AN400

10

Rev. 0.5

3. Whip Antenna for FM/WB Receiver (Si4740/41/42/43/44/45 Only)

A whip antenna is a typical monopole antenna used in AM/LW/SW/FM/WB/RDS receivers. The AM/LW application

is covered in Section "5. Whip Antenna for AM/LW Receiver (Si4740/41/42/43/44/45 Only)" on page 15. The

AM/LW/SW application is covered in "6. Whip Antenna for AM/LW/SW Receiver (Si4742/43/44/45 only)" on page

18. The whip antenna for FM is also used for Si4749. The front-end circuits differ considerably between Si4740-43

and Si4749; so, the whip antenna for Si4749 will be covered in Section "4. Whip Antenna for RDS Receiver

(Si4749 Only)" on page 13.

3.1. FM/WB Whip Antenna Design

A whip antenna is a monopole antenna with a stiff but flexible wire mounted vertically and 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 5 shows the telescopic whip antenna.

Figure 5. Typical Telescopic Whip Antenna Application

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 U.S. FM band. The

antenna capacitance is about 22 pF in the center of the U.S. FM band (98 MHz).

3.2. FM/WB Whip Antenna Schematic

Figure 6. FM/WB Whip Antenna Schematic