Rigid Coax to Get Size Makeover - Radio World

Rigid Coax to Get Size Makeover

The point of the effort is to standardize line sizes to ease compatibility among different products made by manufacturers, keeping possible future applications in mind.
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BRIDGTON, Maine :Faced with a higher occurrence of ill-fitting rigid coax connections, most major U.S. manufacturers of coaxial broadcast products are backing a change in Electronic Industries Alliance standards intended to limit the problem, which often directly affects broadcasters by delaying radio and TV projects and presumably adding to costs.

The companies are banding together to amend EIA rigid coaxial transmission line standards — RS-225 and RS-259 — that haven’t been updated in nearly 30 years. Members of an EIA subcommittee working on the revisions call the standards “obsolete and outdated.”

The point of the effort is to standardize line sizes to ease compatibility among different products made by manufacturers, keeping possible future applications in mind.

While the majority of new FM transmission installations in this country now use some version of semi-flex coaxial transmission line, demand for rigid coax has been up recently with the increased consolidation of FM radio stations into multiplexed antenna and transmission systems, projects that almost exclusively use rigid coax transmission line, industry experts said.

Single FM transmission projects typically call for 3- or 4-inch rigid coaxial. Larger rigid coax, 6 inch and larger, is used for multiplexed FM antenna sites and in nearly every high-power TV installation, industry experts said.


(click thumbnail)Adam Jones of Shively Labs, with Ansoft HFSS and Autodesk’s Inventory software on his computer screen.It’s in those larger dimensions of rigid coaxial transmission line — 6 inch and over — that most of the incompatibility issues are occurring, said Adam Jones, director of manufacturing for Shively Labs.

“Having coax connectors mate properly has been a major issue throughout my time in the business,” Jones said. “I’ve had a fairly easy time convincing [other coaxial manufacturers] that we need a revised standard.”

New standards

Jones specifically mentioned Electronics Research Inc., Dielectric Communications and Myat Inc. as “being supportive of updated rigid coax standards” and being active in the standards process. Representatives of Jampro Antennas/RF Systems Inc., Radio Frequency Systems, Bird Electronic Corp. and Altronic Research Inc. have also been involved with EIA subcommittee meetings, Jones said.

Still to be determined is exactly what the new standards will look like and when the changes could take effect, he said.

Jones, committee chair for the EIA CE 4.1 Rigid Coax Subcommittee, said the group is working through specifications and determining ways to limit the impact of new standards on manufacturers and broadcasters.

“I do not expect these revisions to result in broadcasters seeing a price increase. I can’t speak for all of the manufacturers. There will be some non-reoccurring engineering cost and manufacturing costs that the companies will likely just eat,” Jones said.

A larger issue, he said, is the rising price of copper and other metals, which adds to the cost of transmission projects.

Industry experts say interface issues can occur in both semi-flex and rigid coaxial systems when trying to connect RF components using bolted flanges and inner connectors.

“One of the more fundamental ‘fit-up’ issues is with manufacturers who make slightly different inner and outer tube diameters for the same nominal line impedance,” said Dean Spooner, transmission line engineer, manager for ERI.

(click thumbnail)Dean Spooner of ERI
Spooner said current standards simply define a proper mechanical interface of coaxial flanges and tubes.

“They do not define a proper electrical interface. Neither do they define inner connectors or address thermal expansion differentials,” Spooner said.

Specifically, he often sees connection issues with components such as reducers and directional couplers as well as other specialty interfaces such as antenna inputs and filter or combiner inputs and outputs.

Compatibility

Stephen Kolvek, director of coaxial products for Myat, said, “The purpose of updating the standard is not to redesign rigid coaxial components. It is to standardize line sizes and make some minor corrections to existing designs. Compatibility is one of the committee’s concerns.” Myat manufactures transmission coaxial and a number of other RF products. It and the other vendors are seeking to control the future of rigid coaxial products, Kolvek said.

“If we have the updated standard, that would mean the end users would see systems with clearly marked compatible components.”

Officials at Dielectric Communications believe the most important aspect of standardization is to attempt to anticipate future applications and provide guidance on a standard design procedure in order to avoid fit-up problems.

“Even more new sizes may be appropriate for new applications. A standard that anticipates this can help in the development of new services for our industry,” said Kerry Cozad, senior vice president of broadcast engineering for Dielectric.

“The use of standards will help avoid misunderstandings that can cause significant issues during installation and operation of transmission systems that utilize larger rigid coaxial lines.”

The rigid transmission coaxial subcommittee hopes to address more than just fit-up issues, Jones said. The group also wants to standardize the calculations manufacturers use when marketing their rigid transmission coaxial products, such as “the power rating for rigid transmission line or the attenuation. Right now each manufacturer uses their own general formula,” he said.

“They are all based on basic physics, but one might be using a different safety factor when they rate their coax. Or one might be using a slightly different number for the connectivity of the copper they use.”

The CE 4.1 Rigid Coax Subcommittee hopes to meet this fall at the IEEE Annual Broadcast Symposium in Alexandria, Va., held Oct. 15 to 17.

Jones said his committee is interested in broadcaster feedback and participation in the standard setting process. Interested parties may contact him at Shively Labs.

The subcommittee intends eventually to vote to approve wording in the new standards before turning finalization over to EIA.

“There is a public review and discussion period before anything is adopted. To get the full standard adopted takes some time. We expect to be done with [subcommittee] work by next summer.”

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