AN4190

Antennas for low power applications

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feeding point towards the end. The inverted-F antenna is an inverted-L antenna with a

feeding tap that gives larger antenna impedance. If the antenna is tapped at the right

location, no additional matching circuit is required.

Figure 11.

Inverted-L antenna

Figure 12.

Inverted-F antenna

2.1.3

Loop antenna

The single-turn loop antenna is a metallic conductor bent into the shape of a closed curve,

such as a circle or a square, with a gap in the conductor to form the terminals. A multi-turn

loop or coil is a series connection of overlaying turns. The loop is one of the primary antenna

structures.

The discussion of loop antennas is conveniently divided according to electrical size.

Electrically small loops, those whose total conductor length is small compared with the

wavelength in free space (less than about one-tenth of a wavelength), are the most

frequently encountered in practice. For example, they are commonly used as receiving

antennas with portable radios, as directional antennas for radio-wave navigation, or as

probes with field-strength meters. Electrically larger loops, particularly those near resonant

size (circumference of loop/wavelength

≈ 1), are used mainly as elements in directional

arrays.

The small loop antenna is also known as a magnetic loop since it behaves electrically as a

coil with a small but not negligible radiation resistance due to its finite size. It can be

analyzed as coupled directly to the magnetic field in the near field region, which itself is

coupled to an electromagnetic wave in the far field. Loop antennas with electrically small

circumferences or perimeters have small radiation resistances that are usually smaller than

their loss resistances. Therefore they are very poor radiators, and they are seldom

employed for transmission in radio communication. When they are used in any such

application, it is usually in the receiving mode, such as in portable radios and pagers, where

antenna efficiency is not as important as the signal to noise ratio. The field pattern of

electrically small antennas of any shape (circular, elliptical, rectangular, square, etc.) is

similar to that of an infinitesimal dipole with a null perpendicular to the plane of the loop and

with its maximum along the plane of the loop. As the overall length of the loop increases and

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