What type of antenna consists of two lengths of rod that radiates a doughnut pattern?

The correct answer refers to a Hertz antenna, which is fundamentally characterized by its design of two lengths of rod or metal elements oriented in a linear arrangement. This type of antenna is known for its ability to create a radiation pattern that resembles a doughnut when viewed from above, which is commonly referred to as a toroidal pattern. The radiation pattern results from the physical lengths of the rods being approximately half the wavelength of the signal being transmitted.

Hertz antennas can effectively produce electromagnetic waves through the oscillation of electric charges in the rods, and they are significant in studying basic antenna theory. In practical applications, this radiation pattern allows the antenna to effectively transmit and receive signals in certain directionalities, which can be advantageous for specific communication purposes.

The other options do not accurately match the characteristics described in the question. An isotropic antenna is an idealized antenna that radiates power uniformly in all directions. A ring antenna typically has a different geometric configuration and radiation pattern. The Marconi antenna is a specific type of monopole antenna, which also does not exclusively produce the described doughnut pattern.