Installing BNC Connectors

February 2013

I have an endless list of personality faults...but there's one I finally overcame.

As I was aging (I'd NEVER use the term, "maturing") I utterly hated buying specialized tools.  I figured there was always a way to do it with a hand drill, a hacksaw, and a set of files.

I would bow in cases where there wasn't a way to do it with standard tools, like nicopress fittings.  But for a long time, I'd be hacking away at the workbench, cursing whoever designed the abomination I was trying to build or repair.

The cure was gradual.  A purchase of a cheap drill press opened a whole new world of easier drilling.  A gen-u-wide terminal crimper (vs. the $5 auto shop special) gave me reliable electrical connections.  A band saw, purchased JUST because of some thick stuff aluminum I had to cut, ended up being used on thinner and thinner material, on wood, on plastic, on foam, even on cardboard.

Now, my attitude has switched 100%.  "There's a custom tool for that? Here's my Mastercard!"

The latest has been in the realm of attaching BNC connectors to coaxial cable.  In the bad ol' days, I would do the fancy multi-step stripping of the ends by eye with my trusty pocket knife, buy solder-on type connectors that needed fiddly soldering, and messy screw-on clamps that left shreds of braid everywhere.

About a year ago, when I decided to redo the electrical system on my airplane.  The transponder moved about eighteen inches, and that left its antenna cable too short.  In the past, I'd been REAL lazy.  The local electronics parts emporium sold completed RG-58 cables for a fairly low cost.  The transponder and my comm radio both used this off-the-shelf cables.

But the shop no longer sold pre-built cables.  I'd have to build my own.  Imagine my joy in discovering that $60 worth of custom tools solves all that?

Basics

The Cable

What kind of coaxial cable should you use?  For the vast majority of cases, you'll use RG-58.  Almost all aircraft devices that use RF energy require cables with a 50-ohm characteristic impedance, like RG-58.  There are other 50-ohm cables out there, with lower loss and better shielding, but RG-58 is probably good enough for the typical Fly Baby coax run.

The "50 ohms" by the way, is set by the spacing between the center conductor and the outer shield.  At radio frequency (RF), the signal sees 50 ohms of resistance no matter the length of the cable.  There are other impedances, of course...RG-59 is used for TV cables and antennas, and it has 75 ohms impedance.  But you MUST use RG-58 or another 50-ohm cable for your airplane.

The next decision to make is whether to select RG-58 which has a stranded center conductor, or one with a solid copper wire.  The stranded is RG-58A (or A/U).  Stranded is more flexible than solid, and is more durable over the long run (since the center conductor flexes easier).  On the negative side, connectors for stranded are a bit harder to come by.

WHERE you get your cable might be just as important.  Note the picture below...the top cable is a stranded unit bought from an electronics store that caters to professionals, and the bottom is a solid one bought at a "big box" electronics store.  Note how wimpy the outer braid is on the solid cable.  It probably works...OK....but obviously, the denser weave of the upper cable would be better.  Probably, if I'd bought a solid cable from the professional store, it probably would have had a better braid.

Finally, buy LOTS of cable.  Do you need only six feet?  Buy 25.  The stuff is cheap enough, and you'll never know if you'll want to change the run later and need to make another cable.

The Tools

Two tools will make installing connectors a snap.  They are a coaxial cable stripper, and a coax cable crimper.

The stripper is easy to find... Radio Shack carries them, and they're only $15 or so.  It's part number 278-248.  There are numerous other brands available.  If you can, find a "three step" unit...there's another cut the Radio Shack stripper doesn't make.  It's easily worked around (see below) but if you're going to spend the money, might as well get a tool that does everything.
cable stripper
CrimperSomething to be aware of is that these strippers are designed to work with a variety of cable, so there are some adjustments to make.  You can see the little "X" dial turned to "8" for RG-58 cable.  In addition, the internal cutters will have to be manually positioned to cut the right dimensions.  More on that later.

The second thing is a BNC Cable Crimping tool.  Make sure you get one that covers RG-58 cable.  You also want the kind that ratchets, just a like a professional-quality terminal crimper.  The ratcheting makes sure the fitting is completely crimped; it doesn' let the tool turn loose until it's all the way down.

You can get these from a variety of sources.

Connectors

The standard cable connection used in most aviation radios is a BNC male plug.  "BNC" stands for "Bayonet Neill–Concelman", with the "Bayonet" refering to the little studs on the side of the female end of the barrel that slides into and locks on the plug.

BNC connectors are widely available, but you MUST buy 50-ohm connectors to match the cable.  Also, you must buy the connectors for either stranded or solid conductor coax, depending on what kind you're using.

