s 300MHz to 450MHz Transmitters to Small Loop Antennas

Matching Maxim's 300MHz to 450MHz Transmitters to Small Loop Antennas
The MAX1472, MAX1479, and MAX7044 300MHz to 450MHz ASK transmitter ICs are used in applications that require extremely small
packages, such as automobile key fobs and tire-pressure monitors. Quite often, a small loop is the only antenna that will fit into one of
these packages. Because loops are very small compared to the wavelength at these frequencies, they are extremely high-Q and present
a challenge to good impedance matching.
This application note shows the typical impedance values for small loops and suggests matching networks for these impedances. It
demonstrates the effectiveness of these networks in suppressing harmonics of the transmitted frequency. Most transmitter ICs that
serve these applications, like the Maxim MAX7044, MAX1472, and MAX1479, are biased for maximum efficiency rather than maximum
linearity, which means that the harmonic content of the power amplifier (PA) output may be very high. Regulatory agencies in all
countries where these devices are used restrict spurious emissions, so attenuating harmonic power from the PA is very important.
A complete model for the impedance match of a loop to the Maxim transmitter ICs must include the bias inductor, the output
capacitance of the PA, traces, package, parasitics, etc. These factors will slightly modify the matching component values defined in this
note. The networks detailed here match the MAX7044 transmitter, but also work satisfactorily with the MAX1472 and MAX1479. The
MAX7044 achieves its highest efficiency when driving a 125 load, while the MAX1472 and MAX1479 favor a load of roughly 250 .
Using the MAX1472 and MAX1479 with these networks increases mismatch loss by about 1dB—the networks can be slightly altered to
recover this loss if desired.
Impedance of an Electrically Small-Loop Antenna