At low frequencies (less than 3 MHz), propagation is by ground wave, where the signal hugs the earth’s surface. Signals move from one antenna to another in several ways, based on the radio wave’s frequency. How does the radio signal propagate from transmitter to receiver? Circular polarization is most commonly found in satellites. Often, a helical antenna made of a spiral conductor and a reflector is used.
This again allows frequency reuse by using different polarizations on two different signals. You also can have an antenna that produces right-hand or left-hand circular polarization (RHCP or LHCP). For maximum reception, a circularly polarized receive antenna is needed. Just as the name implies, the polarization rotates continuously during transmission, making it possible to use either horizontal or vertical antennas for receiving. If polarization is a problem in an application, circular polarization may offer a solution. 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 more precisely controlled, the polarization can be used to multiplex two signals on the same frequency. Yet this antenna orientation mismatch does introduce some attenuation.
Furthermore, most signals undergo shifts in polarization over the transmission path due to reflections and other multipath conditions. Real-world antennas are rarely perfectly horizontal or vertical, so some signal is received. Will a vertical antenna receive a horizontally polarized signal or vice versa? In many wireless applications, the antenna is switched between the transmitter and receiver. Any antenna will work for either transmit or receive. Will the same antenna work for both transmit and receive? Thus, the antenna becomes the signal source for the receiver input. At the receiver, the electromagnetic wave passing over the antenna induces a small voltage. The voltage across the antenna elements and the current through them create the electric and magnetic waves, respectively. A voltage at the desired frequency is applied to the antenna. The antenna at the transmitter generates the radio wave. But overall, the far field is the most useful radio wave. The far field breaks away from the antenna and becomes the radio signal.Īpplications 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. The far field is about 10 wavelengths or more from the antenna. The near field is close to the antenna, usually within several wavelengths (λ). Generally, radio waves have a near field and a far field. The antenna is horizontally polarized if it’s horizontal to the earth’s surface.Īre there any other critical features of a radio wave? An antenna is vertically polarized if the electric field is vertical to the earth’s surface. One of the key features is the orientation of the fields with the earth. What are some of the characteristics of a radio wave? According to Maxwell’s well-known equations, they support and regenerate one another along the way, but weaken over distance.
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. Both oscillate at a specific frequency, and they travel together in a direction perpendicular to both fields (Fig. A radio wave is a combination of a magnetic field at a right angle to an electric field.