AN2731 Freescale Semiconductor / Motorola, AN2731 Datasheet - Page 3

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

Basic Antenna Theory
3.1
Basic Antenna Variations
3.1.1
3.1.1 Dipole Antenna
The dipole is one of the most basic antennas. The dipole is a straight piece of wire cut in the center and fed
with a balanced generator or transmission line. As previously stated, this structure is resonant, or
non-reactive, at the frequency where the conductor length is 1/2 wavelength. For the ISM band, this length
is approximately 6 cm or about 2 ½ inches. At this length, the dipole shows resonance, the feed impedance
is resistive, and is close to 73 Ohms. This also holds true for a very thin wire in free space.
Total Length is Approximately ½ Wavelength
(At 2.4 GHz, Length is Approximately 6 cm)
Figure 1. Basic Dipole
A practical dipole of some thickness, loaded with different dielectric materials (PCB etc.), and perhaps
relatively close to ground, shows resonance at a slightly shorter length than calculated, and the radiation
resistance drops somewhat. For dipoles not too close to ground, the shorting factor is typically in the range
of 5-20%, the shorter being more heavily dielectric loaded, and radiation resistance is in the range of 35-65
Ohms.
This dipole setup exhibits a relatively good match to a 50 Ohm generator, but the feed is differential. A
small ceramic balun can be used for single-ended feed. The bandwidth is typically 2-5%, depending on the
return loss required. The radiation pattern in free space is doughnut-shaped, with pronounced dips along
the direction of the wires.To fill out these dips, the outer ends of the antenna can be bent at a 45 degree
angle. Several configurations are possible, including the “broken arrow” shape. Any materials close to the
antenna can distort the radiation pattern.
Compact Integrated Antennas, Rev. 1.2
Freescale Semiconductor
3