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Threading Coax Cables

cdeerinck

Well Known Member
I would like to have three RG400 antennae cables made by Aircraft Spruce. They will have right angle BNC or TNC ends on them, but I won't be able to thread them through the various bulkheads with the ends on.

I am really hoping to avoid having to buy the tool to do the job, and have to learn how to do it well, for only 6 ends.

Will it be safe to un-assemble an end, route it through the fuselage, and then re-assmble it?

If that is a no-go, can I have them strip them, without putting one end on, route them, then put the end on without a custom tool?
 
It's really not that difficult. Watch the Stein video to see how it's done.

You're probably paying Aircraft Spruce $10 per BNC connector for making up the cable. That could pay for the ratcheting crimp tool and dies. Practice stripping on a scrap first and allow a few extra inches of cable in case you spoil the end and need to start over. You can also buy a tool that strips the coax, shield and centre insulation all at once.

Right angle male BNC connectors are quite nice but expensive. You can achieve the same result with a 90 degree adaptor and a straight connector with only a tiny signal loss.
 
Everything that Mark said above, plus no, you can't disassemble and re-assemble an end (at least not a good, crimped end).

Paul
 
I see Radio Install instructions mentioning no sharp bends. But right angle plugs are certainly sharp bends.

Is the advice to prevent weakening the cable or about the radio signal not liking sharp corners?
 
Chuck, you can borrow my crimping tool any time!

Bruce - Aren't you overdue for another visit? If you fly in to Palomar, I can pick you up, show you where I am at with the build, and take you back for lunch at The Landing. That is one of the perks of being a Tech Counselor! Just say the word, and tell me when.
 
I see Radio Install instructions mentioning no sharp bends. But right angle plugs are certainly sharp bends.

Is the advice to prevent weakening the cable or about the radio signal not liking sharp corners?

It's more complicated. The shield is braided and can easily stretch, but the center conductor cannot. With a sharp bend, that center conductor is under tension, it wants to 'cut the corner'. The dielectic is plastic-like, and, over time, the center conductor will slowly move thru the dielectric, moving closer to the shield. With the center wire off-center, the impedance is no longer 50 ohms. RF signals reflect off that impedance bump, reducing the energy transmitted.

The well made right angle connectors are carefully engineered to avoid any changes in impedance along the line, and they are solid metal-nothing can move over time.
 
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There are connectors that can be disassembled and threaded back together, and they were the only type used by the military for years (or better yet, newly assembled on to the end of a cut piece of coax.) The original mil spec UG-88 connector (BNC male, for RG-58, RG-400 and friends) is typical. You need only a thin wrench, a razor blade and a soldering iron. There are right-angle ones, that maintain constant impedance in the angle (unlike bending coax, as Bob Turner mentions.)

But -- hardly anyone uses them anymore because they are expensive and people seldom have the patience to trim the coax and comb out the braid just precisely to use them. And they don't work well with low loss foam dielectric cable (actual RG-58 and RG-400 are fine.)
 
It's more complicated. The shield is braided and can easily stretch, but the center conductor cannot. With a sharp bend, that center conductor is under tension, it wants to 'cut the corner'. The dielectic is plastic-like, and, over time, the center conductor will slowly move thru the dielectric, moving closer to the shield. With the center wire off-center, the impedance is no longer 50 ohms. RF signals reflect off that impedance bump, reducing the energy transmitted.

The well made right angle connectors are carefully engineered to avoid any changes in impedance along the line, and they are solid metal-nothing can move over time.

Not questioning you, just trying to learn, where are you measuring 50 OHM? when I made my cables, the center conductor was .03 OHM? when measured end to end center. RG 400 center is a stranded core center conductor? ya?
 
Bret, you are measuring DC resistance, not RF impedance. The 50-ohm value stated earlier is an impedance value measured at our desired RF frequencies. However since antenna cable is a solid or stranded wire, just like your power cables, it will measure nil DC resistance just as you found.
 
Use an adapter?

Why not use cables with straight BNC ends, and then add 90 degree adapters as required? You can purchase adapters from L-Com.com. They make cable assemblies, too. (I used RG-142B/U with the solid steel center conductor on my plane)
 
Bret, you are measuring DC resistance, not RF impedance. The 50-ohm value stated earlier is an impedance value measured at our desired RF frequencies. However since antenna cable is a solid or stranded wire, just like your power cables, it will measure nil DC resistance just as you found.

