Antenna measurement techniques refer to the testing of antennas to ensure that they meet specifications or simply to characterize them. Typical antenna parameters include gain, bandwidth, radiation pattern, beamwidth, polarization, and impedance. These parameters are essential for effective communication.
The antenna pattern is the response of the antenna to a plane wave incident from a given direction or the relative power density of the wave transmitted by the antenna in a given direction. For a reciprocal antenna, these two patterns are identical. A multitude of antenna pattern measurement techniques have been developed. The first technique developed was the far-field range, where the antenna under test (AUT) is placed in the far-field of a range antenna. Due to the size required to create a far-field range for large antennas, near-field techniques were developed, which allow the measurement of the field on a distance close to the antenna (typically 3 to 10 times its wavelength). This measurement is then predicted to be the same at infinity. A third common method is the compact range, which uses a reflector to create a field near the AUT that looks approximately like a plane-wave.
Far-field range (FF) The far-field range was the original antenna measurement technique, and the simplest; it consists of placing the antenna under test (AUT) a long distance away from the instrumentation antenna. Generally, the far-field distance or Fraunhofer distance, is considered to be
where is the widest diameter of the antenna in any direction, and is the wavelength of the radio wave. Separating the AUT and the standard receiving antenna by this distance reduces the detectable phase variation across the AUT enough to obtain a reasonably accurate estimate of the antenna pattern in the far distance.
The IEEE antenna measurement standard (document i.d. IEEE-Std-149-1979), suggests set-up for measurement and various techniques for both far-field ranges and ground-bounce ranges (discussed below).
Near-field range (NF)
Planar near-field range Planar near-field measurements are conducted by scanning a small probe antenna over a planar surface. These measurements are then transformed to the far-field by use of a Fourier transform, or more specifically by applying a method known as stationary phaseAsymptotic Behavior of Monodromy, Springer Berlin / Heidelberg, 1991, to the Laplace transform . Three basic types of planar scans exist in near field measurements.
Rectangular planar scanning
The probe moves in the Cartesian coordinate system and its linear movement creates a regular rectangular sampling grid with a maximum near-field sample spacing of Δx = Δy = λ /2.
Polar planar scanning More complicated solution to the rectangular scanning method is the plane polar scanning method.
Bi-polar planar scanning The bi-polar technique is very similar to the plane polar configuration.
Cylindrical near-field range Cylindrical near-field ranges measure the electric field on a cylindrical surface close to the AUT. Cylindrical harmonics are used to transform these measurements to the far-field.
Spherical near-field range Spherical near-field ranges measure the electric field on a spherical surface close to the AUT. Spherical harmonics are used transform these measurements to the far-field
Free-space ranges
The formula for electromagnetic radiation dispersion and information propagation is:
where represents distance, power and speed.
The equation means that double the communication distance requires four times the power. It also means double power allows double communication speed (bit rate). Double power is approximately 3 dB increase (or exactly ). Of course, in the real world there are all sorts of other phenomena which complicate the estimated delivered power, such as Fresnel canceling, path loss, background noise, etc.
Compact range A Compact Antenna Test Range (CATR) is a facility which is used to provide convenient testing of antenna systems at frequencies where obtaining far-field spacing to the AUT would be infeasible using traditional free space methods. It was invented by Richard C. Johnson at the Georgia Tech Research Institute. The CATR uses a source antenna which radiates a spherical wavefront and one or more secondary reflectors to collimate the radiated spherical wavefront into a planar wavefront within the desired test zone. One typical embodiment uses a horn feed antenna and a parabolic reflector to accomplish this.
The CATR is used for microwave and millimeter wave frequencies where the far-field distance is large, such as with high-gain reflector antennas. The size of the range that is required can be much less than the size required for a full-size far-field anechoic chamber, although the cost of fabrication of the specially designed CATR reflector can be expensive due to the need to ensure precision of the reflecting surface (typically less than surface accuracy) and to specially treat the edge of the reflector to avoid diffracted waves which can interfere with the desired beam pattern.
Compact radiation measurement systems For the development and verification of small antennas in wireless and IoT devices, compact radiation measurement systems have been developed that operate in a standard laboratory environment without a dedicated anechoic chamber.
These systems use a narrow-beam measurement antenna combined with narrow receiver bandwidth to suppress environmental reflections, enabling measurement of radiation patterns, total radiated power (TRP), antenna gain, efficiency, and effective radiated power (ERP) in frequency ranges used for wireless communications. A typical system can operate in a room as small as 4 × 4 × 3 m, covering frequency bands used by WiFi, Bluetooth, Zigbee, LoRa, and cellular IoT devices.
