Rf Circuitry; Loop Antenna Impedance - Motorola M68HC08 Designer Reference Manual

8.2.2 RF Circuitry

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If the PCB loop antenna is changed in term of trace length or width, the
corresponding loop antenna inductance, L, would be different and the matching
network should be adjusted to maintain the maximum signal transfer condition.
RF
CIRCUITRY
Z1
TRANSMITTER
Figure 8-2. Loop Antenna Impedance
On the transmitter side, the value for matching component C can be easily
calculated by the following equation:
1
2πf = ----------- -
LC
Where:
f = 27 MHz
L = Loop antenna inductance
Table 8-1. Tx Matching Examples
Case
Smaller inductance
Original reference design
Larger inductance
On the receiver side, one of the simple methods to keep the matching condition
for different inductance values is to adjust the capacitance value C such that
the sum of L and C impedance [XL + (– XC)] at 27 MHz is equal to a
pre-matching impedance value (Z4 = 26.3 j).
1
Z4 = jωL + --------- -
jωC
Table 8-2. Rx Matching Examples
Case
Smaller inductance
Original reference design
Larger inductance
Testing and Customization
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LOOP
ANTENNA CIRCUITRY
C14
L1
Z2
L1
200 nH
240 nH
300 nH
@ 26.3j
L2
200 nH (34j)
236 nH (40j)
300 nH (51j)
Testing and Customization
Hardware Customization
RF LOOP
ANTENNA
C46
C45
L2
Z3 Z4
RECEIVER
C = C14
= 173 pF (~180 pF)
= 145 pF (~150 pF)
= 116 pF (~120 pF)
C = C45 + C46
765.5 pF (–7.7j) ~ (470 + 300) pF
430 pF (–13.7j) = (330 + 100) pF
238.6 pF (–24.7j) ~ (200 + 39) pF
DRM055
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