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Coaxial Cables
This page was developed by Mike Morris WA6ILQ from a number
of emails posted by Eric Lemmon WB6FLY (now SK) on the Repeater‑Builder Mailing List
Thanks go to Eric for permission to present this information.
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Some Notes on Double-Shielded Coaxial Cables This page was developed by Mike Morris WA6ILQ from a number of emails posted by Eric Lemmon WB6FLY (now SK) on the Repeater‑Builder Mailing List Thanks go to Eric for permission to present this information. |
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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,
RG‑58 and similar 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 that I'd use as a sample in my repeater talks). In
addition, any cable that has dissimilar metals pressed together will eventually create duplex
grunge. For example, Belden 9913 has an aluminum foil shield rubbing against a copper
braid... and a bit of moisture and you have aluminum oxide rubbing against copper oxide: you
have diodes (grunge generators) inside your coax cable...
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.
If you have to use coax then look at RG‑393 or RG‑214 for the large diameter (i.e RG‑8 / RG‑213 size) and RG‑142 or RG‑400 (RG‑58 size). All are double shielded with silver plated shields and will not cause duplex noise. A bonus: RG‑400 can use the same connectors as RG‑58.
RG‑316 is an interesting special-purpose Mil-Spec (M17/113) cable (but it's single-shield): it's about 1/10 of an inch (2.6mm) in diameter. It's a high loss (8‑9db per 100 feet at VHF, and 16db per 100 feet at UHF), but it will fit in locations where nothing else can.
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 hand and power 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. I've been to mountaintop radio repeater sites that were built in the 1960s and 1970s and the original Phelps Dodge antennas and Heliax feedlines are still in service. 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 can be intermod generators). If you absolutely have to use a coax adapter make sure it's a silver plated adapter.
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... the LMR series cables
have their own set of problems). 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 19-strand 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
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