Why do we need coils to produce electromagnetic waves?

Question

I'm trying to better understand how a simple radio can send/receive EM waves.

I have read that alternating current generates electromagnetic waves. So then if we make a DC circuit and flip the battery around once every second we would be making a 1Hz wave? Like this:

If so then why do we need coils? From this page I can see simple radio circuits all employ a coil: https://www.homemade-circuits.com/spy-bug-circuits/

... The procedures take place rapidly generating a frequency across the coil which is transmitted as carrier waves through the connected antenna.

Can't we just alternate the current on the antenna itself and emit waves? Like this picture: https://physics.stackexchange.com/a/286815

What if we use a coil but then not use an antenna, would the coil itself emit waves?

I guess I'm trying to figure out the relation between a coil and the antenna and why we need both.

Answer 1

If so then why do we need coils? From this page I can see simple radio circuits all employ a coil

The coil is needed to form a simple oscillator as per your linked article: -

C1, C2, C3 and L contribute to what is know as a "tank" circuit. The "tank" can be highly resonant and, when a biased transistor is connected to it correctly it will oscillate at a defined frequency.

What if we use a coil but then not use an antenna, would the coil itself emit waves?

The coil can transmit an alternating magnetic field but, usually its dimensions are so small that it makes an ineffective electric field generator hence, you only get a the M part of an EM wave.

On the other hand, a proper antenna is good for generating the E and M parts of an EM wave in the right proportion to suit the impedance of free space. However, it may not be very suitable as a tuned component in an oscillator hence, we use an inductor for the oscillator and an antenna to transmit the signal.

Answer 2

There's absolutely no need to use a coil to produce oscillations. Using an LC resonant circuit, which includes a coil (the inductor L), is only one way to create radio-frequency signals. There are a lot of other oscillator types: RC oscillators, voltage-controlled oscillators, PLL circuits, crystal oscillators, and even mechanical oscillators (i.e. MEMS). Most modern RF transmitters use a combination of a crystal oscillator followed by a digitally controlled PLL. The RF signal generated by the PLL is then amplified and sent straight into an antenna.

LC oscillators are just way simpler to build for hobbyists, which is why you find them in a lot of DIY transmitter circuits. There's nothing special about them; they're just a way of producing an alternating voltage that can be fed into an antenna. It'd work just as well if you could flip your battery around many million times per second. (But you can't, which is why you need an electronic oscillator.)

Any piece of wire will radiate electromagnetic energy more or less efficiently when you apply an AC waveform. This applies to the wire in your coil as well. The antenna is just way more efficient at radiating your signal into space due to its geometry (its length is matched to the wavelength of the signal). If you remove the antenna, the signal will get much weaker but it won't disappear.