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Small, 915-MHz antenna beats monopole

-May 15, 2003

A 915-MHz data-acquisition project required a small antenna, but the available antennas lacked the necessary characteristics: efficiency, compactness—that is, smaller than a standard 3-in. monopole—with adequate bandwidth, and with amenability to modeling by inexpensive NEC-2 antenna software. (To learn more about NEC antenna software, go to www.nittany-scientific.com.) The result of this design effort, called the "Tab" antenna for its square shape has the following characteristics:

-a square shape, 1.25 in. per side (0.1 wavelengths);

-the ability to be constructed in FR-4 pc board;linear polarization;

-a 2-to-1 VSWR (voltage-standing-wave-ratio) bandwidth of 46 MHz (5% bandwidth);

-the ability to be mounted parallel or perpendicular to a pc board;

-enhanced suppression of second- and third-harmonic radiation; and

-the ability to be mechanically trimmable to resonance.

The Tab antenna is a folded monopole that you miniaturize by forming it into an inverted L with a downward bend at the end (Figure 1). You can solder it perpendicular to a pc board or build it as part of a pc board and place it at a corner. The folded section transforms the 12.5Ω radiation resistance to 50Ω and provides second-harmonic suppression. Third-harmonic suppression comes from an open stub near the base of the antenna. Detailed NEC-2 modeling explores the sensitivity to changes in the dimensions and optimized harmonic suppression. Because NEC models wire antennas in a vacuum and the Tab antenna is built on FR-4, you must incorporate the dielectric properties of the dielectric between the antenna elements into the model.

To incorporate the dielectric properties, you add periodic RC loads between the transmission-line elements. The loads are the small boxes in Figure 2, and each load comprises a 30Ω resistor in series with a 78-fF capacitor. The capacitance of each RC load is equal to the difference between the transmission-line capacitances calculated with FR-4 as the dielectric and with vacuum as the dielectric. You calculate the resistance of each RC load using the published FR-4 loss tangent of 0.02. The following formulas determine the approximate RC values for the transmission-line loads:

where D is the conductor spacing and d is the conductor diameter.

You calculate the RC load parameters for 915 MHz and place them every 0.1 in. (approximately 5°) along the line. You use the following parameters in the calculations:

-D=62 mils,

-d=20 mils, effective Er (dielectric constant) of FR-4=3,

-loss tangent=0.02, and

-tPROP_VACUUM=85 psec/in.

Table 1 shows the effective parameters in air and in the FR-4 medium. You use iterative modeling to determine the antenna dimensions with the following design parameters: The first vertical section and the horizontal section must be of equal lengths, the feedpoint impedance target is 50Ω, and the resonant frequency is 915 MHz. You meet these design criteria with a simulated antenna height of 1.3 in. and a total element length of 3.3 in. The actual element length of 3 in. stems from dielectric loading. You also shorten the actual antenna height to 1.25 in. to compensate for the impedance increase that the dielectric loss causes. Note that the modified NEC model accounts only for the dielectric between the antenna elements.

The folded section, functioning as a shorted 180° transmission line at 1830 MHz, provides second-harmonic suppression. And, although the optimum point for the short circuit is 1.75 in. from the feedpoint, you can achieve second-harmonic suppression of 25 dB by placing the short within 10% of this point. The location of the short has little effect at 915 MHz. An open transmission line at the feedpoint that is 90° at 2745 MHz provides third-harmonic suppression. This line creates a near-short circuit at 2745 MHz and provides harmonic suppression of 15 dB when you trim it to within 5% of the optimum length. Table 2 compares the simulated harmonic suppression of the Tab antenna with that of a quarter-wavelength monopole with a 50Ω source driving both antennas.

The Tab antenna has 20-mil-wide traces on opposite sides of 62-mil FR-4 and mounting pads at three locations to allow soldering the antenna securely to a larger board. You tune the antenna by trimming the open transmission line to provide an impedance minimum at 2745 MHz and then trimming the antenna elements to resonance at 915 MHz. Figure 3 shows that the simulated 2-to-1 VSWR bandwidth is 41 MHz, whereas the measured bandwidth is 46 MHz. Note that the required operating band is only 902 to 928 MHz. The increased measured bandwidth arises from dielectric losses and indicates that the radiation efficiency is approximately 90%. The large bandwidth of the Tab antenna results in low sensitivity to environmental detuning. This design effort yields an antenna only 40% as tall as a standard quarter-wavelength monopole, yet having excellent radiation efficiency, extended bandwidth, and superior harmonic suppression.

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