Antena Rfid

Anatomy of an Antenna
By Dan Galarde
As a RFID implementer, you may find yourself responsible for the maintenance of reader systems that both radiate and receive electromagnetic energy. All RFID reader systems require an antenna to make use of this electromagnetic energy. In this paper, we will discuss antenna characteristics, types, construction, tuning and safety. ANTENNA CHARACTERISTICS Anantenna may be defined as a conductor or group of conductors responsible for radiating electromagnetic energy into space and collecting it from space. The antenna converts electrical energy from the transmitter of the RFID system into electromagnetic energy and radiates it into space. Conversely, the antenna collects electromagnetic energy from space and converts it into electrical energy that isdelivered back to the receiver of the RFID system. This space is known as the air interface. The electromagnetic energy transmitted by the antenna consists of two components known as the electrical field and the magnetic field. The total energy radiated from an antenna is constant in space, except for factors such as absorption by the earth or nearby objects. As the wave advances, the energyspreads out over a greater area causing the amount of given energy in an area to decrease as the distance from the antenna source increases. The actual design of the antenna for transmitting purposes is critical for efficient operation. The antenna must deliver maximum energy to the air interface so that the power from the transmitter is not wasted. An efficient antenna must be cut to exact dimensionsas determined by the operating frequency. The dimensions for a receiving antenna are less important at fairly low frequencies but dimensional importance increases dramatically as the frequency increases. Most practical transmitting antennas are based upon two fundamental characteristics. Half-wave (Hertz) antennas are generally installed above ground and are positioned to radiate eitherhorizontally or vertically. Quarter-wave (Marconi) antennas operate with one end grounded and are mounted perpendicular to the earth or another surface acting as a ground. The half-wave (Hertz) antenna is also referred to as a dipole and forms the basis for more complex antennas generally operating at 2MHz or higher. Quarter-wave (Marconi) antennas are generally used at frequencies below 2MHz. All antennasregardless of their configuration have four basic characteristics defined as directivity, gain, polarization and reciprocity.

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DIRECTIVITY Directivity is a measure of an antenna or array to focus the radiated energy in one or more specific directions. The directivity of an antenna can be determined by viewing the radiation pattern. A given amount of energy from a directional antenna can bepropagated in one or more directions. The elements of a directional antenna can be arranged to radiate energy more evenly or conversely precisely focus radiation in one direction more than others. As shown in Figure 1.1, the patch (linear) and dipole antenna designs propagate radiated energy in specific directions when compared against the loop (omni-directional) antenna design.

The threeantenna types deliver distinct geometrical coverage

Antenna Design and Coverage

Antenna

Dipole

Patch

Loop
coverage zones

Figure 1.1 GAIN The gain of a directional antenna propagating energy in one or more directions is a ratio between the energy focused in these directions and the energy that would have been propagated if the antenna were not directional. The gain of an antenna isconstant for both transmitting and receiving. POLARIZATION The radiation field is made up of magnetic and electric lines of force that are always at right angles to each other. Most electromagnetic fields in space are said to be linearly polarized. The direction of polarization is the direction of the electric vector. That is, if the electric lines of force (E lines) are horizontal, the wave is...