Center conductor question

[Frmn85]

Hi forum members,

I have a question regarding the center conductor of any RF cable.
I understand that most hardline cables are copper coated to save on cost and high frequency tends to ride on the surface of the conductor (skin effect)

1. But what happens If the center conductor is slightly nicked?
2. What is the performance impact to VSWR and or insertion loss proportional to frequency
3. How critical is this for public safety frequencies?
4. Does this impact delay/ timing in BDAs?

Ie. during a hardline connector installation, the hardline foam dielectric was cut with a straight blade around the center conductor. In doing so it made a slight circular cut around the center conductor.

Your assistance in answering this is much appreciated

 

[prcguy]

1. A nicked center conductor can create a VSWR problem, especially if it goes into the base material. Don't nick the center conductor and if you do cut off the bad spot and start over.
2. Above a particular frequency there should be no difference in RF performance compared to solid copper. Makeup of the center conductor will also affect this like copper clad steel will incur more loss below a critical frequency compared to copper clad aluminum due to the difference in conductivity of the base material.
3. The critical frequency where RF starts conducting through the base material should be fairly low like a few MHz or less and well below any public service freqs.
4. I would not expect any difference in timing for VHF/UHF and higher freqs compared to solid copper center conductor.

 

[Frmn85]

Thanks for the timely response.

From what I gather on your response- Any nick or cut that goes through the base (surface coating) material of the center conductor will cause a VSWR issue for any frequencies above a few Mhz.


I ask this because I have seen techs install connectors by using a blade to cut the foam dielectric by using the blade to circle and press into the foam and it often appears the edge of the blade cut through the foam and into the copper coating.-obviously this is a no no but it appears to have never caused an issue?

Also out of curiosity Is there any reference material I can find for this? ie. was any test/research ever conducted conducted where the depth of the cut versus frequency was plotted? I cant seem to find any of this material online. If a test was never conducted on this I may conduct my own just out of curiosity.

 

[merlin]

You can actually see impedance changes using a TDR. Nicks/cuts, kinks, Barrel splices, show up as ripples in the time domain.
Even connectors installed to NASA specs will show that ripple. Generally, the effects are minimal but any installer worth their salt
can properly install connectors.
The newer NANO VNA v4 has TDR functions, you can see the effects of coax damage yourself.

 

[Frmn85]

We have a old school TDR in the shop, we will look for the ripples before and after.

 

[prcguy]

Thanks for the timely response.

From what I gather on your response- Any nick or cut that goes through the base (surface coating) material of the center conductor will cause a VSWR issue for any frequencies above a few Mhz.


I ask this because I have seen techs install connectors by using a blade to cut the foam dielectric by using the blade to circle and press into the foam and it often appears the edge of the blade cut through the foam and into the copper coating.-obviously this is a no no but it appears to have never caused an issue?

Also out of curiosity Is there any reference material I can find for this? ie. was any test/research ever conducted conducted where the depth of the cut versus frequency was plotted? I cant seem to find any of this material online. If a test was never conducted on this I may conduct my own just out of curiosity.

I wouldn't say a nick or cut will cause VSWR problems for any frequency above a few MHz but it will at higher frequencies which I can't predict. Most of my experience with nicks and VSWR problems was at or above 1GHz.

 

[Frmn85]

This is good information to know.

Thanks!!

 

[paulears]

There is a huge difference between the physics and real world usage. What’s lacking is the evaluation and conclusion stages. Yes, there is a difference. Pick a loss figure where you start to worry in a typical system. Let’s pick 1dB. Most business and amateur installations wont even notice a drop of 1dB. Most don’t have equipment accurate enough to measure it reliably. If you have something like an analyser you can see an antenna performance change when you wave your hands near it, and realise that simply attaching the support tube to an antenna is visible, and often changes the signal level by more than 1dB. Can you hear it? No. A nick in the centre core makes a difference, but I don’t have anything sensitive enough to measure. My finger makes more of a difference. Keep in mind that skin effect as frequency goes higher is more important and commercial installers use connectors that fit properly. At 2/70 our soldering probably makes more difference than a small nick. move to microwaves and cables are more plumbing than electronics. Oddly, drop to audio frequencies and the hi fi brigade are convinced surface effect applies to them. They are totally deluded in my book. They quote physics, but miss totally the connection with frequency and wavelength. Hams for years have prized cut off hardline from pro installs, because for us, the discarded cable is better than what we have, but the tower folk consider evert dB worth fighting for.

 

[prcguy]

There is a huge difference between the physics and real world usage. What’s lacking is the evaluation and conclusion stages. Yes, there is a difference. Pick a loss figure where you start to worry in a typical system. Let’s pick 1dB. Most business and amateur installations wont even notice a drop of 1dB. Most don’t have equipment accurate enough to measure it reliably. If you have something like an analyser you can see an antenna performance change when you wave your hands near it, and realise that simply attaching the support tube to an antenna is visible, and often changes the signal level by more than 1dB. Can you hear it? No. A nick in the centre core makes a difference, but I don’t have anything sensitive enough to measure. My finger makes more of a difference. Keep in mind that skin effect as frequency goes higher is more important and commercial installers use connectors that fit properly. At 2/70 our soldering probably makes more difference than a small nick. move to microwaves and cables are more plumbing than electronics. Oddly, drop to audio frequencies and the hi fi brigade are convinced surface effect applies to them. They are totally deluded in my book. They quote physics, but miss totally the connection with frequency and wavelength. Hams for years have prized cut off hardline from pro installs, because for us, the discarded cable is better than what we have, but the tower folk consider evert dB worth fighting for.