Elevated range In an elevated range, the AUT and the measuring antenna are both mounted several wavelengths above ground level as a means of reducing the interference from waves reflected off the ground.
Slant range In a slanted range, the receive antenna is mounted higher above ground than the AUT is, either by having the earth surface of the range sloping downward from the AUT mount, or by placing the receive antenna on a much higher mast. The sloping earth (either actual or effective) serves as a means to eliminate or reduce interference from symmetrical wave reflection, by angling the reflected waves to bounce underneath the receive antenna. In theory, the same technique could be applied in reverse, to bounce most of the earth-reflected waves above the receive antenna.
Antenna parameters
Except for polarization, the SWR is the most easily measured of the parameters above. Impedance can be measured with specialized equipment, as it relates to the complex SWR. Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to put the sensor into the antenna's far field, or an anechoic chamber designed for antenna measurements), careful study of experiment geometry, and specialized measurement equipment that rotates the antenna during the measurements.
Radiation pattern
The radiation pattern is a graphical depiction of the relative field strength transmitted from or received by the antenna. Apart from the mainlobe, it shows sidelobes and backlobes. As antennas radiate in space often several curves are necessary to describe the antenna. If the radiation of the antenna is symmetrical about an axis (as is the case in dipole, helical and some parabolic antennas) a unique graph is sufficient.
Each antenna supplier/user has different standards as well as plotting formats. Each format has its own advantages and disadvantages. Radiation pattern of an antenna can be defined as the locus of all points where the emitted power per unit surface is the same. The radiated power per unit surface is proportional to the squared electrical field of the electromagnetic wave. The radiation pattern is the locus of points with the same electrical field. In this representation, the reference is usually the best angle of emission. It is also possible to depict the directive gain of the antenna as a function of the direction. Often the gain is given in decibels.
The graphs can be drawn using Cartesian (rectangular) coordinates or a polar plot. This last one is useful to measure the beamwidth, which is, by convention, the angle at the −3dB points around the max gain. The shape of curves can be very different in Cartesian or polar coordinates and with the choice of the limits of the logarithmic scale. The four drawings below are the radiation patterns of a same half-wave antenna.
Efficiency
Efficiency is the ratio of power actually radiated by an antenna to the electrical power it receives from a transmitter. A dummy load may have an SWR of 1:1 but an efficiency of 0, as it absorbs all the incident power, producing heat but radiating no RF energy; SWR is not a measure of an antenna's efficiency. Radiation resistance is the part of the resistance to current caused by power lost to radiation by an antenna. Unfortunately, it cannot be directly measured but is a component of the total resistance which includes the loss resistance. Loss resistance is the result of power lost to heat in the antenna materials, rather than to coherent radio waves, thus reducing efficiency.
Efficiency () is defined as the ratio of the power coherently radiated as radio waves () to the total power used by the antenna, which is the sum of the power radiated coherently () and the power radiated as heat ():
The antenna efficiency is also mathematically equal to the radiation resistance () divided by total resistance (real part of the impedance measured at the voltage node, which is often the feed-point):
Bandwidth
IEEE defines bandwidth as "The range of frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard." In other words, bandwidth depends on the overall effectiveness of the antenna through a range of frequencies, so all of these parameters must be understood to fully characterize the bandwidth capabilities of an antenna. This definition may serve as a practical definition, however, in practice, bandwidth is typically determined by measuring a characteristic such as SWR or radiated power over the frequency range of interest. For example, the SWR bandwidth is typically determined by measuring the frequency range where the SWR is less than 2:1 . Another frequently used value for determining bandwidth for resonant antennas is the return loss value, since loss due to SWR is
Directivity
Antenna directivity is the ratio of maximum radiation intensity (power per unit surface) radiated by the antenna in the maximum direction divided by the intensity radiated by a hypothetical isotropic antenna radiating the same total power as that antenna. For example, a hypothetical antenna which had a radiated pattern of a hemisphere (1/2 sphere) would have a directivity of 2. Directivity is a dimensionless ratio and may be expressed numerically or in decibels (dB). Directivity is identical to the peak value of the directive gain; these values are specified without respect to antenna efficiency thus differing from the power gain (or simply "gain") whose value is reduced by an antenna's efficiency.
Gain
Gain as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation in all directions equally, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the Gain or Power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by an hypothetical isotropic antenna: