Antennas for low power applications

AN4190

18/29

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its circumference approaches one free space wavelength, the maximum of the patterns

shifts from the plane of the loop to the axis of the loop which is perpendicular to its plane.

The radiation resistance of the loop can be increased, and made comparable to the

characteristic impedance of the practical transmission line, by increasing (electrically) its

perimeter. Another way to increase the radiation resistance of the loop is to insert, within its

circumferences or perimeter, a ferrite core of very high permeability which raises the

magnetic field intensity and hence the radiation resistance. This form is called ferrite loop.

The loop is entirely different from a monopole, in that both ends of the antenna are

terminated. In this case, the end that is opposite the transmitter (or receiver) is grounded. A

capacitor is used to tune the antenna to real impedance, instead of a coil.

One advantage of a loop is that it is not easily detuned by nearby hand movements. A

disadvantage of loop antennas, besides the poor gain, is the narrow bandwidth which

makes tuning extremely critical.

2.1.4

Spiral antenna

Spiral antennas belong to the class of frequency independent antennas; those with a very

large bandwidth. The fractional can be as high as 30:1. This means that if the lower

frequency is 1 GHz, the antenna may still be in band at 30 GHz, and every frequency in

between. An example of a spiral antenna is shown in Figure 13.

Spiral antennas are usually circularly polarized. The spiral antenna's radiation pattern

typically has a peak radiation direction perpendicular to the plane of the spiral (see

Figure 14).

Spiral antennas are widely used in the defense industry for sensing applications, where very

wideband antennas that do not take up much space are needed. Spiral antenna arrays are

used in military aircraft in the 1 - 18 GHz range. Other applications of spiral antenna include

GPS, where it is advantageous to have right-hand circularly polarized antennas.

The best known spiral antenna is the log-periodic one. This antenna, also known as the

equiangular spiral antenna, has each arm defined by the polar function:

Equation 6

where:

a = parameter that controls the rate at which the spiral antenna grows

rR

0e

a

φ

=