Antennas for low power applications

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Doc ID 023812 Rev 1

length used is one-quarter of the wavelength, called a quarter-wave whip. Half-wave whip

antennas are also common.

A quarter-wave whip antenna has a gain 3 dB (twice in linear) greater than a half-dipole if

mounted above a perfect ground plane. The quarter-wave monopole antenna design and

implementation is shown in Figure 10.

Radiation is maximum when broadside, or perpendicular to a wire, so a vertical whip is ideal

communication in any direction except straight up. The radiation pattern perpendicular to the

whip can be described as omnidirectional. However, the direction of peak radiation has

changed from the x-y plane to an angle elevated from the plane. There is a “null”, or signal

minimum, at the end of the whip.

The whip antenna polarization is vertical, even though, in the real environment, metal

objects and the ground cause reflections, and may cause both horizontal and vertical

polarized signal to be present.

Figure 10.

Quarter-wave monopole antenna design

A simple alternative to the monopole antenna is to make it shorter than a quarter

wavelength and add an inductor near the base of the whip to compensate for the resulting

capacitive reactance. This type of antenna can have a performance nearly equal to that of a

full size whip.

The monopole antenna, as for the dipole, can be made as a trace on a printed circuit board

(PCB). This is very practical at frequencies over 800 MHz. At lower frequencies, a full size

monopole may be too long. The length of the monopole is 10 or 20% shorter than the

calculation, depending on the dielectric characteristics and thickness of the board.

Derivatives of the monopole are the inverted-L and inverted-F antennas, as shown in

Figure 11 and 12.

In the inverted-L antenna, the monopole does not run perpendicularly to the ground plane

over its whole length but is bent parallel to the ground plane after some distance. This helps

to save space, but decreases the radiation resistance because the radiator comes closer to

the ground plane. An additional matching circuit is needed to match the low feed impedance

to the usual transmission line impedance of 50

Ω.

If we proceed from the feed point of the inverted-L antenna to the end, we notice that the

voltage increases (while the current decreases) from a maximum voltage value at the

feeding point to almost zero at the end. This means that the antenna impedance has its

minimum if we feed the antenna as shown in Figure 11 and increases if we move the

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