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Question About Coax Cable

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jpsmith123

Tue Jun 28 2011, 10:49PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

With regard to specifications for coaxial RF cable, does anyone know exactly what is meant by the "minimum bend radius"? Obviously I know in simple terms it means "it's the most you can bend it", but to go beyond that useless tautology, how do manufacturers measure it? Is it the point beyond which some kind of mechanical damage happens, or is it the point where some electrical property e.g., VSWR, goes out of spec., or both, or what?

The reason I ask is because I was contemplating doing an experiment using RG213 cable, where the cable will be wound in a coil, but when I went to look into the properties of the cable, I noticed that the "minimum bend radius" specification varies somewhat between manufacturers.

Proud Mary

Tue Jun 28 2011, 11:20PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Let us consider the case of a single turn.

You will notice that because of the cable's own diameter, this single turn has an inner radius, ra, and an outer radius, rb.

You will notice that if you make the single turn larger and larger, the ratio ra:rb will become more and more nearly the same.

Conversely, if you make the single turn smaller and smaller, the ratio of inner to outer radius will become more and more extreme.

If you imagine your coax turn now being wound forcibly round a nail, you will see that something must give. The outer surface must stretch,
while the inner surface must be compressed.

This distortion of the cable cross-section will damage the dielectric, and perhaps even snap the centre conductor.

So the minimum bending radius is the minimum at which these unwanted effects will not occur to an extent sufficient to alter significantly the cable's characteristic impedance, velocity of propagation, dielectric breakdown voltage, mechanical strength, and so on.

Patrick

Tue Jun 28 2011, 11:25PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

They usually mean this...

the radius of the red circle.

Proud Mary

Tue Jun 28 2011, 11:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Patrick wrote ...

They usually mean this...

the radius of the red circle.

You will note that my observations will apply to any arc of a circle - most pertinently in kinks where they appear in cable runs.

Patrick

Tue Jun 28 2011, 11:33PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

You will note that my observations will apply to any arc of a circle - most pertinently in kinks where they appear in cable runs.

yes, all would be buckled on the inside radius, and stretched on the outer radius.

jpsmith123

Tue Jun 28 2011, 11:57PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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I understand generally that as I try to bend the cable to a smaller and smaller radius, at some point, there will be mechanical damage, yes, no question about that.

My question arises from the empirical fact that different manufacturers apparently have different specs for minimum bend radius; in some cases, significantly different. Yet the cables all have the same outer diameter, the same dielectric, i.e., solid polyethylene, the same type and size of stranded center conductor, and roughly similar copper sheath.

So I'll rephrase my question this way: What accounts for the differences in specified bend radius between different manufacturers for the same type cable made essentially the same way?

Patrick

Wed Jun 29 2011, 12:19AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

jpsmith123 wrote ...

I understand generally that as I try to bend the cable to a smaller and smaller radius, at some point, there will be mechanical damage, yes, no question about that.

My question arises from the empirical fact that different manufacturers apparently have different specs for minimum bend radius; in some cases, significantly different. Yet the cables all have the same outer diameter, the same dielectric, i.e., solid polyethylene, the same type and size of stranded center conductor, and roughly similar copper sheath.

So I'll rephrase my question this way: What accounts for the differences in specified bend radius between different manufacturers for the same type cable made essentially the same way?

Speciffically which manufactures are you camparing? And which products?

jpsmith123

Wed Jun 29 2011, 12:29AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well for example compare this:

With this:

Patrick

Wed Jun 29 2011, 12:53AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I dont see much difference in the quoted numbers, so i dont know.

I would like to find resistance wire RG coax, for making my own oscilloscope probes, but cant find a supplier.

jpsmith123

Wed Jun 29 2011, 01:14AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well in the first example the minimum bend radius is specified as 50mm, and in the product for sale on ebay it's 4 inches (approximately 100 mm); so right there you have a 2:1 difference between the two items which are both called RG213 and look to be physically identical.

I was going to buy the one on ebay and wind it on a piece of 4" diameter PVC pipe, but according to that spec., I apparently shouldn't do that.

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