Aw haa.....gotcha, impedance....wait, what? please continue :rolleyes:
 
Aw haa.....gotcha, impedance....wait, what? please continue :rolleyes:
It's very simple to explain characteristic impedance and VSWR (or S11)... you just need to use a smith chart - which is the most awesome tool, ever :)
smith_chart.jpg
]

PS - I ran all the RG cables in my RV and am putting the connectors on this weekend. It is easy but take your time and do it right. The right angle connectors don't bend the cable so don't worry about that. Make sure you have the right connector for your cable and the right dies for your connector. Also make sure to keep the shield strands out of the center conductor. Good luck!
 
impedance

Let me try a simpler explanation. In a transmission line like coax, there are two conductors (inner wire and 'shield')(but the shield is more than that, it actually carries an equal but opposite current than the inner wire). There is some capacitance between the two conductors (like a capacitor) and also some inductance (like an inductor or coil). These conspire to limit current flow for ac signals. Suppose you have an infinitely long piece of ideal RG58. If I suddenly hook up a 100 volt battery to the two conductors at one end, I will find that 2 amps flows out of the battery. Noting that volts divided by amps is the definition of 'ohms', we say say that RG58 has an impedance of 50 (100 volts divided by 2 amps) ohms. But note that nothing gets hot, no energy is lost, unlike in a resistor. Now for a finite length line, it's more complicated. Suppose the far end is open circuit. When you hook up the battery, a 100 volt/2 amp pulse races down the line at the speed of light, reflects at the open end, races again at the speed of light back to the battery, which now stops putting out the 2 amps. You're left with no current, and 100 volts between the conductors - the usual DC result. And since it all happens at the speed of light you need fairly sophisticated equipment to see it, although a good oscilliscope is sufficient. One last thing- if I put a 50 ohm resistor across the conductors at the far end, all of the energy is turned into heat, and there are no reflected pulses at all, and the battery will continue to put out 2 amps. We say the load is matched to the cable. Now an antenna is not a resistor, but as far as the coax is concerned it could be. A matched 50 ohm antenna will not get hot, but rather will radiate energy. The energy radiated is the current squared times 50 ohms - just like the power converted to heat in a resistor. So the coax cannot tell the difference.

Probably more than you cared to know.
 
I have an RG400 stripping tool. It's a clamp with blades set at different heights to cut the various bits at the proper length. I forget where I purchased it though. I think it was a small vendor.
 
Let me try a simpler explanation. In a transmission line like coax, there are two conductors (inner wire and 'shield')(but the shield is more than that, it actually carries an equal but opposite current than the inner wire). There is some capacitance between the two conductors (like a capacitor) and also some inductance (like an inductor or coil). These conspire to limit current flow for ac signals. Suppose you have an infinitely long piece of ideal RG58. If I suddenly hook up a 100 volt battery to the two conductors at one end, I will find that 2 amps flows out of the battery. Noting that volts divided by amps is the definition of 'ohms', we say say that RG58 has an impedance of 50 (100 volts divided by 2 amps) ohms. But note that nothing gets hot, no energy is lost, unlike in a resistor. Now for a finite length line, it's more complicated. Suppose the far end is open circuit. When you hook up the battery, a 100 volt/2 amp pulse races down the line at the speed of light, reflects at the open end, races again at the speed of light back to the battery, which now stops putting out the 2 amps. You're left with no current, and 100 volts between the conductors - the usual DC result. And since it all happens at the speed of light you need fairly sophisticated equipment to see it, although a good oscilliscope is sufficient. One last thing- if I put a 50 ohm resistor across the conductors at the far end, all of the energy is turned into heat, and there are no reflected pulses at all, and the battery will continue to put out 2 amps. We say the load is matched to the cable. Now an antenna is not a resistor, but as far as the coax is concerned it could be. A matched 50 ohm antenna will not get hot, but rather will radiate energy. The energy radiated is the current squared times 50 ohms - just like the power converted to heat in a resistor. So the coax cannot tell the difference.

Probably more than you cared to know.

That's awesome! RF stuff is voodoo to me. Would love to see a webinar on this subject.
 
There are a number of premade strippers available with adjustable blades. The problems with them are that at the tolerances with Mil Spec aircraft cable (vs. consumer grade RF cables), once you get them adjusted perfectly for one cut, they'll be off for the next one...so on and so forth. They ones I'm referring to look like an old fashioned clothespin (some won't even know what the heck those are I'm afraid)...but we carry a toggle stripper that does a pretty good job.

Also, no reason to spend $130 on a tool to crimp coax cables..unless you plan on working on the shuttle! :)

Anyway, just my 2 cents as usual!

Cheers,
Stein
 
Just think of it as "magic". :D

Not really. We can save the Smith chart for later, and people who know RF can tune out now for this oversimplification. RF sources have a design impedance, which means the value of load that will result in the most power being delivered. A 50 ohm transmitter will deliver the most power to a 50 ohm load (a 50 ohm resistor will get hotter when connected to a 50 ohm transmitter than will a resistor of any other value, for instance.) 50 was chosen years ago as a compromise to feed a bunch of different antenna types; old fashioned AM (and ADF sense) antennas are very high impedance, so they had to make high impedance coax and receiver inputs for them or adapter boxes that are impedance transformers. If your transmitter is 50 ohms and you load it with a 70 ohm antenna, part of the signal will get out the antenna anyway, and the rest will be reflected back into the transmitter which can damage it.

