Issue link: https://resources.mouser.com/i/1541351
57 | different field zones. Each must be carefully applied to reduce errors, while several of these approximations are used only for specific applications or antenna types. The following equations show selected far- field region transition point approximations, where r is the distance to the far-field region, lambda (λ) is the radiation wavelength, D is the largest antenna dimension, and is the maximum antenna length: , where is much larger than , where a transmitting antenna is less than 0.4 of the receiving antenna , to match a wave impedance of 377Ω , used for high-accuracy antennas , satisfies the Rayleigh criterion 1 Basics of RF Signal Measurements RF signals, or conducted measurements, use conductive pathways, mainly transmission lines and waveguides, to transmit or receive test signals (Figure 3). Using conductive measurement methods means that, ideally, all the signal energy is contained within the interconnect, test system, and DUT or SUT. Unlike free-space measurements, where only a fraction of the field energy is exchanged between the test system and test target, conducted measurements can more efficiently exchange energy between the test system and test target, as well as avoid external interference, depending on the loss of the interconnect and the quality of the shielding. Most test equipment uses conductive pathways for some portions of the test apparatus, such as the interconnect and circuit paths from a probe to the transducer. If an RF system or DUT is designed with externally accessible test points, engineers can use conductive probes or an RF interface and interconnects with the exposed test points. This method is often used for quality control and verification testing or tuning RF circuits within a system that is inaccessible via external ports. Figure 3: A schematic of one of the ports of an on-wafer S-parameter VNA test setup. (Source: NASA, redrawn by Mouser Electronics) 2