As with most of modern technology, wireless radio communication also relies on the manipulation of a propagating (moving) electromagnetic field to convey information. Consider an example of a ground station communicating with a satellite, attempting uplink a simple "Hello" message. After the user at the ground station hits the send button, the message gets converted into a stream of electrical 1s and 0s representing a compressed file containing the word "Hello", and then gets sent to the transmitter. The transmitter converts this digital input into an AC current that gets ported to an antenna.
If you've ever taken an introductory electromagnetism course, you've likely encountered the idea that an electric field in a circuit is always accompanied by a surrounding magnetic field. You may have also encountered the concept of an electromagnet - a set of coiled wires that can produce a magnetic field when a current is applied. You'll notice, however, that an electromagnet as a satellite ground station would not be able to produce any significant detectable field at a distance of 400km from a CubeSat in space. In order for information to travel that distance, you need a propagating magnetic field. That is, you need a series of magnetic waves that radiate outwards rather than circle around a solenoid.
A visualization of how a solenoid produces a magnetic field. Note that this magnetic field doesn't radiate!
Antennas are not solenoids. In fact, they're actually open circuits that use alternating currents and thus alternating electric fields as a way to detach magnetic flux lines from their sources.
Consider a dipole antenna: a pair of open wires placed parallel to one another, and attached to a single AC source, as shown in the diagram below.
The basic shape of a dipole antenna
When an alternating current is connected to this set of wires, it moves electrons from one wire to another, causing a charge imbalance between the two. As such, one of the wires essentially becomes positively charged, while the other becomes negatively charged.
An alternating current produces a charge imbalance in a dipole antenna
If we view the image above from an aerial view, we would see a net positive charge on our left and a net negative charge on our right. At this particular instant, we would notice a pretty normal-looking electric field around those two charges as seen below.
In time, as the electrons move into the right wire, these net charges would move closer to the center, getting faster and faster. Eventually, the negative charge would travel to the right side, and the positive would travel to the left, and the two would continue alternating. As the charges get further away from the center, the speed of the motion of the charges slows (as more electrons accumulate on the right side), and eventually stops, after which the motion reverses and the negative charge once again begins to move to the left.
As the charges begin to accelerate, the electric fields they form at particular positions don't resemble the standard shapes that you would expect if they were considered in their stationary representations. Instead, their field lines tend to form small kinks, and the field line maintains its initial shape, particularly as you consider field lines further away from the two charges. Once the charges eventually cross paths, their field lines interfere with one another and end up detaching from the source, propagating away as an electromagnetic wave. In this way, antennas radiate out EM waves and allow signals to travel great distances.
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Animation of two charges converging and producing a propagating EM wave
For a better visual representation of the discussion in this section, refer to the videos below:
https://www.youtube.com/watch?v=ZaXm6wau-jc
https://www.youtube.com/watch?v=FWCN_uI5ygY
What we've described above is actually a better representation of an electromagnetic transmitter rather than an antenna, since we haven't used our antenna for its primary purpose: to amplify signals.