The Pros and Cons of Monopole Antennas
What Is a Monopole Antenna?
A monopole antenna consists of a straight rod or rod-shaped conductor, and it may or may not be mounted perpendicularly to a conductive surface (ground plane). Inverted-F antennas, for example, consist of a monopole antenna parallel to the ground plane and grounded at the mounted end.
A monopole antenna is considered half of a dipole antenna. Quarter wavelength monopole antennas must have a ground system. The classic “whip” antenna is a type of monopole antenna.
The dipole antenna has two conductive elements that are typically bilaterally symmetrical, while the monopole has just one element. The metallic ground plane reflects signals up, causing the monopole to act like a dipole but often with a much shorter antenna because of the ground plane.
A quarter wave whip antenna will have on average 3 dB gain greater than a half dipole if mounted to a ground plane. Putting an inductor near the base of the monopole antenna helps compensate for the capacitive reactance, giving the quarter wave monopole performance almost as good as a half wave monopole.
The Advantages of Monopole Antennas
As the monopole antenna gets longer and the ground losses are reduced, the efficiency of the antenna gets better. Vertical monopole antennas can achieve efficiencies of up to 80%.
A vertical monopole antenna can be used for any frequency shorter than two thirds of the wavelength.
Monopole antennas are a simple omnidirectional antenna that takes up far less space than an array of wheel antennas stacked on top of each other. The monopole antenna can handle communications in any direction except straight up above the antenna.
Monopole antennas are easy to build and install. Passive monopoles are cheap to make and rugged.
If working with frequencies over 800 MHz, the monopole antenna can be made out of the trace on a printed circuit board; this is standard for cell phones.
A monopole antenna has relatively high reactive impedance over most of its frequency range. Put an active amplifier with ah high input impedance, and you can transform the impedance without losing any sensitivity.
The Disadvantages of Monopole Antennas
Because you are radiating equally in all directions, you have equally poor radiation in all directions.
The “torus” shape doesn’t extend to the top of the antenna because the voltage increases as it travels up the antenna. In reality, the signal is sent from the bottom two thirds of the antenna.
Metal objects and the ground itself can cause signal reflections, so you may get a signal that is both horizontally and vertically polarized.
Inside a shielded chamber, monopole antennas can have impedances vary by orders of magnitude, making it difficult to calibrate. Take measurements too close to the tip of the antenna or ground plane, and they will likely be wrong.
A Note on Inverted-F Antennas
Inverted F-antennas’ impedance matching can be done without extraneous parts. They also tend to be shorter and more compact than a simple monopole antenna. Inverted-F antennas can be built into printed circuit board antennas just as monopole antennas can be. The inverted-F antenna is often used for wireless antennas in mobile devices.