AN2731 Freescale Semiconductor / Motorola, AN2731 Datasheet - Page 15

• Manufacturer Part Number: AN2731
• Description: Compact, Integrated Antennas: Designs and Applications for the MC13191 and MC13192
• Manufacturer: Freescale Semiconductor / Motorola

Of the different antenna types, the following are especially suited to interface with the MC13191/
MC13192:
Because the MC13192/92/93 has separate RX and TX ports, two antennas will eliminate RX/TX
switching. The two antennas should ideally be placed at least ¼ wavelength apart to reduce coupling, but
due to the low power requirements, closer spacing can be allowed. The following list shows the results
from testing with dipole antennas and shows the typical isolation. The values are empirical, and depend
somewhat on the surrounding layout etc., but they should provide a reasonable indication of the isolation
obtainable.
For the MC13192/92/93 to show optimum performance, at least 6 dB of TX to RX isolation is required or
the ESD protection diodes in the RX input cause some TX power loss and perhaps also increased 3rd
harmonic output. However, placing the RX and TX dipoles on top of each other, with only 3 dB of isolation
results in the smallest design possible, and only reduces TX power a few dB, which is entirely acceptable
in most cases. The RX side works well with any isolation available. When the antennas are very close, the
coupling results in some interaction in the tuning of the two antennas. With just a few attempts, users
should be able to optimize performance.
Freescale Semiconductor
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Feedpoint impedance of 2-300 Ohm
Easily loaded to smaller size, with PCB or lumped loading
Provide a DC feed to the TX port
Easy to implement in FR-4 PCB
Dipoles with gamma match, or folded dipoles
Half or quarter wave loops
The slot antenna may also prove useful in some cases
On each side of a PCB, on top of each other: – 3 to – 4 dB
Very close, on the same side of the PCB: – 6 dB
15 mm apart: – 10 dB
25 mm apart: – 13 dB
Compact Integrated Antennas, Rev. 1.2
Recommended Antenna Designs
15