User's Manual

Antennas for portable devices include the following designs:

• Internal antenna (invisible or pull-up)

• External antenna, removable and directly connected to the device

• External, remote antenna

5.21.1.1. Internal Antenna (Invisible or Pull-up)

This is the most difficult antenna design scenario. Despite greater physical constraints, an internal antenna must still provide a

gain sufficient to meet network specifications. Metal-cased products cannot have internal antennas since the metal acts as a shield

around the antenna and prevents RF signals from reaching the antenna. (A metal case acts like a Faraday cage.) The antenna should be

positioned so that it is vertically oriented when the device is carried normally. This will ensure that the optimal antenna performance

is available the majority of the time.

Cable routing from the modem to the antenna needs to avoid RF-sensitive circuits and high-level, high-speed clock circuits. The

following points should be considered:

• The location of the antenna to avoid RFI to a computing device.

• Good shielding of the display and other RF-sensitive components.

• The most efficient method of cable routing.

If the above points are not considered, antenna gain can be offset by cable loss. A typical coaxial cable is very thin, such as the

RG178B used in portable devices, and cable loss can be as high as 0.5 dB per foot. Some coaxial cable manufacturers market

relatively thin double-braid coaxial cables. These cables show much better isolation than single-braid cables, typically by 30 to

40 dB. Double-braid cables reduce radiation and RF pick-up when routed inside a portable device.

5.21.1.2. External Antenna, Removable and Directly Connected to the Device

You can design a portable device that can use an off-the-shelf, plug-in antenna, such as a 1/4 wave monopole or 1/2 wave dipole

antenna. The typical gain of these omni-directional antennas is 0 dBi and 2.14 dBi, respectively. Like the internal antenna, these

antennas should be oriented vertically when the device is carried normally to ensure the best antenna performance.

Cabling in this case demands the same consideration as an internal antenna application. In a typical laptop application, the

antenna must be placed as far as possible from the display to avoid deflection. This usually causes a deep null in radiation

patterns.

5.21.1.3. External, Remote Antenna

For remote antenna applications, use the same design approach as internal designs, including the RF cable routing of the external

connector. You can choose an off-the-shelf mobile antenna of omni-directional 1/2 wave length. The antenna should have a gain of

2.14 dBi. Higher gain than this may not be appropriate for portable applications.

A double-braid coaxial cable, (such as RG223) from the device to the antenna is recommended, if the cable length is more than a

few feet long. The difference in cable loss between low-cost RG58 cable and the more expensive RG223 cable is approximately

4.5 dB per 100 feet. If the cable must be routed through noisy EMI/RFI environments, a double-braid cable such as RG223 can

reduce radiation and pick-up by 30 to 40 dB.

5.22. Fixed Devices

Fixed data device applications use the same design recommendations as a portable device with a remote antenna.

The most economical and practical choice for the RF connector of an external antenna is a TNC threaded connector, whether it is

a plug-in type or a remote type. TNC has a good frequency response to 7 GHz, and leakage is low. A mini UHF threaded connector

provides adequate performance and is an economical choice. If the size of the TNC and mini UHF connectors becomes critical,

consider an SMA threaded connector or an SMB snap fit connector. (The SMB connector does not accept RG58 or RG223

cables).

5.23. Antenna Test Methods

Whether portable or fixed, the device antenna is the critical link to the network. A poorly performing antenna reduces the

coverage of the device within the network footprint. The antenna performance must meet the impedance and match the criteria of

the modem (refer to Chapter 8,“Specifications”, page 235.), and have the appropriate amount of gain to meet the network ERP

requirements.