A noncrimp technology for attaching a BNC connector to the cable provides better signal performance and a more secure connection.
By Bruce C. Hauver, Sr.
In the earliest days of electronics (circa 1940), before the transistor was invented, a new type of connector was developed for the burgeoning industry.
The connector interface was originally developed by the team of Bayonet, Neill and Councelman1 to provide a secure, yet easily operated interconnect mechanism for linking coaxial cables to the large cabinets that held vacuum tube electronics. The beginning letters of the developers' last names represented the designation given the new connector: BNC.
A primary feature of the original BNC connector design was its capability for quick connect and disconnect through the use of a sliding bayonet-lock mating system. From this basic design, BNC-style connectors have continued to evolve along with the electronics industry, progressing through many changes as they have been adapted to meet the demands of a range of coaxial interconnects, including video equipment, cable television (CATV) "headend" transmitters and local area network (LAN) Ethernet applications. It is in the CATV industry, however, that BNC-style connectors have undergone their most significant change, including cable-to-connector attachment methods for improving or simplifying the installation and improvements to the materials of internal and external connector components.
Early on, most coaxial type "F" male connectors used in CATV applications utilized some form of mechanical crimp connection method to attach the connector body to the cable in the "drop" portion of the CATV system (the cables that connect the main CATV "hardline" cable to individual residences). Although the type F male crimp-type connectors were easy for most technicians to use and required simple installation tools, the resulting connection was not always completely effective in sealing out the environment, allowing moisture and contaminants to enter the connector and eventually cause corrosion. Over time, this could degrade the performance of the connection and, ultimately, the CATV system itself.
Ingress and Egress
Approximately 15 years ago, the CATV industry became more aware of ingress and egress problems because of this type of corrosion in the drop system. Ingress is identified as signals entering the CATV system. It is caused when the F male and F female interface corrodes, the cable becomes damaged, the F fitting is loose or the F connector is improperly installed — all problems that allow signals to "enter" into the cable system. Egress is caused by the same basic problems, but the signals propagate in all directions from within the shield of the cable to the "outside" or exterior of the CATV system. The egress level is mandated by the Federal Communications Commission (FCC) to minimize interference. Both ingress and egress have impact on system performance, especially with today's CATV services such as cable modems and pay-per-view services.
To support the CATV industry's demand for improved environmental requirements, a new type F male drop fitting (the Snap-N-Seal) was developed by LRC.2 The technology represented an advancement over crimp-type connectors in that it featured a true 360° radial compression of the connector body around the cable for improved moisture and radio frequency interference (RFI) integrity. The 360° radial compression provides an even distribution of circumferential compression forces acting between the cable outside diameter surface and the connector inner diameter surface, while distributing this compression force over a given length.
The 360°compression maximizes the mechanical, electrical and environmental characteristics as compared to a hex-crimp design. The hex-crimp design incorporates six evenly spaced high-compression areas, and usually includes six very low-compression areas over a given length.
Instead of deforming the metal of the connector body around the cable's outer jacket to make a mechanical connection, as with a crimp-style connector, the design of the Snap-N-Seal compression connector uses a detachable plastic sleeve that slips over the outer jacket of the cable. A compression tool forces the sleeve into the body of the connector, providing the compression that effectively seals the connector around the cable. When this connector system is used in an exterior connection, such as in a typical CATV drop system, it results in a sealed system to prevent moisture from migrating into the connection and causing corrosion.
Return Loss
More importantly for the BNC-style connectors, the design of the cable-to-BNC interface provides reliable high-frequency return loss performance of the connector. As CATV systems have become more sophisticated, carrying not only video signals but Internet data and phone traffic as well, the frequency at which the systems are required to operate has increased almost to the Gigahertz level. High return losses at these frequencies (often caused by improperly installed connectors) can result in signal degradation. The connectors exhibit return losses of -30 dB at up to 1 GHz.
Many BNC connectors are typically used in the CATV headend — the equipment that processes the satellite, off-air and local origination "signals" for transmission through the CATV system. RF and digital signal integrity starts at the headend equipment, especially in high-bandwidth CATV systems.
The signal integrity at the headend is critical to ensure a quality signal is distributed throughout the cable system. Anything less than a quality signal only provides a less-than-desired signal quality throughout the CATV system.
Recently, the Snap-N-Seal-type interface was added to a series of BNC plug connectors for headend cable applications. Combining these two connector technologies resulted in a precision BNC connector that exhibits improved performance, while providing a reliable connection. The interface represents advancement in RFI integrity over previous clamp/crimp-type BNC connectors.
Making the Selection
Selection of a BNC connector requires some research by the end users. Factors to consider include:
- Application (equipment)
- Cable type (based on manufacturer and/or cable construction)
- Connector design advantages and materials (based on manufacturer)
- Number of loose connector pieces (dependent on specific manufacturer designs or series)
- Ease of installation (including craft sensitivity)
- Overall performance (again, dependent on manufacturer designs or series)
A thorough review of the requirements will help users to choose a connector designed to meet their application, installation and performance needs.
References
1. Allen Nemetz, "A Designer's Guide to RF Connector Selection," RF Design, September/ October, 1980.
2. LRC, formerly a division of the Augat Communications Group, is a part of the Thomas & Betts Cable Communications Div.
BRUCE C. HAUVER, SR. is Product Manager — LRC Drop Products, Thomas & Betts Corp., 8155 T&B Blvd., Memphis, TN 38125; (800) 920-0328; Fax: (901) 680-5999; Web site: www.tnb.com.
SPEC SHEET
End Applications:
CATV headend signal processors and modulators, signal combiners, video equipment, high-density patch panels, LAN Ethernet applications
Related Products:
Coaxial cable and connectors
Main Point:
Many different versions of BNC connectors are used in CATV, video and Ethernet applications. A noncrimp technology for attaching a BNC connector to the cable provides simplified installation and a more secure connection. Factors to consider in the selection of a BNC connector include the application, cable type, connector design advantages and materials, number of loose connector pieces, ease of installation and overall performance.
