic waves.
If f = 450 MHz (the wave cycles 450 million times in a second) then λ = 3x 10
8
/
450 x 10
6
or .666 meters for a full wavelength.
If f = 2500 MHz (the wave cycles 2500 million times in a second) then λ = 3x
10
8
/ 2500 x 10
6
or .120 meters for a full wavelength.
It’s important to note that as the frequency of a wave increases, its wavelength
decreases. Keeping in mind the introduction section where we talked about
antenna size, lets consider the “old” days of radio. The common use of low
frequencies meant much longer wavelengths and significantly larger antennas
for reception. Today's modern electronic devices tend to operate at much
higher frequencies and thereby require smaller antennas to operate properly.
Determining the Resonant Frequency of the Antenna
Let’s explore another factor in antenna as well as radio design, the resonant
frequency of the circuit. Recalling that we would like our discone antenna to
work over a large range of frequencies, we need the antenna system to be
optimized for the full desired range. Resonance in an antenna circuit occurs
when the antenna length exactly matches the wavelength of the desired
frequency. To make an antenna resonant over a range of frequencies, it needs
to look like a multitude of lengths.
Looking at the desired waveform, the shortest length of wire that will resonate
at a given frequency is one which is just long enough to permit an electric
charge to travel from one end to the other and then back again in the time of