Antennas how do they work

Signals sent on low frequencies, often times AM stations, rely on the use of ground wave propagation to operate. These ground waves can travel a great distance when used in the lower frequencies of the spectrum. The ionosphere is home to free electrons which help to reflect signals and allow messages to be directed across the entire globe.

To help you visualize the importance of the ionosphere, imagine our world before satellites. Satellites float through our skies and receive then transmit signals back, using their scope to send messages across distances that our ancestors could never have imagined. When you imagine the journey of the message, it goes up, probably at an angle, is received, then beamed down to the receiver.

It forms a sort of triangle whose base can span more than half of the globe. The ionosphere serves a similar role. Imagine that you had a message that you could send as far as you wanted, but only in a single direction with no effect from gravity.

Before satellite technology existed, we used this method to establish the first worldwide broadcasts. Understanding how an antenna works is an essential first step toward understanding how our world works. Antennas are used for both sending and receiving radio transmissions.

Technologies such as radio and TV, mobile phones and Wi-Fi, connected cars, global positioning systems GPS , space communications, radars etc. The simplest and most widely used antenna is a dipole antenna. A dipole is simply made of two wires, which are equal in length and connected to a receiver or a transmitter at one end. When an alternating voltage is applied to the antenna, the electric charges oscillate along the wire, shifting between positive and negative over time. The electric charges, and the electrons carrying the charges electric current , generate magnetic and electric waves that carry signals and information through the air at the speed of light.

The number of times per second the charged electrons change polarity along the antenna wire the number of oscillation of charges , defines the frequency of the dipole antenna. Frequency is measured in Hertz. One Hertz Hz represents one oscillation in one second. To give you an idea, for a W i-Fi antenna, the polarity changes 2. The antenna size and length is a function of its operating frequency and wavelength.

The lower the frequency the larger the wavelength and the larger the antenna. The radiation pattern of a dipole antenna. The antenna is oriented vertically along Z-axis. The energy radiated by an antenna is represented by the radiation pattern of the antenna. Radiation patterns are diagrammatical representations of the distribution of radiated energy into space, as a function of direction.

The radiation pattern of the dipole antenna, for example, is called omnidirectional as the antenna radiates equally in all directions around the wire. The directions at which an antenna does not radiate are called nulls. Transmitter and receiver antennas are often very similar in design. For example, if you're using something like a satellite phone that can send and receive a video-telephone call to any other place on Earth using space satellites , the signals you transmit and receive all pass through a single satellite dish—a special kind of antenna shaped like a bowl and technically known as a parabolic reflector , because the dish curves in the shape of a graph called a parabola.

Often, though, transmitters and receivers look very different. But you don't need anything that big on your TV or radio at home: a much smaller antenna will do the job fine. Waves don't always zap through the air from transmitter to receiver. Depending on what kinds frequencies of waves we want to send, how far we want to send them, and when we want to do it, there are actually three different ways in which the waves can travel:.

Artwork: How a wave travels from a transmitter to a receiver: 1 By line of sight; 2 By ground wave; 3 Via the ionosphere. Photo: This telescopic FM radio antenna pulls out to a length of about 1—2m 3—6ft or so , which is roughly half the length of the radio waves it's trying to capture. The simplest antenna is a single piece of metal wire attached to a radio. The first radio I ever built, when I was 11 or 12, was a crystal set with a long loop of copper wire acting as the antenna. I ran the antenna right the way around my bedroom ceiling, so it must have been about 20—30 meters 60— ft long in all!

Photo: Antennas that use line-of-sight communication need to be mounted on high towers, like this. You can see the thin dipoles of the antenna sticking out of the top, but most of what you see here is just the tower that holds the antenna high in the air.

Most modern transistor radios have at least two antennas. One of them is a long, shiny telescopic rod that pulls out from the case and swivels around for picking up FM frequency modulation signals. The other is an antenna inside the case, usually fixed to the main circuit board, and it picks up AM amplitude modulation signals.

If you're not sure about the difference between FM and AM, refer to our radio article. Why do you need two antennas in a radio?