SI4706 Просмотр технического описания (PDF) - Silicon Laboratories

Номер в каталоге

Компоненты Описание

производитель

SI4706

Si47XX ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

Silicon Laboratories 

AN383

This equation shows that radiated power increases as the square of the antenna length and as the square of the

delivered current. Every effort should therefore be made to maximize the length of the antenna and maximize

antenna current. Minimizing circuit losses will maximize antenna current and can best be achieved by selecting a

tuning inductor with a Q > 30. Note maximizing antenna current is equivalent to maximizing antenna voltage, a

point that will be discussed later in this section.

The antenna reactance models the antenna capacitance for electrically short monopole antennas and is given as

follows:

XANT = 1-2---9---8-f--L-c-

where L = meters, c = 3 x 108 m/s

This equation shows that antenna reactance decreases, and therefore capacitance increases, linearly with

increasing antenna length. This capacitance should be kept in the range that can effectively be resonated by the

tuning inductor. In practice this is easily achieved and is only mentioned for completeness.

The following section, “ Electrically Short Monopole Interface Model”, discusses the ranges of capacitance and

inductance that should be expected.

As the name suggests, the electrically short vertical monopole antenna over an infinite ground plane assumes an

antenna oriented vertically, or perpendicularly, to a ground plane that is infinite in size. As the antenna

implementation deviates from this model, the effective antenna length decreases, resulting in a decrease in

radiated power. As a practical matter, the geometry of the antenna and ground plane is dictated by physical

dimensions of the device and the closest approximation to the ideal model is achieved by keeping the antenna as

far away from ground and power planes as possible. This applies equally to a wire antenna and a PCB trace

antenna. For brevity, the remainder of this document refers to this model as simply the "electrically short

monopole."

Electrically Short Monopole Interface Model

The electrically short monopole model can be extended to incorporate the FM Transmitter output buffer

capacitance, external tuning inductor and PCB capacitance as shown in Figure 38. At lengths below 1/2

wavelength, the antenna reactance XANT is capacitive and is replaced with capacitance CANT.

FM Transmitter Buffer Tuning Inductor

PCB

Antenna

CBUFFER

IBUFFER

RBUFFER

LTUNE

RTUNE

CPCB

CANT

RANT

IANT

Figure 38. Simplified Model of the Electrically Short Monopole

Rev. 0.8

57

Share Link: