On fake 50 ohm coaxial cables
I guess by now everyone is already aware that cheap no-name items often aren't what they claim to be. I've recently found out that BNC coaxial cables sold as having a 50 Ω characteristic impedance often are in fact 75 Ω. Since this is something that is not trivial for the customer to measure and a 75 Ω cable will sort of work even in a 50 Ω system I guess it's easy for sellers to get away with this kind of scam.
This article goes into the details of how to measure characteristic impedance of a transmission line. I used the Smith chart method. I connected each cable to the (50 Ω) NanoVNA CH0 on one end and a Zref = 50 Ω calibration load on the other end. I then measured S11 over the frequency range from 10 to 200 MHz. NanoVNA was calibrated using the calibration kit that came with it.
I found the point where the S11 plot on the Smith chart crosses the real axis. This is when the cable acts as a quarter-wave transformer. The point on the chart marks the impedance Zt. It is marked in orange on the plots below. Characteristic impedance Z0 of the cable can then be calculated using the following formula:
The Seafront 100 cm BNC cable sold as "50 Ω Oscilloscope Test Probe Cable Lead" measured 71.1 Ω using this method. The second cable I got in the pair had a bad ground connection and couldn't be measured at all. The reviews on the Amazon product page mention bad mechanical construction of the connectors. The cable has a readable SYV 75--3--2 marking that already raises suspicions that this is indeed a 75 Ω cable.
The Goobay 50 cm BNC cable sold as 50 Ω RG-58 measured 79.6 Ω. This cable in fact has RG58 50 OHM COAXIAL CABLE printed on it. It was either marked incorrectly from the factory or was relabeled later. There is a review on the Amazon product page from R. Reuter that also reports that the cable they received was 75 Ω based on a NanoVNA measurement.
The 60 cm LMR-200 SMA cable that I got with the NanoVNA measures 50.4 Ω. I'm showing it here for comparison with the two results above. The cable construction is not that great - one of the two such cables I got in the kit wasn't crimped properly and the connector fell off the cable after a few uses. However the cable actually has the correct characteristic impedance within a reasonable tolerance.
I'm curious why this is happening. I'm guessing because 75 Ω cables are cheaper for some reason. Maybe there is a bigger demand for 75 Ω cables for video applications and 50 Ω used in RF systems is more of a niche application? Also the cables I measured didn't have good matching even for use in 75 Ω systems. Maybe they're in fact quality control rejects from a failed factory run? Anyway, it's good to be aware of this practice and double check if using a cable in an application that is sensitive to this sort of thing.
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Well there are simple physical reasons why 75-Ohm cables can be manufactured cheaper than 50-Ohm cables. The impedance of the cable mainly depends on the inductance and the capacity of the cable per unit of length, if we ignore ohmic losses and conductivity of the dielectric, we get Z = sqrt(L/C). Making a 50-Ohm cable requires a lower inductance and/or a higher capacity than a 50-Ohm cable. Reducing the inductance requires a thicker inner conductor (compared to the outer diameter) and therefore requires more copper (which is rather expensive). Increasing the capacity requires more dielectric material, so you need massive plastic instead of a cheap foam consisting of 90% air and only 10% plastic.
https://en.wikipedia.org/wiki/Coaxial_cable#Fundamental_electrical_parameters