We’ve all see those tall antennas on top of cop cars or on older cars. But i never understood how it worked or whats in them. Are they literally just a long flexible wire? Does the signal emit from the top or throughout the length of the wire?
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The antenna is usually just a wire with a powerful amplifier attached at one end (but sometimes it’s a loop with electrical connections at both ends). The amplifier pushes waves of electrical current into the wire which slosh back and forth in the wire.
Moving electrical charges emit electric and magnetic fields. At any instant in time, there’s a pattern to the movement of charges in the antenna. Because of the pattern, when you sum up the fields emitted by all the charges in the whole antenna, the result is a wave that radiates away from the antenna.
There are a lot of interesting details. Like with a whip antenna, the signal is strong in directions perpendicular to the antenna but weak in the direction along the antenna, which is why the antenna is mounted vertically not horizontally. And there are antenna shapes with complicated sets of cross-arms in different lengths. Adding up the fields generated by the charges moving in all of those makes for a field that is very strong in one direction only.
And also it works the same way in reverse –. A radio wave passing an antenna forces electrical charges in the antenna to slosh back and forth. An electrical detector connected to one end will detect a signal that matches the signal that was used to make that radio wave.
So the signal is emitted by the whole antenna. But another detail is that you can design an antenna to take advantage of conductive materials nearby that aren’t part of the antenna. Like a car roof or the ground (there’s always enough water in dirt so that it’s conductive). The radio wave generated by the antenna causes electrical charges to slosh around in the car roof, and those moving charges emit fields that reinforce the signal.
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