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Maxim > Design Support > Technical Documents > Application Notes > Wireless and RF > APP 3401

Keywords: Loop antenna, PCB antenna, MAX1472, MAX7044, MAX1479, antenna matching, ISM band TX, ISM

transmitter, 315MHz transmitter, 433MHz transmitter, 433.92MHz transmitter

APPLICATION NOTE 3401

Matching Maxim's 300MHz to 450MHz Transmitters to

Small Loop Antennas

Dec 21, 2004

Abstract: 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 125Ω

load, while the MAX1472 and MAX1479 favor a load of roughly 250Ω. 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

The radiation resistance of a small, printed circuit-board loop with area A at a

frequency whose wavelength is

is given by:

The dissipative resistance in the loop, ignoring dielectric loss, is calculated by the

loop perimeter (P), the trace width (w), the magnetic permeability (µ = 400

nH/meter), the conductivity ( , 5.8 x 10

Ω/meter for copper is typical), and the frequency (f):

The loop's inductance is determined by the perimeter (P), the area (A), the trace width (w), and the magnetic

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permeability (µ):

These three quantities are derived from expressions in antenna theory textbooks.

1,2

A typical printed circuit- board loop, from which dimensions are used to calculate representative resistive and reactive

values for a small loop, is shown in the drawing of Figure 1. It is roughly rectangular, 25mm by 32mm, with a trace

width of 0.9mm. At 315MHz, these dimensions lead to the following values for the three quantities listed above:

For the other commonly used frequency of 433.92MHz, the values are:

Figure 1. Small loop on a printed circuit board.

The radiation resistance is extremely small. In addition, the resistance arising from the dissipative losses can be more

than ten times the radiation resistance, which means that the best radiation efficiency possible with this loop is about

8% at 315MHz and 27% at 433.92MHz.The matching network must minimize mismatch losses and additional

dissipative losses from the matching components. Typically, a small loop may be able to radiate only a few percent of

the power that comes from the transmitter.

Basic Matching Network

The simplest matching network is the "split capacitor," described in a recent Microwaves & RF article.

Connecting this

network to the PA output with the bias inductor, as shown in Figure 2 below, makes it possible to adjust the value of

so that it resonates with the parallel combination of L

, the PA-related capacitance, and the residual reactance

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