Repairing Coaxial Cable - A Difficult Task? 13

[muthabored]

I'd been having trouble with my high speed internet (the connection would suddenly drop on its own for what appeared to be no reason). After speaking with my ISP's tech support, my attention was directed to the coaxial cable that comes in from the outside (the cable is frayed). They offered to send someone out to take a look at it/repair it but I'm wondering if this is something that I could do on my own. I have spare cable and I think I'd have to get one of those connectors (F81) and then something to cut/crimp the cable. Can anyone refer me to a website that might assist me with this? Thanks in advance!

 

[LawnBoy]

and what I was taught

Do you still have your textbook?
I understand that squeezing the dielectric will change the frequency response of the cable. No argument, I have seen it on an o'scope myself. This does not mean the dielectric carries the signal. It means the capacitive reactance of the cable has changed, thereby changing the frequency response.

 

[edfair]

I must admit jumping the gun.

One additional reference is "how things work coaxial cable" which duplicates Greg's description. That there is, in fact, electomagnetic field propogation in the dielectric.

Not to be contradictory, but my next stop is my old trusty ARRL handbook to see what the theory was 50 years ago. And to see what has changed in theory or what I've forgotten.


Ed Fair
Give the wrong symptoms, get the wrong solutions.

 

[diogenes10]

http://www.freepatentsonline.com/5539361.html

In this link, is the last sentence in the second paragraph under background of the invention, useful to this discussion?

 

[edfair]

From the trusty 1958 ARRL Handbook c13 p336 transmission lines:
"In the time of one cycle the energy will travel a distance of one wave length along the line wires. The current at a given instant at any point along the line is result of a voltage that was applied at some earlier instant at the input terminals. Hence the instantaneous amplitude of the current is different at all points in a one-wave-length section of line. In fact, the current flows in opposite directions in the same wire in adjacent half-wavelength sections."
"The result of all this is that the current (and voltage) travels along the wire in a series of waves having a length equal to the velocity of travel divided by the frequency of the a.c. voltage. On an infinetely-long line, or one properly matched at the load, an ammeter inserted anywhere in the line will show the same current, since the ammeter averages out the variation in current during a cycle.It is only when the line is not properly matched that the wave motion becomes apparent."

From this it appears that the current does really reach the far end and that the 2 references are in error. I would propose that somebody perform an experiment and put the last couple of inches of cable hanging through the air and see if the signal gets through. Not having the dielectric in contact with the termination would prove it to everybody's satisfaction.

I may do it with a thinnet section of the houwe network just to satisfy my own curiousity.


Ed Fair
Give the wrong symptoms, get the wrong solutions.

 

[diogenes10]

Wouldn't that just say air instead of foam is the dielectric and the signal is still being carried in the electromagnetic stuff created by the electricity in the wire?

 

[edfair]

Yeah, that is what the test proposes. Or even better is to take the 2 conductors in a 180 degree loop before entering the bnc connector.
It is more to put at rest that the signal is carreid by the dielectric. Gives me an opportunity to disign a test since it is a challange to build it and still have the network work.

Ed Fair
Give the wrong symptoms, get the wrong solutions.

 

[diogenes10]

You just went beyond what little I have been able to understand about this.

What does taking the conductors in a loop do or test?

 

[edfair]

It would eliminate any possibility of any signal propogated from the dielectric finding its way into the bnc connector on a direct path.

This is really an excuse for me to try using a short piece of RG59U as an extension cord. Thinnet specs call for RG59AU but Office Depot and CompUSA sell U, which works sometimes and not sometimes. Or it works for a while and you move the cable or hook another computer in the string and everything falls apart. In my case, I added a machine in the house to the end of the string by 5 ft and 2 other machines disappered from a network that had been working for 5 years.Ended up restringing 3 segments with AU to get them back. So, if I know that 5 feet of U will work when everything else is AU then potentially I would not need to make a parts run on an emergency basis but could patch in a short jumper of U from CompUSA temporarily until my next trip to the customer. This would be no use at the house as I have a spool here, but I don't carry it in the service car with me.

Ed Fair
Give the wrong symptoms, get the wrong solutions.

 

[gbaughma]

I'm taking a stab here.... but I *believe* what is going on is that the center conductor and braiding act as an antenna and gound plane, respectively, to propagate the signal through the dialectric.

Ahh... confirmed.

http://en.wikipedia.org/wiki/Coaxial_cable

First section, under "Signal Propagation"



Just my 2¢

"What the captain doesn't realize is that we've secretly exchanged his dilithium crystals for new Folger's Crystals." -- My Sister
--Greg

http://parallel.tzo.com

 

[biglebowski]

From what I undestand the signal is still travelling down the centre conductor but is radiating and the braiding is acting as a mini faraday cage to stop this radiation. The signal is actually being carried on the surface of the centre conductor and this skin effect transmission is due to the higher frequency.

When I was born I was so suprised I didn't talk for 18 months

 

[edfair]

And as a "ham" you will dispute the work of your "bible"? Dispute the work of fellow hams who measured the current flow through the wire with meters and thingys?

Are you sure you want to do this? Your fellow hams may petition the FCC to withdraw your license for "dis-belief" if it gets out.

Can I believe you were taught that the signal is generated in the dielectric? Yes. I've been involved with teaching technical stuff and I've seen plenty of stuff taught by people that didn't understand what they were teaching.
Can I believe the encyclopedia was wrong? Yes. The editors are not RF engineers.
Can I believe that Wikipedia is wrong? Of course. Can I believe that "how things work" is wrong? Of course.

Does all this matter to me? Yes, because it forced me to revisit some RF theory I hadn't bothered to keep up with for nearly 50 years.

