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. Hold the file near an AM radio and you will hear a lot of static. In the early days of radio, the transmitters were called spark coils, and they created a continuous stream of sparks at much higher voltages e.
The high voltage created big fat sparks like you see in a spark plug , and they could transmit farther. Today, a transmitter like that is illegal because it spams the entire radio spectrum , but in the early days it worked fine and was very common because there were not many people using radio waves.
As seen in the previous section, it is incredibly easy to transmit with static. All radios today, however, use continuous sine waves to transmit information audio, video, data. The reason that we use continuous sine waves today is because there are so many different people and devices that want to use radio waves at the same time. If you had some way to see them, you would find that there are literally thousands of different radio waves in the form of sine waves around you right now -- TV broadcasts, AM and FM radio broadcasts, police and fire radios, satellite TV transmissions, cell phone conversations, GPS signals, and so on.
It is amazing how many uses there are for radio waves today see How the Radio Spectrum Works to get an idea. Each different radio signal uses a different sine wave frequency , and that is how they are all separated.
The transmitter takes some sort of message it could be the sound of someone's voice, pictures for a TV set , data for a radio modem or whatever , encodes it onto a sine wave and transmits it with radio waves.
The receiver receives the radio waves and decodes the message from the sine wave it receives. Both the transmitter and receiver use antennas to radiate and capture the radio signal. A baby monitor is about as simple as radio technology gets. There is a transmitter that sits in the baby's room and a receiver that the parents use to listen to the baby. Here are some of the important characteristics of a typical baby monitor:.
Don't worry if terms like "modulation" and "frequency" don't make sense right now -- we will get to them in a moment. A cell phone is also a radio and is a much more sophisticated device see How Cell Phones Work for details. A cell phone contains both a transmitter and a receiver, can use both of them simultaneously, can understand hundreds of different frequencies, and can automatically switch between frequencies. Here are some of the important characteristics of a typical analog cell phone:.
You can get an idea for how a radio transmitter works by starting with a battery and a piece of wire. In How Electromagnets Work , you can see that a battery sends electricity a stream of electrons through a wire if you connect the wire between the two terminals of the battery.
The moving electrons create a magnetic field surrounding the wire, and that field is strong enough to affect a compass. Let's say that you take another wire and place it parallel to the battery's wire but several inches 5 cm away from it. If you connect a very sensitive voltmeter to the wire, then the following will happen: Every time you connect or disconnect the first wire from the battery, you will sense a very small voltage and current in the second wire; any changing magnetic field can induce an electric field in a conductor -- this is the basic principle behind any electrical generator.
One important thing to notice is that electrons flow in the second wire only when you connect or disconnect the battery. A magnetic field does not cause electrons to flow in a wire unless the magnetic field is changing.
Connecting and disconnecting the battery changes the magnetic field connecting the battery to the wire creates the magnetic field, while disconnecting collapses the field , so electrons flow in the second wire at those two moments.
To create a simple radio transmitter, what you want to do is create a rapidly changing electric current in a wire. You can do that by rapidly connecting and disconnecting a battery, like this:. A better way is to create a continuously varying electric current in a wire. The simplest and smoothest form of a continuously varying wave is a sine wave like the one shown below:. By creating a sine wave and running it through a wire, you create a simple radio transmitter. It is extremely easy to create a sine wave with just a few electronic components -- a capacitor and an inductor can create the sine wave, and a couple of transistors can amplify the wave into a powerful signal see How Oscillators Work for details, and here is a simple transmitter schematic.
By sending that signal to an antenna, you can transmit the sine wave into space. If you have a sine wave and a transmitter that is transmitting the sine wave into space with an antenna, you have a radio station.
The only problem is that the sine wave doesn't contain any information. You need to modulate the wave in some way to encode information on it. There are three common ways to modulate a sine wave:. Pulse Modulation - In PM, you simply turn the sine wave on and off.
This is an easy way to send Morse code. PM is not that common, but one good example of it is the radio system that sends signals to radio-controlled clocks in the United States. One PM transmitter is able to cover the entire United States! Amplitude Modulation - Both AM radio stations and the picture part of a TV signal use amplitude modulation to encode information.
In amplitude modulation, the amplitude of the sine wave its peak-to-peak voltage changes. So, for example, the sine wave produced by a person's voice is overlaid onto the transmitter's sine wave to vary its amplitude.
Frequency Modulation - FM radio stations and hundreds of other wireless technologies including the sound portion of a TV signal , cordless phones, cell phones, etc. The advantage to FM is that it is largely immune to static. In FM, the transmitter's sine wave frequency changes very slightly based on the information signal.
One characteristic of a sine wave is its frequency. The frequency of a sine wave is the number of times it oscillates up and down per second.
When you listen to an AM radio broadcast, your radio is tuning in to a sine wave with a frequency of around 1,, cycles per second cycles per second is also known as hertz. The center pin is the data pin to transmite the signal. The Vcc pin should be powered with a regulated 5V supply.
The operating current of this module is less than 5. The pins Dout and Linear out is shorted together to receive the Mhz signal from air.
This signal is then demodulated to get the data and sent out through the data pin. HT12E is an encoder IC that converts the 4-bit parallel data from the 4 data pins into serial data in order to transmit over RF link using transmitter. HT12D is a decoder IC that converts the serial data received by the RF Receiver into 4-bit parallel data and drives the output accordingly. The RF modules can also function without the need of Encoder and Decoder modules.
Simply power on both the modules with the corresponding voltage mentioned above. But, there is a big drawback in this method. You can have only one button on the sender side and one output on the receiver side. So to have more inputs and outputs, the encoder and decoder modules are required. As said they have 4-data bit and 8-addresss bit, these 8 address bits has to be set same on both the encoder and decoder to make them work as a pair.
Today, many avionics devices are small enough to be mounted in the instrument panel, which is customary on most light aircraft.
Because of the number of communication and navigation aids, as well as the need to present an uncluttered interface to the pilot, most complicated aircraft retain an area away from the flight deck for the mounting of avionics. The control heads of these units remain on the flight deck. A transceiver is a communication radio that transmits and receives.
The same frequency is used for both. When transmitting, the receiver does not function. The push to talk PTT switch blocks the receiving circuitry and allows the transmitter circuitry to be active. In a transceiver, some of the circuitry is shared by the transmitting and receiving functions of the device. So is the antenna. This saves space and the number of components used. Transceivers are half duplex systems where communication can occur in both directions but only one party can speak while the other must listen.
VHF aircraft communication radios are usually transceivers. VHF aircraft communication transceivers.
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