WIFI Antenna Design

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Cell Phone Antenna Design

WIFI antennas are on just about every cell phone these days. The WIFI frequencies are the highest of all frequency on the device (relative to cell frequencies, gps, nfc, etc). The WIFI frequency is divided into two bands: 2400-2484 MHz (which also includes bluetooth) and 5150-5850 MHz.

All WIFI antennas support the 2400-2484 MHz band, and typically the WIFI antenna is connected to a chip that does both WIFI and bluetooth. This means designing antennas for WIFI and bluetooth is basically the same thing. Phones now are shipping with the 5150-5850 MHz band included in WIFI, so that the WIFI antenna is often dual band. It is also possible to have two wifi antennas, one for 2.4 GHz and one for the 5 GHz band, although this is less common.

Because WIFI is the highest frequency on the mobile device, the WIFI antenna will be the smallest antenna. A half-wavelength at 2.4 GHz is 6.25 cm (2.5"), and a half-wavelength at 5 GHz is 3 cm or just over an inch. Hence, using quarter-wavelength antennas leveraging the devices groundplane can make for very small wifi antennas. Good quarter-wavelength antennas for WIFI include the IFA antenna and the PIFA antenna.

WIFI antennas are simultaneously used for transmit and receive. Hence, WIFI antennas must abide by FCC and governmental SAR rules. In addition, there are peak antenna gain rules that are specified in dBm. Typically, antenna gain is specified in dB, such as peak antenna gain equals +2 dB. This means that the efficiency times the directivity is +2 dB.

FCC rules often specify gain in dBm. This means that they might say the wifi antenna gain must be less than 10 dBm. This means that the peak Effective Isotropic Radiated Power (EIRP) must be less than 10 dBm. This number is a function of the conducted power (the power the radio outputs at its terminals, which is typically something on the order of 15 dBm for WIFI), the wifi antenna efficiency, and the directivity of the antenna.

Peak gain [dBm] is basically equal to Conducted_Power [dBm] + Antenna Efficiency [dB] + Directivity [dB]. For instance, if the conducted power is 15 dBm, the antenna efficiency is -3 dB, and the directivity is +4 dB, then the peak gain would be 16 dBm. If the spec for peak gain was 14 dBm, the system could achieve the spec by dropping the conducted power to 14 dBm (a drop of 2 dB).

WIFI antenna efficiencies for handheld mobile devices are typically on the order of -6 dB to -2 dB. The efficiency is decreased due to antenna-antenna coupling and lossy resistance of all the components around the antenna (camera, PCB, glass on the screen, etc).

The WIFI antenna is typically located on the top of the device (near the GPS antenna and diversity cellular antenna). Note that there are no TRP specs, so in general you don't want the WIFI antenna to have too high of an efficiency, or the SAR and peak gain values will require a large conducted power backoff.

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