Ed Fair
Give the wrong symptoms, get the wrong solutions.

 

[gbaughma]

I give up. I've given three sources, PLUS drawn on what I've been taught and has been demonstrated in class before my own eyes.

I give up.



Just my 2¢

"What the captain doesn't realize is that we've secretly exchanged his dilithium crystals for new Folger's Crystals." -- My Sister
--Greg

http://parallel.tzo.com

 

[biglebowski]

The symmetry of the coaxial cable and the fact that the outer conductor surrounds the inner conductor make it a shielded structure. At high frequencies, signal currents concentrate near the inside surface of the outer conductor and the outer surface of the inner conductor. This is called skin effect. The depth to which currents penetrate decreases with increasing frequency. Decreased skin depth improves the cable's self-shielding and increases transmission loss. This loss (expressed in decibels per kilometer) increases approximately as the square root of frequency because of the skin effect.

http://www.answers.com/topic/coaxial-cable

http://www.vhfman.freeuk.com/radio/current.html

http://www.w8ji.com/coaxial currents/coaxial_line_and_shielded_wires.htm

When I was born I was so suprised I didn't talk for 18 months

 

[LawnBoy]

Oh, lord, you're not trying to pass off wikipedia as any kind of authoritative resource, are you? That is user-generated content, not peer-reviewed science.

 

[Freestone]

This is my understanding after reading the ARRL's Antenna book, and Wikipedia's entries of subjects coaxial cable and electromagnetic radiation.

One needs to make a distinction here. In a coaxial cable, the electrical current flows through the wire, the resultant electromagnetic wave flows through (or is contained within) the dielectric. The dielectric is taking the place of air's role in an open wire transmission line.

Once the current reaches its termination point, the energy is transferred to that medium, and the dielectric role ceases at that point. If we are talking about a dipole antenna, the two legs of the antenna receive this energy and release it as another (or a continuance of the same wave if you will) electromagnetic wave to the ether.

73

 

[gbaughma]

Freestone:

And that's what I've been saying! The Dielectric actually carries the signal, with higher frequencies being more towards the outside, and lower frequencies being more toward the inside (near the center conductor).

THEREFORE, pinching the dielectric essentially pinches off the upper frequencies.

Wait... didn't I say I was giving up on this? I'm done. Honestly this time.

Bottom line about the post: Let them repair the cable.



Just my 2¢

"What the captain doesn't realize is that we've secretly exchanged his dilithium crystals for new Folger's Crystals." -- My Sister
--Greg

http://parallel.tzo.com

 

[edfair]

Very good Freestone, but would add that the decay (at RF frequencies) within the dielectric pushes the current along the conductor.

I have no problem quoting or linking Wikipedia but find that sometimes there is a disconnect between what it has to say and how others say it. I will include Greg's link again with a parallel explanation from a manufacturer.

http://www.en.wikipedia.org/wiki/Coaxial_cable

http://www.gemelec.com/technica.htm

I agree that there is an electromagnetic field genererated and that the field is generated within the dielectric between the center conductor and the shield.

We disagree from there. Greg stated that the electromagnetic field is transmitted down the line through the dielectric. I claim that the signal is transmitted down the center conductor by the effects of the collapse of the EMF in the dielectric when the instantaneous voltage at a point on the center conducter falls below that on the corresponding position on the shield. In this view it would appear that the tranmission is through the dielectric as the field would build and die as the voltages on the center conducter build and die. Remember that there is pressure being applied at one end of the cable so the current flow genereated would in general be to the other end.

Ed Fair
Give the wrong symptoms, get the wrong solutions.

 

[Sympology]

This is way over my heard, but from what I'm reading doesn't quite add up.

If electrons are shooting down the copper and radio wave down the dialectric, then surely they would go out of synch as radio wave travel far slower than copper conducted electrons.

I'm thinking along the lines Fibre, where the shield is vital to the operations of the fibre as it is used to bounce back in the "lost" signal, thus a damaged shield will actually prevent the core from working properly.

But like I said, this is way above me...


Only the truly stupid believe they know everything.
Stu.. 2004

 

[LawnBoy]

I'd like to congratulate everyone on remaining civil during this debate. It is refreshing.

Any wire carrying an electric current will produce an electromagnetic field around it, coax or not. If I understand him correctly, Greg is saying that it is this field, and not the current in the wire, that actually contains the signal. Am I right so far?

If so, this field (which is the real signal) exists within the dielectric of a coax, or in the air surrounding an unshielded wire. Am I right so far?

Therefore, the signal delivered by my cat5 cabling is actually delivered by the air around the wire, and not the wire itself?

Greg, as to your three references, one of them is discussing antennas, not the signal on a cable TV coax (which is not feeding an antenna). The other two references are to encyclopedias, which are layman's guides not technical references. I will happily concede a lack of understanding on my part if you could produce a hard technical reference. The burden of proof is on you, since you made the assertion. Obviously we're not going to accept each other's arguments, but I will accept an argument from authority.

 

[LawnBoy]

By "hard technical reference", I mean something like this:

Excerpted from the book "Commercial Radio Operator Theory Course" by Martin Schwartz, former instructor at American Radio Institute, copyright 1972 by Ameco Publishing Corp. Updated 1981. ISBN # 0-912146-16-8.

The primary reason for using waveguides at high frequencies is that they have low losses. At high frequencies, coaxial cable exhibits very high dielectric losses. Waveguides use air as the dielectric, making the dielectric losses almost negligible.

So the dielectric in a coax causes signal loss at high frequencies. It is not responsible for signal propagation.