AN2731 Freescale Semiconductor / Motorola, AN2731 Datasheet - Page 2

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

Antenna Terms
2
Antenna Gain
Decibel (dB)
Radiation Resistance
3
Every structure carrying RF current generates an electromagnetic field and can radiate RF power to some
extent and likewise an external RF field can introduce currents in the structure. This means that
theoretically any metallic structure can be used as an antenna. However, some structures are more efficient
in radiating and receiving RF power than others. The following set of examples explains this concept.
Transmission lines (striplines, coaxial lines etc.) are designed to transport RF power with as little radiation
loss as possible because these structures are designed to contain the electromagnetic fields. To obtain any
appreciable radiation from such a structure, requires excessively high RF currents which causes low
efficiency due to high losses. Likewise, the ability to introduce RF currents into the structure is of
importance, described by the feed point impedance. If the feed point impedance is very high, low, and/or
highly complex, it is difficult to introduce RF current with good efficiency.
The antenna structure should be of reasonable size compared to the wavelength of the RF field. A natural
size is half a wavelength, which corresponds to approximately 6 cm at the 2.4 GHz ISM band. This size is
effective because when fed with RF power at the center point, the structure is resonant at the half wave
frequency. Reducing the size below 6cm tends to make the antenna less visible to the RF field and not
resonant which causes low efficiency. Not all structures make an efficient antenna.
Numerous structures have been devised that provide good efficiency and impedance match, but most of
these are derived from a few basic structures. A short description of these basic antennas, and some good
advice on how to implement these with success is provided later in this note.
This note does not include complicated formulas concerning antenna theory because it is beyond the scope
of this note. The intention of this note is to provide basic information about how antennas work, which
should allow users to achieve reasonable performance with a minimum number of attempts.
If users are interested in performing complex calculations and antenna simulations, they should consult the
abundant and widely available literature concerning antenna theory and design. Note that simply copying
an existing design does not necessarily ensure reasonable performance. A lot of external factors affect
antenna tuning, gain, radiation patterns, etc. An antenna tuned for one set of environmental factors may
not perform at all if put into a new environment, and may require a lot of tuning to achieve even reasonable
performance.
2
Antenna Terms
Basic Antenna Theory
A measure of how well the antenna radiates the RF power in a given direction,
compared to a reference antenna, such as a dipole or an isotropic radiator. The gain
is usually measured in dB’s. A negative number means that the antenna in question
radiates less than the reference antenna, a positive number means that the antenna
radiates more.
A logarithmic scale used to represent power gain or loss in an RF circuit. 3 dB is
a doubling of the power, -3 dB is half the power. -6 dB represents half the voltage
or current, and quarter the power.
The part of antenna’s impedance which produces radiated power. The measured
impedance of an antenna is comprised of radiation resistance and loss.
Compact Integrated Antennas, Rev. 1.2
Freescale Semiconductor