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:

enter image description here

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

enter image description here

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.

  • \$\begingroup\$ What you have drawn is a coil too, it just has one loop of wire. \$\endgroup\$
    – Justme
    Jan 23, 2022 at 16:21
  • \$\begingroup\$ right, but does it not emit waves on its own? why does it still need an antenna? \$\endgroup\$
    – Dan
    Jan 23, 2022 at 16:24
  • \$\begingroup\$ Coils can also form antennae ... en.wikipedia.org/wiki/…. \$\endgroup\$
    – user16324
    Jan 23, 2022 at 16:54
  • \$\begingroup\$ There's absolutely nothing special about a coil, it's just a length of wire rolled up into a convenient shape. It doesn't behave any different than a really long loop of plain wire would. \$\endgroup\$
    – TooTea
    Jan 23, 2022 at 17:32
  • \$\begingroup\$ flip the battery around once every second we would be making a 1Hz wave ... no, that would be 1/2 Hz \$\endgroup\$
    – jsotola
    Jan 23, 2022 at 19:21

2 Answers 2


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: -

enter image description here

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.

  • \$\begingroup\$ Thank you for your answer. Could I also ask, why antennas are usually open wire (either dipole or monopole), why can't it be a closed circuit like my battery diagram above? \$\endgroup\$
    – Dan
    Jan 23, 2022 at 16:47
  • \$\begingroup\$ They can be. There are many different types of antennas. Some don't even look like antennas. The world of antennas is vast @Dan \$\endgroup\$
    – Andy aka
    Jan 23, 2022 at 17:02
  • \$\begingroup\$ @Dan They can. Look up folded dipole. \$\endgroup\$
    – Graham Nye
    Jan 23, 2022 at 20:15

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.

  • \$\begingroup\$ Consider the LASER. Each atom emits EMR, no coil needed. However, a coil is a useful way of putting a lot of wire in a small space to increase inductance. One could use a delay line or a crystal to make an RF oscillator, or a Wien bridge cicuit: electronics-tutorials.ws/oscillator/wien_bridge.html . \$\endgroup\$ Jan 24, 2022 at 0:11

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