SI4721 datasheet(29/69 Pages) SILABS

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AN383

Rev. 0.8

Figure 16. Rectangular Loop (Wire) Antenna—Orthogonal View

5.1.2.3. Design Guidelines



Route the antenna as shown in Figure 16.



A > 5 mm



B + C + D + E > 13 cm



C > 3 cm



Maximize antenna length (B+C+D+E >13 cm) to provide sufficient radiating power for transmit and maximize

incident voltage for receive.



Keep the antenna as far from the ground plane and shield as possible (A > 5 mm), and make the enclosure

from non-conductive material (plastic), to minimize parasitic capacitance and maximize radiation for transmit or

maximize incident voltage for receive.



Antenna inductance for an ideal loop antenna is given by Lant = n²µ0r[ln(8r/b)].

r: loop radius (m)

n: number of turns

µ0: permeability (4 x 10–7 N/A2)

b: wire radius (m)

Number of turns greater than one usually results in a high inductance loop with which the varactor cannot resonate.

It is acceptable to place two loops in a parallel structure to reduce the effective inductance.

For a loop with a small radius used in cellular handset or mp3 applications, the loop antenna equation can be

approximately applied to a rectangular loop of the same circumference.



Use an ideal vertical loop (wire) antenna as a reference point to measure the performance of the loop antenna.

Antenna performance will degrade in a practical application where the loop antenna is bent parallel to the GND

plane.

Radius (cm)

Turns

Total Length (cm)

Lant (nH)

12.6

111.5

18.8

182.6

25.1

257.9

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