In large cable/satellite distribution system where the center conductor is nicked in the same spot (by the same idiot) on lots of cables the problem becomes additive, very noticeable and much worse than 1dB.

On skin effect at audio frequencies, it is there and it is perceivable. You will not hear it on a 1980s vintage Kenwood/Pioneer/Sansui stereo with matching vintage speakers but you can hear it in A/B comparisons with very high end audio equipment. I've heard many things that seem impossible on some really high end audio systems like differences in RCA cables, speaker cables and even swapping power cables. Some of the differences are not subtle at all and anyone can hear it.

 

[davidgcet]

i work in cellular and honestly a nick at 1.9ghz causes more issues with rtwp than vswr. i frequently fight receive issues where the center is nicked or the connector is not torqued to spec(over is just as bad as under) causing a port to go 10 or even 20db too hot, yet will still have a 1.1 vswr.

BTW, tower hardline is rarely coated. usually it is pure copper. now 1/2"-5/8" does often have a coated center conductor but most of us don't call those hardline except when asking if a jumper is hardline or superflex. for cellular superflex is absolute crap, install crews love it for ease or dressing, maintenance crews love it because they WILL be getting lots of work, techs hate it because outside it frequently cracks from even minor but repeated movement in the wind.

 

[paulears]

On skin effect at audio frequencies, it is there and it is perceivable. You will not hear it on a 1980s vintage Kenwood/Pioneer/Sansui stereo with matching vintage speakers but you can hear it in A/B comparisons with very high end audio equipment. I've heard many things that seem impossible on some really high end audio systems like differences in RCA cables, speaker cables and even swapping power cables. Some of the differences are not subtle at all and anyone can hear it.

I'm afraid here, I have to disagree. I've spent my working life in a wide range of facilities ranging from the BBC, who have years of scientific research to back up specs, to the clever wonderful audiophile places. I'm afraid I just cannot subscribe to the physics on this one. It's fine, I know many people claim to hear it - but my personal opinion is that it's far too subjective. At audio frequencies, the improvements, for me are not audible, and won't float.

 

[prcguy]

I'm afraid here, I have to disagree. I've spent my working life in a wide range of facilities ranging from the BBC, who have years of scientific research to back up specs, to the clever wonderful audiophile places. I'm afraid I just cannot subscribe to the physics on this one. It's fine, I know many people claim to hear it - but my personal opinion is that it's far too subjective. At audio frequencies, the improvements, for me are not audible, and won't float.

We don't claim to hear it, its obvious and you can hear it too if I send you the place where I and many others have heard it. The key is having a playback system that can resolve the fine details and apparently you have not had the opportunity to listen on one that will let you hear the differences.

 

[paulears]

Sorry but we will have to differ. I remember many hours if BBC double blind tests of cables and the conclusion was that the only cable issue was impedance related capacitance loading and at 600 ohms and below the change to frequency response was insignificant at studio cable lengths. The BBC designed many cables over the years. For speakers the focus was cross sectional area. I have rejected the practice of using my ears and not linking them to proper statistical scrutiny, in fact many of the tests I endured were delivered to two groups simultaneously with the results predicted in advance based on how the tests were delivered. Every member present was positive they did the test absolutely properly. As a result I simply rely on the science. Which states skin effect is measurable at high frequencies way above the audio band. I’m very happy others have a different opinion. But mine will not change now.

 

[prcguy]

Sorry but we will have to differ. I remember many hours if BBC double blind tests of cables and the conclusion was that the only cable issue was impedance related capacitance loading and at 600 ohms and below the change to frequency response was insignificant at studio cable lengths. The BBC designed many cables over the years. For speakers the focus was cross sectional area. I have rejected the practice of using my ears and not linking them to proper statistical scrutiny, in fact many of the tests I endured were delivered to two groups simultaneously with the results predicted in advance based on how the tests were delivered. Every member present was positive they did the test absolutely properly. As a result I simply rely on the science. Which states skin effect is measurable at high frequencies way above the audio band. I’m very happy others have a different opinion. But mine will not change now.

This needs its own thread. I'm sorry you have never listened on a system that will resolve the differences, its quite a mind blowing experience. Come to So Cal USA and I will take you to one.

 

[Ubbe]

Prcguy are probably correct but it isn't noticed in a $10,000 equipment and you have to go to $50,000 and higher to start to make it have an impact on sound. But when it comes to a mains cable, that are just a few percent of the whole chain of mains wiring in the wall to the fuse box and out on the street to a transformer and then to a bigger transformer station, then it's harder to swallow. In the device that are connected you have capacitors between phase and neutral and ground that takes care of any high frequency components from the mains and then comes the inductor transformer that are a bandpass filter in itself as it cannot transfer higher frequencies between its windings and its efficiency goes down at lower frequencies below the mains frequency. Then comes the rectifier that have capacitors over the diodes to get rid of any transients from the switching of polarity and then a huge reservoir capacitor to give a steady DC voltage, with smaller capacitors in parallel to pick up any residual AC components. It's hard to believe that anything from a mains cable can have any impact on the resulting DC voltage.

But what can happen are that a mains cable are manipulated in some way to not have any ground, that might be illegal and dangerous, and the device will then instead be grounded from its audio cables going to another device that then are properly grounded and both devices then share a common ground and gets rid of any ground loop issues that might create hum and distortion that can have an affect on the sound.

/Ubbe