AN851 Datasheet, PDF (14/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

AN851
6. Air Loop Antenna
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.
6.1. Air Loop Antenna Design
Figure 9 shows an example of an air loop antenna.
Figure 9. 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 µH. Therefore, in order to interface with the air loop
antenna properly, a transformer is required to raise the inductance into the 180 to 450 µH range.
T1 is the transformer to raise the inductance to within 180 to 450 µH range. A simple formula to use is as follows:
L_equivalent = N²L_AIRLOOP
Typically a transformer with a turn ratio of 1:5 to 1:7 is good for an air loop antenna of 10–20 µH 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 toroidal shape core to prevent
noise pickup from interfering sources.
A few recommended transformers are listed in Table 6.
Table 6. Recommended Transformers
Vendor
Part Number
Type
Primary Coil Turns (L1)
Secondary Coil Turns (L2)
Wire Gauge
Inductance (L2)
Transformer 1
Jiaxin Dianzi
SL9x5x4MWTF1
Surface Mount
12T
70T
ULSA / 0.07mm x 3
380 µH ±10% @ 796 kHz
Transformer 2
UMEC
TG-UTB01527S
Surface Mount
10T
55T
n/a
184 µH min, 245 µH typ
@ 100 kHz
Q
130
50
Transformer 3
UMEC
TG-UTB01526
Through Hole
10T
58T
n/a
179 µH min, 263 µH typ
@ 100 kHz
75
14
Rev. 0.2