BNCs: 50 Ohm vs. 75 Ohm

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The traditional usage of 50 Ohm BNC connectors on 75 Ohm cable in analog video and analog telephony central office equipment has little distortion effect on the signal at frequencies below 300 MHz.

However, digital signals in video and telephony applications have necessitated the usage of 75 Ohm connectors with 75 Ohm cable. Mismatched connectors cause attenuation of the digital signal resulting in slower rise time of square waves. This distortion of the signal can cause transmission errors. Also, impedance mismatched connections cause reflections of the signal returning to the source known as return loss. Perhaps one mismatched connection will not have a noticeable effect on system performance, but multiple mismatched connections in the link between the source and destination have a cumulative effect causing possible distortion of the transmitted signal.

MilesTek BNC FAQs

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Is the quality of MilesTek BNC’s the same as other brands?
Depending on the brand, MilesTek BNC’s are equal to or exceed the quality of other competitors. For example, the MilesTek BNC plug bodies are machined from virgin brass in accordance with MIL-C39012 and then plated with bright nickel. Many other less expensive brands have cheaper die cast bodies. Furthermore, the MilesTek BNC body incorporates a beryllium copper wave washer to allow the BNC to flex during mating and to provide firm uninterruptible continuity of the shield signal. Teflon, the best dielectric, provides the proper dielectric for 75Ω impedance and securely holds the captive center pin.

How is your BNC center pin different than some other brands?
In order to be true 75Ω impedance, the center pin has the same diameter in the crimp area as the mating area. Our captive BNC pins are plated with 30 µ” of gold in accordance with MIL-G-45204. More than 30 µ” is unnecessary for normal use. Some competitive brands have a 75Ω body with 50Ω center pin to eliminate the necessity of a separate center pin crimper. If tested, the connector with 50Ω pin and 75Ω body produces an impedance of approximately 68Ω .

Should the BNC body have enclosed beryllium copper spring fingers?
One manufacturer promotes this idea. Unless the BNC is subject to abuse such as thousands of matings and foreign objects poking into the interface of the connector, this design is unnecessary.

What about cable pull strength?
Pull strength is important on small diameter cables such as 735A. MilesTek BNC plugs have diamond knurling for maximum cable retention. Out of 3,000 to 4,000 coaxial cable assemblies, MilesTek’s BNC’s for 735A recorded pull test performance of 49.3 Ibs. The minimum specification is 30 Ibs.