Antennas for low power applications

AN4190

20/29

Doc ID 023812 Rev 1

The ‘a’ parameter, called flare rate, is the rate at which the spiral grows with angle. If it is too

small, the spiral is tightly wrapped around itself: in this case it behaves more like a capacitor

giving poor radiation.

The feed structure determines the high end of the operating band. How tightly the spiral can

be wrapped in on itself determines how small the wavelength can be that fits on the spiral

and still maintains spiral antenna operation. The highest frequency in the spiral antenna's

operating band occurs when the innermost radius of the spiral is equal to lambda/4.

2.1.5

Helical antenna

A basic, simple and practical configuration of an electromagnetic radiator is that of a

conducting wire wound in the form of a screw thread forming a helix. In most cases the helix

is used with a ground plane.

The geometrical configuration of a helix, as shown in Figure 15, usually consists of N turns,

diameter D and spacing S between each turn. Another important parameter is the pitch

angle

α which is the angle formed by a line tangent to the helix wire and a plane

perpendicular to the helix axis.

Helical antennas can operate in one of two principal modes: normal mode or axial mode.

In the normal mode of operation the field radiated by the antenna is maximum in a plane

normal to the helix axis and minimum along its axis. To achieve normal mode operation, the

dimensions of the helix are usually small compared to the wavelength. These simple and

practical antennas were primarily designed to replace very large antennas. Their reduced

size is therefore most suitable to mobile and portable communication systems.

A more practical mode of operation, which can be generated with great ease, is the axial or

end fire mode. In this mode of operation, there is only one major lobe and its maximum

radiation intensity is along the axis of the helix, as shown in Figure 16. The minor lobes are

at an oblique angle of the axis. To excite this mode, the diameter and spacing must be large

fractions of the wavelength. Most often the antenna is used in conjunction with a ground

plane, whose diameter is at least lambda/2. The helical antenna that works in axial mode

produces circular polarization. In radio transmissions, circular polarization is often used

where the relative orientation of the transmitting and receiving antennas cannot be easily

controlled, such as animal tracking and spacecraft communications, or where the

polarization of the signal may change. Since large helices are difficult to build and unwieldy

to steer and aim, the design is commonly employed only at higher frequency.