And of COURSE make sure to select the crimp-on kind.  And buy enough so that you're not scrambling to find them if you suddenly find you need to make another cable.

The Process

OK, let's go through the process of installing the connectors.

Cable
            strippingStripping the Cable

First step?  Making sure you've got some spare cable to practice on.

Second step is to set up your cable stripper to cut the right dimensions for the connectors you're using.  The diagram to the right illustrates the typical coax make-up.  The black is the outside shell of the coax, the yellow is the braided shield, the white is the dielectric (insulator) around the center conductor, and the copper-colored bit is the center conductor itself.

The dimensions cause some discussion.  The figure to the right shows two dimensions, one by a connector provider, and the other some "real world" experience by an RV-10 builder.  I kind of like the fact that the "real world" values are easier to check.  But as you can see, there's some variation.

The "C" dimension, especially, isn't critital.  It's the amount the center conductor sticks out past the dielectric, is manually trimmed later.

You'll need to set up your cable stripper to approximate these dimensions.  The instructions are included with the Radio Shack unit.  You remove the pin that the cutters pivot on, and reposition the blades to the desired positions.

Once the cutters are in the right place, adjust the depth of cut.  The Radio Shack tool includes a small Allen wrench that fit hex sockets in the bottom of the tool.  One socket controls the depth of the cutter that cuts the outer shell, braid, and dielectric away from the center conductor.  The other one controls the cutter that strips the outer shell from the top of the braid.

I like to adjust both so they both barely FAIL to cut all the way through.  They just leave a thin skin of plastic holding the pieces together.  This way, you run less chance of scoring the center conductor or cutting the braid free.

To cut, hold the tool in your right hand with your thumb atop the lever that opens the jaws.  Open the jaws, and insert the cable into the left side of the tool so that the end comes out on the right side, and let the jaws close again.  The amount the cable sticks out on the right side is not critical; you're going to probably be trimming away center conductor anyway.

Hold the cable on the left side of the tool.  Stick your finger in the hole in the end of the tool, and twirl it around the cable...four times to the left, four times to the right.

The open the jaws and remove the cable.  You should see two scored lines around the cable.  The black outer shell and braid should remove easily from the scored area closest to the end of the cable.  The outer shell should still be barely holding on for the other section... use a pliers or set of side cutters to gently tug it free.

One problem you'll note is that the Radio Shack cable stripper only makes two cuts...not the three that the standard diagrams show.  As you see above, the connector companies want the braided shield trimmed back about 1/8" from the end of the dielectric.  You can fluff up the braid a bit and shorten a bit with a nail trimmer.  This is where the "three step" cable stripper is an advantage.

Center PinInstalling the Center Pin

Next step is to install the center pin, or "Male Contact" as it's sometimes called.  The key point here is that the center conductor must be long enough to go all the way inside the pin, but the open end of the pin must butt up against the white dielectric.  A couple of test fits are usually necessary, and the end of the center conductor can just be trimmed up with a set of side cutters.

How do you fasten the pin?  With the crimper.  Here's an extreme close-up of the business end.  Note the teeny little hole between two of the bigger ones.  This is set up to take the center pin.  Stick the pin tip-first into the tool, and slowly close the tool until the pin is just held (notice that it ONLY crimps in a short area inside the tool.  Make sure the cable is inserted fully into the hole, then compress the tool the rest of the way.
crimper

sleeve and
        bodyAdd the Sleeve and the Body

The sleeve is a metal tube that came with the connector.  Slide it over the end, and slide it down out of the way on the cable.

Open the end of the braid a bit, and make sure there aren't any stray strands going towards the tip that might make contact with the center conductor.

Then, slide the main body of the connector over the center dielectric, UNDER the braid.  The end of the body is a knurled tube which is the part that goes under the braid.  Again, make sure you don't have any stray strands going under the knurled section.

Crimp the Connector

Ready to CrimpSlide the sleeve back up towards the end and over the braid.  Note the phantom view to the right... the braid is between the sleeve and the knurled section of the connector.  Using the 0.213" hole on your crimp tool, crimp the sleeve down against the cable and connector.  It's important that it's crimped against both the open braid section (for the electrical connection) and the bit of the outer protector (for the mechanical connection.  I crimp close to the head of the connector, then do another (if necessary) at the opposite end.

Last step, when both ends are on:  Use a ohmmeter to ensure that the electrical connections are proper.  Make sure the center pins aren't shorted to the shell, and that both ends are electrically connected.

Ron Wanttaja


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