Measurements in 75 Ohm Coaxial Transmission Lines Using a 50 Ohm VNA

One application of a vector network analyzer (VNA) is the measurement of 75 Ω coaxial transmission lines. In this article, we will discuss making these measurements using the most common type of VNAs, which have 50 Ω test ports, in conjunction with 50Ω‐to‐75Ω Minimum Loss Pads (MLP), i.e. impedance‐matching attenuators with insertion loss of 5.7 dB. As 50Ω VNAs we suggest using the VNAs manufactured by Copper Mountain Technologies: PLANAR TR1300/1, PLANAR 304/1, and PLANAR 804/1 (as of 2012). The use of an MLP affects accuracy of measurements by changing the calibration error and, depending on the location of the attenuator in the measurement circuit, impacts stability of measurements related to test cable bending.

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