AN2731 Freescale Semiconductor / Motorola, AN2731 Datasheet - Page 6

Manufacturer Part Number

AN2731

Description

Compact, Integrated Antennas: Designs and Applications for the MC13191 and MC13192

Manufacturer

Freescale Semiconductor / Motorola

Datasheet

1.AN2731.pdf (26 pages)

Impedance Matching

For heavily loaded antennas and antennas close to ground, the radiation resistance may deviate

considerably from 50 Ohms which causes a poor match. An Inductive/Capacitive (LC) matching network

may be employed, but better efficiency is possible by raising the feed impedance.

These techniques may also be employed if an impedance higher than 50 Ohm is required.

The current and voltage distribution on a dipole is such that the impedance is low in the center and raises

towards the ends. By tapering the dipole at some distance from the center, an appropriate match can be

found. The tapering may take the form of Gamma, Delta or Capacitive tapping as shown in Figure 4. This

allows for matching impedances from 2 up to 300 Ohms. Some loading may be required to take out the

reactance introduced by the tapering, or the antenna could be slightly offset tuned to compensate for the

added reactive component.

Gamma Impedance Match

Delta Impedance Match

Capacitive Impedance Match

Figure 4. Impedance Matching

Another approach is using the folded dipole. This is where two parallel wires are placed closely together.

Due to the tight coupling, the current distribution is approximately proportional to the surface area of each

wire. This means that in two equal wires, the current in the feeding wire is approximately half the value of

the wires together. Half the current at the same power means twice the voltage, or four times the impedance

of 73 Ohms (292 Ohms). In practice, the impedance is somewhat lower, as in the normal dipole case.

however, by changing the relative wire diameter, or even introducing several wires, it is possible to tune

the impedance from less than 100 Ohms to several hundred Ohms.

Compact Integrated Antennas, Rev. 1.2

Freescale Semiconductor