We use coax cable with the same surge impedance as the impedances of the source and load, to preserve this match. A 50 ohm cable connected to a 50 ohm antenna will present a 50 ohm load to the transmitter feeding it, we call that behaving as a "transmission line" which means it's not resonant (like an antenna is) and small differences in length don't matter. There are places in airplanes where we use tuned lengths of different impedance cable, mostly in nav antenna baluns and splitters. In this case we use the cable to transform one impedance to another, but that's another chapter.

Connectors have a characteristic impedance too. There are 75 ohm BNCs common in video, for instance, and they are not the same as the 50 ohm BNCs we use in nav and comm although you can make them fit together. Besides not fitting 50 ohm cable properly, they will introduce an impedance mismatch. It might not be too bad at VHF but can cause real problems at high power, such as for a transponder or DME antenna. Not magic, just physics; instead of a constant voltage along the transmission line, you get peaks of voltage and current. The voltage peaks can cause arcing, the current peaks can melt wire.

And yes, you could use right-angle adapters but please don't. Every connector interface introduces loss, again due to the unavoidable impedance mismatch (plus the reliability issue of all the mating surfaces oxidizing.)
 
Is it easy to mix up video and radio BNC connectors or are they pretty distinct? I purchased mine from Stein, so I'm sure they're correct, but just curious how different they are.
 
...people who know RF can tune out now for this oversimplification.
I earned an EE degree a l-o-o-o-n-n-n-g time ago, but Smith Charts are still a great mystery to me. I guess if we'd had some good ways to measure antennas back then (like the MFJ-266), it all would have made more sense to me. That said, I cut my own full dipole antenna for my COMM radio using the formula in the ARRL Antenna Handbook and it's worked out well - no complaints from ATC. I used a 1/4 wave "bazooka" type of balun at the antenna. I'm glad it worked because it's permanently sealed in my (fiberglass) vertical stabilizer :eek:
 
There are a number of premade strippers available with adjustable blades. The problems with them are that at the tolerances with Mil Spec aircraft cable (vs. consumer grade RF cables), once you get them adjusted perfectly for one cut, they'll be off for the next one...so on and so forth. They ones I'm referring to look like an old fashioned clothespin (some won't even know what the heck those are I'm afraid)...but we carry a toggle stripper that does a pretty good job.

Also, no reason to spend $130 on a tool to crimp coax cables..unless you plan on working on the shuttle! :)

Anyway, just my 2 cents as usual!

Cheers,
Stein
Hi Stein,

Do you mean that mil spec cables are not precise enough for a cutting tool's settings to work repeatedly or that our RG400 cable isn't Mil Spec?

Also, I looked on the Steinair site for a Toggle Stripper, but couldn't find one.

Thanks!
Russ
 
There are a number of premade strippers available with adjustable blades. The problems with them are that at the tolerances with Mil Spec aircraft cable (vs. consumer grade RF cables), once you get them adjusted perfectly for one cut, they'll be off for the next one...so on and so forth. They ones I'm referring to look like an old fashioned clothespin (some won't even know what the heck those are I'm afraid)...but we carry a toggle stripper that does a pretty good job.

Also, no reason to spend $130 on a tool to crimp coax cables..unless you plan on working on the shuttle! :)

Anyway, just my 2 cents as usual!

Cheers,
Stein

So that is the reason for variation! I just adjusted with # of spins and left a little to stretch/break.

http://www.vansairforce.com/community/showthread.php?t=109494
 
This is some great info! so for the center pin should we be soldering these onto the center cable on RG 400 or is a correct crimp sufficient?
 
Hi Stein,

Do you mean that mil spec cables are not precise enough for a cutting tool's settings to work repeatedly or that our RG400 cable isn't Mil Spec?

Also, I looked on the Steinair site for a Toggle Stripper, but couldn't find one.

Thanks!
Russ

Both. Every mil spec has a tolerance of some percents - and some are tighter than others per batch. Then the plastic strippers...well, their tolerances are difficult to repeat twice in a row, much less multiple times. Not a big deal on fat 75 cable like RG59 and such, or when its feeding things like a TV but much more important on the smaller 400/142 we all typically use where the certified GPSes or transponders can be fairly picky.

Example, we just had a customer who swore up and down his coax cables were good (made himself). After sending them here for testing we quickly found one had a miniscule tiny little short in it. Didn't seem like much and a typical continuity check didn't pick it up, but it was enough for him to blow up two transponders before he sent everything here.

Anyway, not saying prefab'd tools won't work because they will. You just need to be careful. Probably more important than the stripping is the assembly of them as that's usually what catches people. It only takes one or to little hairs of the shield touching the wrong thing to make things go poof! :)

Just my 2 cents as usual!

Cheers,
Stein
 
Is it easy to mix up video and radio BNC connectors or are they pretty distinct? I purchased mine from Stein, so I'm sure they're correct, but just curious how different they are.

I very much doubt that Stein would be selling 75 ohm connectors, except for the old widescreen TV's we had in the aft lounge of all our planes...

You can see the difference pretty clearly, see this photo snipped from Wikipedia
https://en.wikipedia.org/wiki/BNC_connector#/media/File:BNC_50_75_Ohm.jpg
 
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