AN4190

Antennas for low power applications

Doc ID 023812 Rev 1

19/29

Figure 13.

Spiral antenna

Figure 14.

Spiral antenna 3D radiation pattern

The total length of the spiral (or the outer radius) determines the lowest frequency of

operation for the spiral antenna. The lowest operating frequency of the spiral antenna is

commonly approximated to occur when the wavelength is equal to the circumference of the

spiral.

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.

The total length of the spiral (or the outer radius) determines the lowest frequency of

operation for the spiral antenna. The lowest operating frequency of the spiral antenna is

commonly approximated to occur when the wavelength is equal to the circumference of the

spiral.

AM16437v1

AM16438v1