Electronic Design

Antennas 102: More Questions And Answers

What is antenna isolation?
Isolation refers to keeping multiple antennas separate from one another to avoid interaction effects. In some devices—especially cell phones, where multiple antennas must be squeezed into a small space—coupling between antennas can reduce their effectiveness. Coupling detunes the antenna, introducing mismatch losses and otherwise lowering efficiency.

The key to good isolation is greater physical separation between the antennas. One good way to improve isolation is to use a ceramic dielectric antenna. In some cases, ceramic dielectric antennas may be placed within a few millimeters of one another without drastic results. Isolation is usually expressed in dB, and a good typical isolation is in the 20- to 30-dB or greater range.

Will an antenna designed for one frequency work on another frequency?
Maybe. If the second frequency is close to the antenna design frequency, it may work, though poorly, depending on the bandwidth of the antenna. Also, the SWR on the transmission line will be higher, creating losses. Some antennas will work well on harmonic frequencies. An antenna for 7 MHz will work fine at the second harmonic of 14 MHz, but it will have a different impedance and need to be matched.

In general, it’s really best to use a separate antenna for each transmitter or receiver. Cell phones have multiple antennas, such as the main cellular band antenna, a Bluetooth antenna, and maybe even a GPS antenna. A separate antenna typically is used with each. Some antenna manufacturers addressed this problem with some wide-bandwidth models that can handle two frequencies if they’re close.

What is a ceramic antenna?
Also called a dielectric antenna, a ceramic antenna puts the antenna conductors on a dielectric material, usually ceramic. This greatly reduces antenna size at a given frequency. Antenna length is directly proportional to the wavelength (λ)—the shorter the wavelength, the shorter the antenna (see the figure).

For a ceramic antenna, the length is proportional to λ/√e, where e is the dielectric constant. Therefore, ceramic antennas are much smaller, and they’re widely used in cell phones and other portable equipment.

What is an active antenna?
An active antenna is integrated with some kind of electronic circuit. Most antennas are typically passive, but an active antenna may add an RF amplifier or some kind of tuning arrangement that allows the amplifier to operate over a wider range of frequencies.

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