AN2731 Freescale Semiconductor / Motorola, AN2731 Datasheet - Page 8

• Manufacturer Part Number: AN2731
• Description: Compact, Integrated Antennas: Designs and Applications for the MC13191 and MC13192
• Manufacturer: Freescale Semiconductor / Motorola

Impedance Matching
The ground plane should be a reasonably sized area compared to the antenna, and should be reasonably
continuous. If a monopole is used on a very small PCB, perhaps even with only a small area of copper,
efficiency suffers, and the antenna is difficult to tune. Components and tracks introduce additional losses
and affect the feed point impedance.
As for the dipole, resonance is obtained at a length slightly shorter than one quarter wavelength, typically
5-15% shorter. Typical lengths are slightly more than an inch or two or 3 to 5 cm. The radiation resistance
is caused by bending the antenna, and like the dipole, the marked dip in the radiation pattern can be
eliminated. By bending the antenna closer to ground, the radiation resistance and efficiency drops, so the
antenna should not be placed too close to ground. Like the dipole, the monopole can also be folded and
bent around corners, if board space requires this, or it can be loaded with series coils.
Of the many variations that exist, the following sections highlight the most common.
4.1.1
If board space allows, a full-size quarter wave antenna is quite efficient and often provides a reasonable match to
a 50 Ohm system. Slight folding or bending of the ends has negligible impact on performance.
4.1.2
If the monopole is bent and traced along the ground plane, it will be more compact and the null in the
radiation pattern is partly eliminated. The antenna should not bee too close to ground, preferably not closer
than 1/10 wavelength (1 cm), or efficiency suffers too much. At this close spacing, the radiation resistance
is so low (in the order of 10 Ohms) that a matching network is usually needed. If the monopole is very
close to ground, it resembles a transmission line, with little or no radiation at all.
4.1.3
The F-antenna can be thought of as a tilted whip, where impedance matching is done by tapping the
antenna at the appropriate impedance point. Because this antenna is reasonably compact, has an
omnidirectional radiation pattern, good efficiency, and is very simple, it is used extensively in applications,
including the mobile communications business. It should be noted that the currents in the ground leg are
high, and that a good sized ground plane is necessary to provide good efficiency.
8
PCB Whip, Quarter Wave Monopole, or Quarter Wave
Open Stub, Tilted Whip
The F-Antenna
Figure 7. Tilted Whip and F - Antenna (Note the Ground Plane Area)
Compact Integrated Antennas, Rev. 1.2
Freescale Semiconductor