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AN555 View Datasheet(PDF) - Silicon Laboratories

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AN555
7. Air Loop Antenna for AM
An air loop antenna is an external AM antenna (because of its large size) typically found on home audio
equipment. An air loop antenna is placed external to the product enclosure making it more immune to system noise
sources. It also will have a better sensitivity compared to a ferrite loop antenna.
7.1. Air Loop Antenna Design
Figure 20 shows an example of an air loop antenna.
Figure 20. Air Loop Antenna
Unlike a ferrite loop, an air loop antenna will have a smaller equivalent inductance because of the absence of ferrite
material. A typical inductance is on the order of 10 to 20 uH. Therefore, in order to interface with the air loop
antenna properly, a transformer is required to raise the inductance into the 180 to 450 uH range.
T1 is the transformer to raise the inductance to within 180 to 450 uH range. A simple formula to use is as follows:
Typically a transformer with a turn ratio of 1:5 to 1:7 is good for an air loop antenna of 10 to 20 uH to bring the
inductance within the 180 to 450 uH range.
Choose a high-Q transformer with a coupling coefficient as close to 1 as possible and use a multiple strands Litz
wire for the transformer winding to reduce the skin effect. All of this will ensure that the transformer will be a low
loss transformer.
Finally, consider using a shielded enclosure to house the transformer or using a torroidal shape core to prevent
noise pickup from interfering sources.
A few recommended transformers are listed in Table 18.
Rev. 0.2
33
 

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