The antenna is the device that transforms the high-frequency signals generated by the transmitter into electromagnetic waves that leave the antenna and travel through the air.
The antenna is also the device that transforms received incoming electromagnetic waves back into Alternating Current electrical energy. For most antennas it holds true that if it can receive well it can also transmit well.
Antennas will pick up incoming radio waves on a broad range of frequencies. However, depending on the specific shape and size of the antenna it can be made very sensitive for a certain frequency. When this happens we say that the antenna is resonant.
The resonant length of an antenna for a given frequency is related to the frequency it needs to be resonant for.
To grasp this concept a little better we take a quick dive into Frequency Wavelengths.
The speed of radio waves traveling through air is about as fast as the speed of light. If an oscillator changes polarity of a signal 1 million times per second, the resulting wave form looks something like this:
|sine wave at 1MHz on an oscilloscope|
The vertical dotted lines are 400 nanoseconds apart, which means that one sine wave (from top to top) takes about 1000ns (or 1 microsecond) to complete.
If we calculate the distance that light travels in 1us we get a length of about 300 meters.
The formula to calculate the wavelength from the frequency is as follows:
|Wavelength (in meters)||= Speed of light (in 1000’s km/sec) / Frequency (in MHz)|
|= 300 / 1|
|= 300 meters|
For an antenna to work well we commonly use an antenna length of a half-wavelength of the frequency we are interested in. This means that in our 1MHz example, the antenna would be 150 meters long!
Now, let’s crank up the frequency a bit and see what happens at 100MHz (this is the frequency where many commercial FM radio stations are broadcasting).
|Wavelength (in meters)||= 300 / Frequency (in MHz)|
|= 300 / 100|
|= 3 meters|
As you can see, when the frequency goes up, the wavelength gets shorter.