Fundamentals of ISM-Band and short range device antennas, Part 2

Part 1 focuses on antenna basics. Part 3 covers RF Propagation issues.
Click here for Part 4: Examples

Antenna Types and Their Features
The following common antennas are covered in detail in this section: half-wave dipole, quarter wave monopole, transversal mode helical, and small loop antennas.

Half-Wave Dipole

The half-wave dipole antenna (Figure 6) is the basis of many other antennas and is also used as a reference antenna for the measurement of antenna gain and radiated power density.

At the frequency of resonance, i.e., at the frequency at which the length of the dipole equals a half-wavelength, we have a minimum voltage and a maximum current at the terminations in the center of the antenna, so the impedance is minimal. Therefore, we can compare the half-wave dipole antenna with a series RLC resonant circuit as given in Figure 2. For a lossless half-wave dipole antenna, the series resistance of the equivalent resonant circuit equals the radiation resistance, generally between 60 Ω and 73 Ω, depending on the ratio of its length to the diameter.

The bandwidth of the resonant circuit (or the antenna) is determined by the L-to-C ratio. A wire with a larger diameter means a larger capacitance and a smaller inductance, which gives a larger bandwidth for a given series resistance. That's why antennas made for measurement purposes have a particularly large wire diameter.

As opposed to the (only hypothetical) isotropic radiator, real antennas such as the half-wave dipole have a more or less distinct directional radiation characteristic. The radiation pattern of an antenna is the normalized polar plot of the radiated power density, measured at a constant distance from the antenna in a horizontal or vertical plane.

Figure 7 shows the radiation pattern of a half-wave dipole antenna.

Since the dipole is symmetrical around its axis, the three-dimensional radiation pattern rotates around the wire axis.

The isotropic gain of a half-wave dipole antenna is 2.15 dB. Therefore, in the direction perpendicular to the wire axis, the radiated power density is 2.15 dB larger than that of the isotropic radiator. There is no radiation in the wire axis. The half-wave dipole produces linear polarization with the electrical field vector in line with, or in other words parallel to, the wire axis.

Because the half-wave dipole is often used as a reference antenna for measurements, sometimes the gain of an antenna is referenced to the radiated power density of a half-wave dipole instead of an isotropic radiator. Also the effective radiated power (ERP), which is the power delivered to an ideal dipole that gives the same radiation density as the device under test, is used instead of the EIRP. The relations G_dipole = G_isotropic - 2.15 dB and ERP = EIRP - 2.15 dB can be applied.

The half-wave dipole needs a differential feed because both of its terminations have the same impedance to ground. This can be convenient if the transmitter output or the receiver input have differential ports. A balun will be used along with the half-wave dipole in case of single-ended transmitters or receivers, or if an antenna switch is used. For external ready-manufactured half-wave dipoles, the balun is visually built-in to the antenna and provides a single-ended interface.

The half-wave dipole is an electrical antenna. This means that it is easily detuned by materials with a dielectric constant larger than 1 within its reactive near field. If, for instance, the housing of a device is in the reactive near field, the housing has to be present when the antenna is matched. The human body has a large dielectric constant of approximately 75. As a result, if an electrical antenna is worn on the body or held in the hand, it can be strongly detuned.