AN4190 Datasheet, PDF (16/29 Pages) STMicroelectronics – Antenna selection guidelines

Antennas for low power applications
AN4190
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
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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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