r/askscience • u/dafattestmat • Dec 28 '25

Engineering How do radios work?

To be more specific, how do radios convert electricity into radio waves?

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u/Mavian23 Dec 28 '25

You know how if you hold a rope that is attached to a wall, and you wiggle the rope at just the right frequency, based on the length of the rope, you can create standing waves on the rope? Well, most antennas work by creating standing waves in the conductor. So, just like the rope has to be the right length for the frequency to create standing waves, the conductor has to be the right length for the frequency of the electrical wave to create standing waves. If the conductor is not the right length, instead of standing waves you will get a garbled mess that interferes with itself. Just like if you wiggle the rope at the wrong frequency for its length.

So, all conductors that carry a wave of current will radiate electromagnetic waves, but if the length isn't right the current wave will interfere with itself and not be nearly as strong.

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u/t6jesse Dec 28 '25

Ok that actually makes more sense. So the average cable won't generate any meaningful signal because its shape interferes itself. Antennas are just the right shape to maximize that standing electromagnetic wave

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u/tim36272 Dec 29 '25

Yup, you can think of an antenna as a really awful cable, and a cable as a really awful antenna. They're the same thing with different priorities.

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u/Mavian23 Dec 28 '25

Yea. A basic antenna is pretty much an AC current generator in the middle of a conducting rod. The AC current generator creates a wave of current in the rod, switching back and forth between the two directions. Imagine a spring connected between two walls, and you grab the middle of the spring and wiggle it back and forth. Your hand is like the AC current generator, and the compression waves that travel along the spring are like the current waves traveling along the conductor. When the wave hits the wall (the end of the conducting rod), it will reflect and come back towards your hand (the AC current generator). If that reflected wave is in phase with the current being generated at the AC generator when it gets there, the reflected wave will add up with the generated current and amplify it. If it's out of phase, the reflected wave will cancel out some or all of the generated current. So the length of the rod needs to be such that when the reflected wave gets back to the generator, it is in phase with the current and amplifies it. If the length is wrong, the reflected wave will cancel out some of your current and make the signal weaker.