BNC Cables 50 Ohm & 75 Ohm Distinctions, Applications, Connectors

50 Ohm and 75 Ohm are Two Distinct Types of BNC Connectors & Cables

  • BNC cables and connectors are manufactured in 50 Ohm and 75 Ohm specification versions.
  • 75-ohm cables/connectors are made for high quality digital video (CCTV) and are able to scale their output depending on the input received. 
  • 75-ohm cables can also be effectively used on the older analog video formats, which makes them more versatile and flexible - as they can be used in any situation.
  • 75 Ohm BNC cables/connectors are used when Low Loss of Signal is Especially Important.
  • 50-ohm cables/connectors work well with old analog video formats. If you are looking for a high quality video output, 50 Ohm will not give you a high quality level.
  • Connection of the two types of connectors is possible but not best practice: Mixing will not give you the best output.
  • 50 Ohm BNC connectors are used with 50 Ohm cables. 75 Ohm connectors are used with 75 Ohm cables. 

75 Ohm BNC applications are mostly in the fields of:

  • Satellite, high definition televisions, and cable TV receiver box.
  • AM/FM radio receivers.
  • Police scanners.
  • RG-179 coaxial cable uses the 75 Ohm BNC connector type and it is used in high temperature situations.

Applications of 75 Ohm BNC Cables using RG-179

  • RG-179 coaxial cable uses a 75 Ohm BNC connector.
  • Is specially designed for high temperature environments: With a TFE taped outer jacket. It can easily withstand in the temperature up to 200 degree Celsius.
  • RG-179 is widely used in such high temperature environment applications as:
    • Medical equipment in hospitals and clinics
    • Video security cameras for safety purposes
    • Audio security systems.

Other applications of BNC Cables by Coax Type

There are wide range applications of the BNC connectors. It has different applications according to the type of the connector.

  • RG-59 coaxial cable are used to transmit video signals from a CCTV camera to the monitor room. RG-59 coaxial cables use 75 Ohm BNC connector.
  • RG58 coaxial cable use 50 Ohm BNC connector and it has wide range of applications due to its performance. This cable is lower priced.
  • RG-174 coaxial cable also use 50 ohm BNC connector, it is very thin and has wide applications in GPS navigations antennas of the cars and many vehicles.
  • RG-213 coaxial cable is also has 50 Ohm BNC connector, and it used where high power handling is needed:  RG-213 can handle 1000 watts and more. RG-213 coaxial cable is the thickest coax of BNC coax types: It is  almost half an inch thick.

50 Ohm and 75 Ohm Cables: Differences / Distinctions

50 Ohm and 75 Ohm coaxial cables are identified according to their impedance measured in Ohms, the unit that measures electrical resistance. The radio frequency signals that are transmitted down these cables carry an alternating current (AC) rather than direct current (DC). With AC signals, both the magnitude and phase of the transmitted signal are countered and contained by the cable as it flows down its length. The impedance rating for coaxial cable therefore comprises:

  1. Resistance: the amount of resistance to the flow of the current.
  2. Inductance: the amount of voltage generated by the magnetic field of the electrical current.
  3. Capacitance: the amount of charge that is contained or retained within the cable while the current flows.

Coaxial cable is specifically designed to support signal transmission and is structured to balance resistance, capacitance, and inductance for consistent performance in radio frequency circuits. The composition of a specific coaxial cable will determine its impedance with the characteristic impedance of the cable down to the dielectric constant of the insulating layer and the radii of the outer and inner conductors.

Why 50 Ohm and 75 Ohm?

The use of 50 and 75 Ohms as standard characteristic impedances for coaxial cable for most radio frequency applications is essentially a compromise for optimal power handling and the lowest feasible signal loss. These key impedances were identified through extensive testing in the early 20th century. These experiments found though excellent voltage and power handling could be achieved with 30 Ohm cable, the lowest attenuation was achieved with 77 Ohm coax.

