Image 1 Caption: "An animation of a half-wave dipole antenna (a stardard radio antenna) emitting radio waves (radio band fiducial range 3 Hz -- 300 GHz = 0.3 THz, 0.1 cm -- 10**5 km) showing the electric field lines. The electric dipole, in the center, consists of two vertical metal rods with an alternating current (AC) at its resonance frequency applied at its center from a radio transmitter (NOT shown). The electrical potential (AKA voltage) alternately charges the two ends of the antenna positive (+) and negative (-). Standing waves of electrical current (red arrows: ↑ ↓) flow up and down the rods. The alternating electrical potential (AKA voltage) on the rods creates waves of electric field with a loopy structure as shown by the electric field lines in black: the electric field lines pinch off into closed loops and radiate away from the antenna at the speed of light. These are the radio waves. The radiated power is greatest in the horizontal direction, perpendicular to the antenna, and decreases to zero above and below the antenna, on the antenna axis. The animation only shows the electric field in a single plane through the antenna axis: the electric field is actually has axial symmetry about the antenna and we are just seeing a cross section. The action is shown slowed down drastically in the animation: real radio waves oscillate with frequencies in the fiducial range 3 Hz -- 300 GHz = 0.3 THz." (Somewhat edited.)

Features:

1. Recall that vector fields are visualized using the visualization/conceptual tool of field lines. For a fullish expliction of field lines, see figure vector_field_field_lines.html. For an explication specialized to electric fields and magnetic fields, see figure em_field_lines.html.

2. Actually, the radiated electromagnetic radiation (EMR) has a magnetic field as well as an electric field. Only combined electric fields and magnetic fields can self-propagate. The magnetic field lines are NOT shown for simplicity in the animation. But a small sample of them are shown in Image 2 below.

   

3. Image 2 Caption: We see both the electric field lines (with symbol E and color purple for electric field) and a small sample of the magnetic field lines (with symbol B and color red for magnetic field).

   In fact, the electric field and magnetic field have values at all points in spacetime, but in animations and diagrams only a representative sample of field lines can be shown of course.

   In Image 2, the sample of magnetic fields is rather skimpy.

   In fact, the magnetic field lines will be everywhere perpendicular to the electric field lines and form circles around the antenna axis (see Wikipedia: Near and far field: Summary of regions and their interactions).

4. Note that the magnetic field has much less direct effect on radio transmission (as determined by the Lorentz force) in most technological applications than the electric field.???? However, its indirect effect is equal since, as aforesaid, only combined electric fields and magnetic fields can self-propagate.

5. In the far-field limit (i.e., far from the antenna), a point receiver would detect the radio waves as forming plane waves to high accuracy/precision.

6. By the by, AM radio (typically 530--1700 kHz) (see Wikipedia: Medium wave) imposes information on the radio transmission signal by amplitude modulation of the radio waves. On the other hand, FM radio (typically 88--108.0 MHz) (see Wikipedia: FM broadcast band) imposes information on the radio transmission signal by frequency modulation of the radio waves.

7. Another by the by, all the behavior discussed above follows from classical electromagnetism which is summarized by Maxwell's 4 equations of classical electromagnetism.

Images:

1. Credit/Permission: User:Chetvorno 2015 / Public domain.
   Image link: Wikimedia Commons: File:Dipole xmting antenna animation 4 408x318x150ms.gif.
   
2. Credit/Permission: © User:Maschen, 2012 / CC BY-SA 1.0.
   Image link: Wikimedia Commons: File:Felder um Dipol.svg.