What makes cheap coax cheap, and expensive coax expensive?

[Blaise]

To clarify the click-baity question a bit:

I recently watched an online debate/put-down session on someone who claimed to be able to make cheap coax just as efficient and low-loss as super-expensive stuff just by doing things like running it in copper tube, wrapping in metal mesh, etc.

This seems a bit naive/uninformed, but it does make me ponder questions like:

• *Is* it possible to decrease losses in existing cable via external means?
• What is actually going on internally that makes one cable less lossy than another?  Is is just thickness of conductors?  Geometry tricks? Shielding?
• Is it possible to build something equivalent to or better than "good" coax to get your signal where it's going in controlled circumstances, like a fixed installation on a roof?  Does the answer to this question include the "ladder line" I keep seeing hams post about?  Do DIY ideas I've seen like extra-insulated 'cheap' coax, home-made braided wire, or a pipe with insulated grounding cable inside hold water?
• I have some antenna resources, but they don't go into much detail on this topic beyond 'here's what you do'.  Is there an in-depth but still relatively accessible primer on the science of signal propagation in cables, etc?

So yeah, just a million questions.  I sound really demandy.  I'd like fifteen thousands words on my desk by Monday!
 

[Lscott]

On 2/24/2023 at 11:52 AM, Blaise said:

What is actually going on internally that makes one cable less lossy than another?  Is is just thickness of conductors?  Geometry tricks? Shielding?

It's split mainly between dielectric losses in the isolating material between the center conductor and skin effect losses in the conductors. The geometry has an indirect effect on both.

https://www.resonac.com/solution/tech/transmission-loss.html

 

[Sshannon]

42 minutes ago, Blaise said:

To clarify the click-baity question a bit:

I recently watched an online debate/put-down session on someone who claimed to be able to make cheap coax just as efficient and low-loss as super-expensive stuff just by doing things like running it in copper tube, wrapping in metal mesh, etc.

This seems a bit naive/uninformed, but it does make me ponder questions like:

• *Is* it possible to decrease losses in existing cable via external means?
• What is actually going on internally that makes one cable less lossy than another?  Is is just thickness of conductors?  Geometry tricks? Shielding?
• Is it possible to build something equivalent to or better than "good" coax to get your signal where it's going in controlled circumstances, like a fixed installation on a roof?  Does the answer to this question include the "ladder line" I keep seeing hams post about?  Do DIY ideas I've seen like extra-insulated 'cheap' coax, home-made braided wire, or a pipe with insulated grounding cable inside hold water?
• I have some antenna resources, but they don't go into much detail on this topic beyond 'here's what you do'.  Is there an in-depth but still relatively accessible primer on the science of signal propagation in cables, etc?

So yeah, just a million questions.  I sound really demandy.  I'd like fifteen thousands words on my desk by Monday!
 

This is a pretty good starting point in understanding how Coax works.  It won't answer all of your questions, but it will give you a start.

[KAF6045]

43 minutes ago, Blaise said:

To clarify the click-baity question a bit:

I recently watched an online debate/put-down session on someone who claimed to be able to make cheap coax just as efficient and low-loss as super-expensive stuff just by doing things like running it in copper tube, wrapping in metal mesh, etc.

This seems a bit naive/uninformed, but it does make me ponder questions like:

• *Is* it possible to decrease losses in existing cable via external means?
• What is actually going on internally that makes one cable less lossy than another?  Is is just thickness of conductors?  Geometry tricks? Shielding?
• Is it possible to build something equivalent to or better than "good" coax to get your signal where it's going in controlled circumstances, like a fixed installation on a roof?  Does the answer to this question include the "ladder line" I keep seeing hams post about?  Do DIY ideas I've seen like extra-insulated 'cheap' coax, home-made braided wire, or a pipe with insulated grounding cable inside hold water?
• I have some antenna resources, but they don't go into much detail on this topic beyond 'here's what you do'.  Is there an in-depth but still relatively accessible primer on the science of signal propagation in cables, etc?

So yeah, just a million questions.  I sound really demandy.  I'd like fifteen thousands words on my desk by Monday!
 

COAX was created explicitly to be IMMUNE to the surrounding environment (electrical behavior, not immune to fire, etc.). It is meant to be run through conduits and around wiring bays.

So, no -- copper tube/metal mesh won't change it... Unless the coax connections are NOT unbalanced, and there is signal (losses) on the outside of the coax shield (in which case a tube or mesh is just going to couple to the unwanted/lossy part of the signal). When used with a balanced antenna (dipole), one normally needs a balun/choke to force the signal into the coax core and block it from the outside of the shield.

Ladder line tends to run 450-600 ohm impedance, and is a balanced line (no balun needed for dipole -- BUT you need a matching network to convert from that high impedance down to native antenna [dipoles tend to run between 33-75 ohm depending upon height] and if your radio doesn't have a balanced input, you'll need a balun to convert down to the unbalanced/50 ohm connector). Also ladder line (window line, 300 ohm TV twin-lead) MUST be kept away from metal objects as there is no shielding and it will couple to any metal running nearby.

[Blaise]

On 2/24/2023 at 11:52 AM, Blaise said:

What is actually going on internally that makes one cable less lossy than another?  Is is just thickness of conductors?  Geometry tricks? Shielding?

Anyone have details on this question?  It's really the crux of what I was wondering...

[WRQC527]

On 2/24/2023 at 8:52 AM, Blaise said:

Is it possible to build something equivalent to or better than "good" coax to get your signal where it's going in controlled circumstances, like a fixed installation on a roof?  Does the answer to this question include the "ladder line" I keep seeing hams post about?  Do DIY ideas I've seen like extra-insulated 'cheap' coax, home-made braided wire, or a pipe with insulated grounding cable inside hold water?

Ladder line is wierd stuff. It's 300 ohm or higher, and requires baluns, transformers, tuners, etc to work on a radio that needs 50 ohms. But Mostly used on HF. There are military NVIS antennas out there that use a specially designed support pole as coax. All very specialized and probably not worth experimenting with for anything but HF. Coax is cheap, easy and reliable. DIYing feedline to save money is probably an uphill battle.

[Blaise]

36 minutes ago, Lscott said:

It's split mainly between dielectric losses in the isolating material between the center conductor and skin effect losses in the conductors. The geometry has an indirect effect on both.

https://www.resonac.com/solution/tech/transmission-loss.html

 

Thank you!  This, and the searches it inspired, have provided *much* digestible info...

[Blaise]

33 minutes ago, WRQC527 said:

Ladder line is wierd stuff. It's 300 ohm or higher, and requires baluns, transformers, tuners, etc to work on a radio that needs 50 ohms. But Mostly used on HF. There are military NVIS antennas out there that use a specially designed support pole as coax. All very specialized and probably not worth experimenting with for anything but HF. Coax is cheap, easy and reliable. DIYing feedline to save money is probably an uphill battle.

Yeah, I'm less interested in the DIY aspect than the science of it.  I hate just accepting specs without knowing why...

 

[axorlov]

2 hours ago, Blaise said:

Anyone have details on this question?  It's really the crux of what I was wondering...

Two things, mostly, make one cable less lossy than another. First: geometry, the thicker the cable (more distance between central conductor and shield) the less the loss in dielectric material. Second: the dielectric material. Thicker cables have more of copper in them (and everything else), thus they are more expensive. Times Microwave developed their formulation of the foam dielectric 30 years ago, and continues to make money on that RND investment.