  • 50 Ohm coaxial cable would therefore have a good power handling profile as well as low attenuation. Over the subsequent decades, 50 Ohm coax had come to the fore as the primary solution for cable with good power handling, especially for 100 watts or more. It is often used for antenna cables in applications that involve transmitters and transceivers across amateur and broadcast radio, cellular and wireless networking.
  • 75 Ohm cable was preferable for applications that require low signal loss, capacitance, and signal distortion. It is the coaxial cable of choice for applications where an efficient signal transfer is needed with minimal loss. These cables are often used in applications where a connection is made to a receiver and primarily video applications, which are low power and do not require the power handling of 50 Ohm cable. Key applications include cable television, HDTV, and CCTV. 75 Ohm coax can also be used for coaxial digital audio making 75 Ohm coax capable of transferring audio, for example, in a home theater system.

75 Ohm cable and digital transmissions

75 Ohm cable is often perceived as a legacy cable type, but it has surprising utility and demand for transmission of digital signals. Digital signals have a distinctly square waveform compared to the smooth sine wave undulations of analog waves. Cables with high capacitance can distort the transmission of digital signals, diminishing the distinction between the stepped peaks and troughs, but 75 Ohm coaxial cable has been found to have great utility for digital transmissions as it has a lower capacitance than 50 Ohm coax and ensures that the whole of the digital transmission gets down the line with minimal loss and distortion. 75 Ohm coax is preferred for digital audio and video broadcast applications including:

  • Direct broadcast satellite dishes.
  • Over-the-air High Definition Television (HDTV) antennas.

Ensure the correct matching of cables and connectors

If self-assembling 75 Ohm cables, the BNC connectors must have the same impedance as the cable. If 50 Ohm BNC connectors are attached to a 75 Ohm cable, an impedance mismatch is created which leads to the reflection of energy back to the source and the generation of standing waves. A standing wave leads to signal loss and multiple reflections due to impedance mismatch will eventually degrade the signal and the image received until they are unrecoverable.

BNC 75 Ohm & 50 Ohm Connector

Types: Male & Female

BNC Connectors' Composition

  • 75 Ohm BNC connectors
    • Use teflon as the dialectric
    • Their outer spring fingers are directed by air.
    • Their pin has a very consistent diameter, and this applies both at the front and back. 
  • 75 Ohm BNC connectors
    • Use delirin in place of teflon.
    • The diameter of their center pins are not consistent: It tends to increase as you get nearer to the crimp region

BNC connector (Bayonet Neill–Concelman) is a miniature quick release and quick connect RF connector for terminating a variety of miniature to standard RG coax cables. The female connector has two lugs that latch positively or in reverse with a quarter turn to quickly connect or disconnect. Connectors largely have a brass body, they may feature gold plating for external use to increase durability, and nickel plating for terminating onto a soldered circuit board. The connectors are lightweight and reliable.

Terminating a coax cable to the connector can be done either by clamping or crimping.

BNC connectors come in a variety of 50 Ohms and 75 Ohms impedance, to accommodate termination with matching cable and equipment impedance requirement. 75 Ohms connectors are set to use up to 2 GHz frequency signals. 50 Ohm connectors conduct signals of up to 4 GHz. Both male and female connectors are slotted and use a plastic dielectric material that increases loss of signal above 4 GHz, this is the reason for their relatively moderate frequency rating. Both 75 Ohms and 50 Ohms connectors can be interchanged with no significant loss in performance, and with no damage to connectors.

The most remarkable feature of the BNC connectors is the ability to effortlessly disconnect and connect securely with just a quarter turn. This means devices can have easily retractable and reliable antennas or points of connection for rapid swapping.

BNC connectors are available for commercial, military and industrial use. BNC was originally developed for the military, it has now gained popularity for use in RF and video signal equipment. Some common applications of BNC connectors and cables are:

• Aerospace electronics
Antennas
• Automotive wireless devices
• Broadcast (75 Ω)
• Cable Modems
• Computer LAN equipment
• Medical Equipment
• Parallel and serial video signal devices
• Telecom

A 95 Ohms type of BNC cables and connectors also exists but is rarely used aside from glass displays in some aircraft. 

Posted by George Hardesty on 9th Dec 2020


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