Another common half-wavelength antenna is the dipole (i.e. two poles) antenna. Here are two examples for 144MHZ and 10MHz respectively:
|vertical dipole||horizontal dipole|
The dipole antenna is fed in the middle of the two radiating elements. In the middle, where the elements meet, there is a small gap so that the elements don’t touch. For completeness we sometimes refer to these antennas as Center-Fed Dipole.
A simpler, schematic version of the dipole antenna could be drawn as follows:
Where the lower-case letter L is the length representing a half-wavelength of the resonant frequency, and each leg being one-quarter of the wavelength. The two vertical lines are not to scale and indicate the feed line (aka transmission line).
Often, the greek letter λ (pronounced as ‘Lambda’) is used for wavelength. Therefore, when we speak of a half-wavelength dipole antenna we commonly write this as a ½ λ dipole antenna.
Earlier we showed how to calculate the wavelength (in free space; more on that later) of a frequency. Let’s use this to calculate the radiating element lengths of a ½ λ dipole antenna.
Below are the full and half-wavelengths for the common ISM bands:
|Frequency||Full Wavelength||½ Wavelength|
So is this all the info we need to make a dipole antenna? Almost. The next section will take velocity factor into consideration.
There is now a wideband PCB Dipole available for 868-915mhz: