The Basics of Antennas

 

A little understanding of antenna theory can go a long way to boosting the performance and reliability of wireless links. This article explains some of the basics of antenna specifications.

An antenna is essentially a conductor that transmits electromagnetic waves into the air when a voltage is applied. It also receives incident electromagnetic waves, which produce a voltage in the antenna.

Antennas do not radiate equally in all directions; rather, they are directional to a greater or lesser degree. This directionality depends on the design of the antenna, its size, the environment and some other factors. Antenna datasheets usually provide diagrams showing the extent of the directionality using radiation pattern diagrams like the one below, which is for an Antenova chip surface-mount GPS antenna. For most applications, an antenna that radiates as well as possible in all directions is desirable.

Gain is a measure of an antenna’s directionality, that is, it’s a measure of the signal transmitted in a certain direction compared to one of two standard antenna types – either compared to a simple dipole antenna, or to an isotropic antenna (a theoretical construction which radiates equally in all directions). Gain is usually expressed in decibels – either decibels over a dipole (dBd) or decibels over an isotropic antenna (dBi). When comparing antennas, it’s worth noting that gain in dBi is 2.1 dB higher than the dBd measurement, since a dipole’s gain over an isotropic antenna is 2.1 dBi. If an antenna’s gain figures are high, this means it will transmit a larger signal than one with a lower gain for the same power, but only in that direction. That is, high gain antennas are very directional, so do consider the application’s requirements carefully.

Antenna gain in every direction for the Antenova A10137 chip GPS antenna when
mounted as per the company’s reference design.

 

 

Types of Antennas

One of the most basic types of antenna is a monopole, that is, a straight rod-shaped conductor mounted perpendicular to a ground plane (which might be the Earth for a radio transmitter, or the roof of a car for an FM radio antenna, or a PCB ground plane). The feed for the antenna (the connection to the rest of the system) is at the lower end of the antenna, between it and the ground plane, with the other half of the feed connected directly to ground. A monopole transmits equally well in all directions perpendicular to the antenna, but is strongly directional in the Z axis, with zero signal transmitted along the antenna axis. The overall pattern is skewed away from the ground plane.

Dipoles are one of the most commonly used types of RF antenna. This type has two symmetrical terminals (poles), with the feed in the middle between the two – each half of the feed is connected to one of the terminals. A basic dipole has a radiation pattern which looks like a toroid (doughnut) –similar to the monopole pattern, zero signal is transmitted along the antenna axis, but since there isn’t a ground plane, the shape is more symmetrical in the Z direction.

Operating frequency and impedance of monopole and dipole antennas is dependent on the length of the conductors. Quarter wave is the most common size of monopole, that is, with length equal to a quarter of the wavelength of the signal in that conductor, not the wavelength in free space. Dipoles are typically half wave, so each terminal is a quarter wave. This produces resonance at the desired frequency of signal, so that standing waves travel between the two ends of the antenna.

There is a wide variety of variations of the basic monopoles and dipoles available which trade off gain, size and bandwidth (the range of frequencies over which it works), with more complex radiation patterns.

Aside from gain and directionality, other electrical properties of antennas that may be compared include efficiency and voltage standing wave ratio.

Efficiency refers to how well the antenna converts electrical power it receives into radiated power (the efficiency of the Antenova GPS antenna considered above is around 70% for the setup they describe).

The voltage standing wave ratio (VSWR) is a measure of how well the antenna’s impedance is matched to the impedance of what it’s connected to. Most antennas come with standard impedance of 50 Ohms to match the standard transmission line impedance, and some contain impedance matching circuits to achieve this. VSWR of 1.0 is ideal, but the smaller the VSWR, the better the antenna is matched to the transmission line, so there is less reflection of the signal and more power actually reaches the antenna.