Antennas are much more than simple devices connected to every radio. They’re the transducers that convert the voltage from a transmitter into a radio signal. And they pick radio signals out of the air and convert them into a voltage for recovery in a receiver.
Typically taken for granted and left for the last minute in a design, antennas are nonetheless critical for establishing and maintaining a reliable radio connection. They may look complex and enigmatic to most engineers, especially EEs working with wireless applications for the first time—not to mention that they come in a seemingly infinite variety of sizes and shapes. However, a brief review of the essentials can help allay any design worries.
Just what is a radio wave anyway?
A radio wave is a combination of a magnetic field at a right angle to an electric field. Both oscillate at a specific frequency, and they travel together in a direction perpendicular to both fields (Fig. 1). These electromagnetic fields move at the speed of light (about 300 million meters per second or about 186,400 miles per second) through free space. According to Maxwell’s well-known equations, they support and regenerate one another along the way, but weaken over distance.
What are some of the characteristics of a radio wave?
One of the key features is the orientation of the fields with the earth. This is called polarization. An antenna is vertically polarized if the electric field is vertical to the earth’s surface. The antenna is horizontally polarized if it’s horizontal to the earth’s surface.
Are there any other critical features of a radio wave?
Generally, radio waves have a near field and a far field. The near field is close to the antenna, usually within several wavelengths (?). The far field is about 10 wavelengths or more from the antenna. The far field breaks away from the antenna and becomes the radio signal.
Applications like radio-frequency communication (RFID) and near-field communications (NFC) use the near field, which is more akin to the magnetic field around a transformer primary winding. But overall, the far field is the most useful radio wave.
How does an antenna work?
The antenna at the transmitter generates the radio wave. A voltage at the desired frequency is applied to the antenna. The voltage across the antenna elements and the current through them create the electric and magnetic waves, respectively. At the receiver, the electromagnetic wave passing over the antenna induces a small voltage. Thus, the antenna becomes the signal source for the receiver input.
Will the same antenna work for both transmit and receive?
Yes. We call that antenna reciprocity. Any antenna will work for either transmit or receive. In many wireless applications, the antenna is switched between the transmitter and receiver.
Will a vertical antenna receive a horizontally polarized signal or vice versa?
In most cases, yes. Real-world antennas are rarely perfectly horizontal or vertical, so some signal is received. Furthermore, most signals undergo shifts in polarization over the transmission path due to reflections and other multipath conditions. Yet this antenna orientation mismatch does introduce some attenuation.
If more precisely controlled, the polarization can be used to multiplex two signals on the same frequency. In some satellites, a vertically polarized antenna can transmit one signal while simultaneously transmitting or receiving on a separate horizontally polarized antenna on the same frequency. If polarization is a problem in an application, circular polarization may offer a solution.
What is circular polarization?
Just as the name implies, the polarization rotates continuously during transmission, making it possible to use either horizontal or vertical antennas for receiving. For maximum reception, a circularly polarized receive antenna is needed.
You also can have an antenna that produces right-hand or left-hand circular polarization (RHCP or LHCP). This again allows frequency reuse by using different polarizations on two different signals. Often, a helical antenna made of a spiral conductor and a reflector is used. Circular polarization is most commonly found in satellites.
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