AN2731 Freescale Semiconductor / Motorola, AN2731 Datasheet - Page 13

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

the two output pins. Because the balun is a discrete device, it is bidirectional. The balanced port can be
both input or output.
Several discrete circuits are available that perform as baluns. Most of them are sensitive to input and output
loading and PCB layout which requires cumbersome fine tuning. And all of these require at least two chip
inductors. In the 2.4 GHz band, small ceramic baluns exist which are easy to use and are less sensitive to
the PCB layout. Standard output impedances are 50, 100 and 200 Ohms.
The cost of a discrete balun is comparable to, or higher than, the ceramic balun, and the ceramic balun
requires less board space. Therefore, the ceramic balun is recommended for most designs.
To interface with the MC13192/92/93, the standard component 50–200 Ohm balun is recommended. A
50–400 Ohm device provides slightly better performance, but it is not an off-the-shelf device.
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As previously stated, reducing antenna size results in reduced performance. Some of the parameters that
suffer are:
As stated, several performance factors deteriorate with miniaturization, but some antenna types tolerate
miniaturization better than others. How much a given antenna can be reduced in size depends on the actual
requirements for range, bandwidth, and repeatability. In general, an antenna can be reduced to half its
natural size without much impact on performance. However, after a one half reduction, performance gets
progressively worse as the radiation resistance drops off rapidly. As a rule, one half the antenna size equals
one quarter the radiation resistance. As loading and antenna losses often increase with reduced size, it is
clear that efficiency drops off quite rapidly.
The amount of loss that can be tolerated depends on the range requirements. Bandwidth also decreases,
which causes additional mismatch losses at the band ends. The bandwidth can be increased by resistive
loading, but this often introduces even more loss than the mismatch loss. The low bandwidth combined
with heavy loading requires a spread analysis to ensure adequate performance with variations in
component values and PCB parameters. As shown by these facts, it is often better not to reduce antenna
size too much, if board space allows. Even if range requirements do not require optimum antenna
performance, production problems and spread are minimized. It is also best to keep some clearance
between the antenna and nearby objects. Although the antenna may be retuned to compensate for the
loading introduced by the surroundings, tuning becomes more critical, and the radiation pattern can be
heavily distorted.
Freescale Semiconductor
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Miniaturization Trade-offs
Reduced efficiency (or gain)
Shorter range
Smaller useful bandwidth
More critical tuning
Increased sensitivity to component and PCB spread
Increased sensitivity to external factors
Compact Integrated Antennas, Rev. 1.2
Miniaturization Trade-offs
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