The performance of log periodic antennas is governed by a set of well-defined international and industry standards, primarily focused on ensuring consistent measurement of their key electrical characteristics. These standards, established by bodies like the IEEE and ANSI, provide the critical framework for manufacturers, test labs, and engineers to specify, verify, and compare antenna performance with accuracy and repeatability. Without these standards, claims about gain, bandwidth, or radiation patterns would be subjective and unreliable. The core standards are IEEE Std 149-1979 for test methods and IEEE Std 149-2021 for definitions, which outline the precise procedures for measuring an antenna’s performance in controlled environments, such as anechoic chambers, to eliminate reflections and interference.
One of the most fundamental parameters governed by these standards is the antenna’s impedance, which is typically designed to be 50 ohms. Proper impedance matching is crucial for maximizing power transfer from the coaxial cable to the antenna elements. A Voltage Standing Wave Ratio (VSWR) is the most common metric used to quantify impedance matching. A VSWR of 1:1 indicates a perfect match, but in practice, a VSWR of 2:1 or better across the operating band is considered excellent. This means that at least 90% of the power is delivered to the antenna. Standards dictate that VSWR be measured using a vector network analyzer (VNA) with a calibrated setup. The following table shows typical VSWR performance for a well-designed Log periodic antenna across its frequency range.
| Frequency Range (MHz) | Minimum VSWR | Maximum VSWR | Typical Performance |
|---|---|---|---|
| 200 – 400 | 1.1:1 | 1.8:1 | 1.5:1 |
| 400 – 800 | 1.2:1 | 2.0:1 | 1.6:1 |
| 800 – 1500 | 1.3:1 | 2.2:1 | 1.7:1 |
Gain and Radiation Pattern Standards
Gain, measured in decibels relative to an isotropic radiator (dBi), is a measure of how effectively the antenna directs radio frequency energy in a specific direction. The standards require that gain be measured using the comparative method, where the antenna under test is compared against a standard gain antenna with known performance. The radiation pattern, a graphical representation of the antenna’s radiation properties, is another critical aspect. Standards mandate that patterns be measured in both the E-plane (the plane containing the electric field and the direction of maximum radiation) and the H-plane (the plane containing the magnetic field and the direction of maximum radiation). For a log periodic antenna, the pattern should exhibit a directional “beam” with a well-defined main lobe and suppressed side lobes. The half-power beamwidth (HPBW), the angular width of the main lobe where the power is at least half its maximum value, is a key specification derived from the radiation pattern. A typical UHF log periodic antenna might have a gain of 6-8 dBi and an HPBW of 60-80 degrees.
Bandwidth and Frequency Response
The defining feature of a log periodic antenna is its wide bandwidth, often achieving a 10:1 frequency ratio. Standards govern how this bandwidth is defined and measured. It’s not just about the frequency range over which the antenna functions, but the range over which it meets all its specified performance criteria (VSWR, gain, pattern stability). The frequency response must be smooth, without significant dips or peaks that would distort signals. Measurements are taken at numerous discrete frequency points across the band to ensure compliance. For instance, a log periodic antenna specified from 800 MHz to 2.5 GHz would be tested at intervals of perhaps 50 or 100 MHz to verify consistent performance.
Front-to-Back Ratio and Polarization
The front-to-back ratio (F/B ratio) is a critical parameter for directional antennas, expressing the ratio of power radiated in the forward direction to the power radiated in the opposite direction (180 degrees). A high F/B ratio is essential for rejecting interference from the rear. Standards specify how this is measured from the radiation pattern data. A well-designed log periodic antenna should have an F/B ratio greater than 20 dB. Polarization, the orientation of the radio wave’s electric field, is also standardized. Most log periodic antennas are linearly polarized, with the elements parallel to the boom creating horizontal polarization. The standard requires that the polarization purity be measured, ensuring minimal cross-polarization (unwanted radiation in the opposite polarization).
Environmental and Mechanical Standards
Beyond pure electrical performance, standards also cover mechanical and environmental durability. These are often defined by standards like MIL-STD-810 for military equipment or IEC 60068-2 for industrial equipment. They ensure the antenna can withstand real-world conditions. Key tests include:
Vibration Testing: Simulates stresses encountered during transportation or when mounted on moving vehicles. The antenna must not suffer mechanical failure or degradation in electrical performance.
Temperature and Humidity Cycling: The antenna is subjected to extreme temperatures (e.g., -40°C to +85°C) and high humidity to test material integrity and connector corrosion resistance. Performance parameters like VSWR must remain within specification.
Wind Load and Survival: For outdoor antennas, standards specify maximum wind speeds the antenna can withstand without permanent deformation or failure. This involves calculations of surface area and force, often requiring the antenna to survive winds of 125 mph or higher.
Ingress Protection (IP) Rating: Standards like IEC 60529 define the level of protection against solid objects and water. An IP67 rating, for example, means the antenna is dust-tight and can be immersed in water temporarily.
Standard-Compliant Testing Setups
Adhering to performance standards requires a specific test environment. The gold standard is the anechoic chamber. Its walls, ceiling, and floor are lined with RF-absorbent material to create a reflection-free space that simulates an antenna operating in free space. The measurement distance must satisfy the far-field condition, which is typically calculated as 2D²/λ, where D is the largest antenna dimension and λ is the wavelength. For a large VHF antenna, this distance can be over 50 meters. The test setup also requires precision equipment: a vector network analyzer for S-parameters (VSWR), a spectrum analyzer with a tracking generator, and a positioner that rotates the antenna with high angular accuracy to plot the radiation pattern. Calibration of all equipment traceable to national standards is mandatory for valid results.