AN738 Datasheet, PDF (26/37 Pages) Silicon Laboratories – Si4825/36-A ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

AN738
5. Whip Antenna for FM Receiver
A whip antenna is a typical monopole antenna.
5.1. FM Whip Antenna Design
A whip antenna is a monopole antenna with a stiff but flexible wire mounted vertically with one end adjacent to the
ground plane.
There are various types of whip antennas including long, non-telescopic metal whip antennas, telescopic metal
whip antennas, and rubber whip antennas. Figure 14 shows the telescopic whip antenna.
Figure 14. Telescopic Whip Antennas
The whip antenna is capacitive, and its output capacitance depends on the length of the antenna (maximum length
~56 cm). At 56 cm length, the capacitance of the whip antenna ranges from 18 to 32 pF for the US FM band. The
antenna capacitance is about 22 pF in the center of the US FM band (98 MHz).
5.2. FM Whip Antenna Schematic
Figure 15. FM Whip Antenna Schematic
L1 (56 nH) is the matching inductor and it combines with the antenna impedance and the FMI impedance to
resonate in the FM band.
C5 (1 nF) is the ac coupling cap going to the FMI pin.
U3 is a required ESD diode since the antenna is exposed. The diode should be chosen with no more than 1 pF
parasitic capacitance, such as the California Micro Device CM1213.
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