Figure: Radio wave emission and explication

Image 1 Caption: A half-wave dipole antenna (a standard radio antenna) emitting radio waves showing a sample of the electric field lines which are loops in purple (and marked by E) and a smaller sample of magnetic field lines which are circles in red and orange (and marked by B).

The image seems to be just the back half of the partially spherical emission of electric field waves and a three-dimensional depiction of the magnetic field waves.

At every point in space, the electric and magnetic field lines are perpendicular to each other.

The emission has axial symmetry about the antenna.

The image only illustrates the near field limit of emission: i.e., near to the antenna. The is also a far field limit. For more explication of these limits, see see Wikipedia: Near and far field: Summary of regions and their interactions.

The frequency of emitted waves (i.e., frequency of waves passing any point in space) is actually very high. It is somewhere in the radio band fiducial range 3 Hz -- 300 GHz = 0.3 THz, 0.1 cm -- 10**5 km.

Further explication:

1. There is actually a continuum of electric and magnetic field lines expanding outward from the antenna, but that cannot be illustrated.

2. The antenna in the center, consists of two vertical metal rods with an alternating current (AC) at the antenna's resonance frequency applied at its center from a radio transmitter (NOT shown).

3. The electrical potential (AKA voltage) alternately charges the two ends of the antenna positive and negative. Standing waves of electrical current flow up and down the rods.

4. The alternating electrical potential (AKA voltage) on the rods creates waves of electric field with a loop structure as shown by the electric field lines. The electric field lines pinch off into closed loops and radiate away from the antenna at the vacuum light speed. These are the radio waves. The radiated power is greatest in the horizontal direction which is perpendicular to the antenna and decreases to zero on the antenna axis.

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

6. For self-propagation, electromagnetic radiation (EMR) (of which radio is particular frequency band) requires coupled (i.e., interacting) electric fields and magnetic fields.

7. Note, 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 coupled electric fields and magnetic fields can self-propagate.

8. 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.

9. 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.

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