The Pros and Cons of Dipole Antennas
What is a Dipole Antenna?
The dipole antenna is the most common type of wire antenna. The most common length of the dipole antenna is half the wavelength of the frequency it is supposed to receive. The dipole antenna is connected to the receiver or transmitter via a coaxial cable. The radio frequency voltage is provided to the center of the antenna, between the two connectors or “poles”.
A dipole antenna is used as a reference antenna for measuring antenna gain. At the resonant frequency or half wavelength of the frequency it receives, the antenna experiences its minimum voltage and maximum current at the center of the antenna.
The Advantages of Dipole Antennas
The biggest advantage of a dipole antenna is its simplicity. Dipole gap sizes have almost no impact on the practical performance of the antenna.
Dipole antennas are very efficient when used at their resonant frequency.
Dipole antennas are omnidirectional when sending and receiving signals. They have a donut shaped radiation pattern, with the antenna sitting in the middle of the hole. You’ll receive almost any signal on that frequency range without worrying about its direction.
Many dipole antennas can be shortened and resonated using a loading coil. You get the best results when a loading coil is located at the center.
Dipole antennas a relatively easy to make and install. You can connect dipole antennas in an inverted V or drooping dipole, sloping dipole and other configurations. The “inverted V” antennas are most stereotypically represented by the “rabbit ears” on top of old TVs to pick up the signal. The inverted V dipole arrangement also allows you to receive balanced signals and sort out conflicting signals without impacting reception quality.
Telescoping dipoles let you receive a much wider frequency range than a telescoping monopole antenna.
The Disadvantages of Dipole Antennas
For very low frequencies, the wavelength is too long to make a dipole antenna practical unless you use a quarter-wave dipole.
For frequencies below 27 MHz, the efficiency of the dipole antenna begins to fall as the antenna length decreases. And it starts to become mismatched from the intended frequency because of capacitive reactance.
Inverted-V antennas have reduced efficiency compared to standard dipole antennas. However, if space is limited or the height of the antenna is limited by regulations, the inverted-V antenna lets you operate on frequencies not possible with a full-sized dipole antenna.
Folded dipoles are slightly more complicated than monopole antennas, though they are not as complicated as yagi or quad patch antennas.
A Note on Folded Dipole Antennas
A folded dipole antenna is made from a basic dipole antenna with an added connector that connects the two dipoles, creating a loop. The folded dipole antenna is half a wavelength wide, with each dipole element being a quarter wavelength long. Folded dipoles have significantly greater feed impedance.
Folded dipole antennas have a flatter frequency response. There is no difference in radiation, since they both act like a standard dipole.
Their rounded edges are less likely to create a corona discharge, a source of interference on antennas.
The folded dipole can be considered the main element of a yagi antenna, but the yagi itself is a directional antenna. Folded dipoles are also the driven elements or feed antennas for log periodic and phased array antennas.