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Some Notes on Double-Shielded Coaxial Cable This page was developed by Mike Morris WA6ILQ from emails posted by Eric Lemmon WB6FLY on the Repeater‑Builder Mailing List Thanks go to Eric for permission to present this information.

Some comments from Mike Morris WA6ILQ:
The type of cable you use in your repeater system can have a huge effect on the performance.   Many hams that come to the repeater environment from the HF world are used to RG-8 and RG-58 cables.   Well, those loose-weave-braid cables just don't work in a duplex environment, and besides both of the Mil standards (the "RG") have been retired and just about anything can be sold as "RG-58" or "RG-8" (I used to have a length of RG-8 that had maybe 80% braid coverage).   In addition, any cable that has dissimilar metals pressed together will create duplex grunge.   For example, Belden 9913 has an aluminum foil shield rubbing against a copper braid.   In comparison, RG-214 has two silver-plated braids and a silver-plated inner conductor for maximum noise rejection. Both RG-142 and RG-400 are a smaller diameter version of RG-214 but similar construction and will give good results.

My personal preference is to use nothing but Heliax, Superflex and Mil-Spec RG-214 on the antenna side of the duplexer, and nothing but Superflex, Mil-Spec RG-214 and RG-400 on the radio side.   Yes, it's expensive, but like my late father used to say about tools, "Buying quality only hurts once".   Good cable paired with good connectors are going to last a long, long, long time, and it's one less thing to worry about.   And when you do make cables, make them with the correct silver-plated connectors on each end, so that you do not have to use any adapters.   Use a right-angle connector rather than a normal connector and a right angle adapter.   All it takes is one chrome or nickel plated connector or adapter to ruin your day (chrome and nickel are both ferrous metals, and are intermod generators).

From: Eric Lemmon WB6FLY Subject: [Repeater‑Builder] Double-Shielded Coax In recent postings to the list there have been several references to the need for double-shielded coaxial cable when hooking up components within a repeater cabinet. Although RG‑142/U is widely used by repeater builders, perhaps a better choice for some applications is RG‑400/U (note that RG‑400 is NOT the "LMR‑400" cable). At first glance, RG‑142/U and RG‑400/U appear identical, with see-through brown jackets, but there is a significant difference between the two: RG‑142/U has a solid steel center conductor that is silver plated. RG‑400/U has a stranded copper center conductor, with each strand silver plated before twisting into the center conductor. Both types of cable have double, silver-plated copper braided shields, and a Teflon dielectric. Both types have similar RF characteristics. Because of its flexibility, RG‑400/U is better suited for bench test leads or to connect any repeater component that is mounted on a movable panel or on the inside of the cabinet door. When flexed repeatedly, the steel center conductor of RG‑142/U will fracture and become a noise generator. I personally have settled exclusively on crimp-type connectors made by RF Industries, and I swear by them. Whether N or BNC, I use only the silver-plated connectors with gold-plated center pins and Teflon dielectric. It helps to have the proper tools to strip and crimp each connection, and I am very pleased with the quality. 73, Eric Lemmon WB6FLY

From: "Eric Lemmon" Subject: RE: [Repeater-Builder] RG-214/U Genuine, MIL-C-17 RG-214/U coaxial cable has double concentric silver-plated copper shields. Several companies manufacture an RG-214 "TYPE" cable that is very similar, but without the silver plating. As you would expect, it's a lot cheaper than the genuine RG-214/U stuff. Such "TYPE" cable may also have less braid coverage than the genuine cable. Genuine RG-214/U has to meet the Mil Spec Number MIL-C-17. If the manufacturer is going to ignore one part of the specification (the silver plating) what is preventing him from ignoring another part (the outer jacket UV resistance, or the braid density)? If you don't see RG-214/U Mil Spec Number MIL-C-17 printed on the jacket, it's not real RG-214/U cable.

From: "Eric Lemmon" Subject: RE: [Repeater-Builder] RG-142, RG-400 va RG-223 I should have mentioned that RG-223 has a solid silver-coated copper center conductor, while RG-400 has a stranded silver-coated center conductor, which makes RG-400 better suited where flexibility counts. RG-142 has a solid steel center conductor that is silver coated and copper clad, but it should not be used where it will be flexed after installation. The big disadvantage of RG-223 is the power-handling capability. RG-223 is rated for just 86 watts at 400 MHz, while RG-400 (and RG-142) are rated for 1100 watts at 400 MHz. RG-223 has 50% greater attenuation at 50 MHz and 15% greater attenuation at 400 MHz. RG-223 will be okay in most applications, but the power-handling limitations and its attenuation should be considered. Use silver-plated connectors that are specifically designed for that cable, and avoid using any barrels or between-series adapters. 73, Eric Lemmon WB6FLY

Comments from Mike WA6ILQ (12-Nov-03): The above statements are only applicable when applied to true mil-spec RG cable. Beware of any "RG‑(nnn)-TYPE" or "RG‑(nnn)-LIKE" cable (where (nnn) is any coax number) as a cable labeled that way does not have to meet ANY specifications.   It may have a loose braid weave, or minimal UV resistance, no silver plating, or worse.   A cable labeled "RG‑142-TYPE" or "RG‑142-LIKE" may have a copper center conductor (which may or may not be silver plated).   "Real" RG‑142 has a silver plated steel center conductor - the edge of a knife can scrape the silver plating (if it's there), and the core is easily tested with a simple magnet. RF Industries main web page is at http://www.rfindustries.com Their connector page can be reached via the main page or at http://www.rfcoaxconnectors.com Eric's comment above that "When flexed repeatedly, the steel center conductor of RG‑142/U can fracture and become a noise generator" brings this article to mind: "Help!! I have a crackling noise in my repeater" ... well worth reading as to how duplex noise is generated... And it's not "can fracture" but "will fracture eventually". Personally, I have been using RG‑400 for a couple of years, have replaced many jumpers with it and noticed improved performance in many cases - RG‑400 is now my "standard" jumper cable inside the repeater cabinets - everything on the receive side of the duplexer, plus anything at 50 watts or less on the transmit side.   Note that RG-400 is spec'd at 9.6 dB of loss per 100 feet at 400 MHz.   This is lossy cable!   If you put 100 watts into a three-foot long jumper you will get about 93-94 watts out the other end, with a 6-7 watt loss in just THREE feet.   It will get warm!   It is jumper material, not feedline!   Despite this loss factor, it is my standard jumper cable for anything at 50 watts or less.   At higher power levels I use RG‑393 cable. RG‑223, RG‑400 and many others are available from many sources, including "The Wireman". His coax page is at http://www.thewireman.com/coaxp.html.   Their web site has on-line ordering with secure credit card payment.   I have purchased from them at past ARRL conventions and will patronze them again - in fact at the next convention I'll be ordering another 100' spool of RG‑400. Lastly, my mentioning "The Wireman" here is intended only as a individual expressing satisfaction with an individual vendor and is NOT to be taken as an endorsement by Repeater‑Builder (the web site) or Repeater‑Builder (the company).