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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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 Posted: Fri Mar 07, 2008 6:19 am Post subject: Coax termination and electrical theory explained. |
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I have been dabbling with the physics of current moving along a wire. One of the remarkable things about physics that I have to keep re-learning is that seemingly simple phenomena have wildly complicated reasons for working as they do. But understanding these reasons keep us on track .
You would think that the “theory” of electricity has been well worked out by now. But here’s your test:
1)What carries the energy in a simple electrical circuit?
a-Electrons,
b-Holes,
c-God knows!
2)What returns on the ground or return wire?
a-Electrons,
b-Holes,
c-God knows!
3)In a coaxial cable, exactly where is the energy carried?
a-In the center conductor,
b-In the shield,
c-God knows!
If you answered “c” for the three questions, you are close to correct but we can shed considerable light on the subject and only a few miracles will be invoked.
Let’s start with some simple truths:
1) You can't expect electrons to carry energy to the load because a coulomb (a miniscule amount of electrons), a meter from another coulomb electrostatically repel each other with about a million tons of force. (So there's probably some kind of taxpayer-funded DARPA electronic weapon based on this fact in our future....).
2) Electrons don’t carry signals or information either because they are far too slow. It takes days for an electron to go a mile. Electromagnetic fields do the job.
3) The notion of ground current as used-up tired electrons slowly returning home to battery rehabilitation is wrong. BOTH the high potential and ground return conductors do identical things, but with opposite polarities—even in DC circuits.
4) Electrons don’t carry energy to the load because they usually don’t get there anyway. Most circuits just don’t connect power and load in any contiguous way.
5) Coaxial cables are waveguides because the energy transmitted is almost entirely within the inner insulation. An amazing proof of this is that the wave velocity is almost exactly the speed of the electromagnetic wave in the particular material out of which the insulation is made. The importance of this is directly proportional to frequency. For audio purposes it matters little.
“Skin Effect” is a 1957 tribute to Bridgette Bardot, and also what happens to electrons that increasingly drift towards the surface of a conductor at high frequencies. Why do we care about this when electrons don’t do much anyway? This is interesting only because the center of conductors at high frequencies have no function. This is easily seen in HF induction coils made of hollow tubing instead of solid.
The "pigtails" where the coax shield is twisted into a lead and both the inner conductor and twisted up shield lead are treated as separate wires is deadly to high frequency signals >Megahertz. The shield should be terminated by any method that does not decrease the impedance of the shield or the inner conductor. There are many ways to do this, but they all use some collar arrangement to terminate the messy coax shield. Any coax catalog has lots of solutions. Remember that above a megaHertz, the inner INSULATION carries the bulk of the energy!
(As an aside--50 Ohms and 75 Ohms are the impedance of vacuum and air respectively. Early coaxial cables were one or the other.)
Bob, in my humble opinion, needs a little nudge on this. Here's how NOT to terminate coaxial cable--
http://aeroelectric.com/articles/shldwire/shldwire.html
But yikes....! So it turns out that the simplest DC circuit operation is wildly difficult to explain, but basically the electromagnetic fields carry the energy, not the electrons. They are just there to move and cause the magnetic field. The electrons have only an electric field, and it is only the electrons movement through space that generates a magnetic field. If you followed alongside an electron, you wouldn’t see ANY magnetic field. This is the difference between static electricity and the regular stuff. Static electricity has no magnetic field because the electrons aren’t moving with respect to the observer.
http://science.uniserve.edu.au/school/curric/stage6/phys/stw2002/sefton.pdf and if this (excellent!) paper doesn't blow your mind, try--
http://sites.huji.ac.il/science/stc/staff_h/Igal/Research%20Articles/Pointing-AJP.pdf
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part6/page3.html
Textbook, Electromagnetics Explained: Part of the EDN Series for Design Engineers by Ron Schmidt
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Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
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echristley(at)nc.rr.com
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| Posted: Fri Mar 07, 2008 7:08 am Post subject: Coax termination and electrical theory explained. |
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Eric M. Jones wrote:
the same.
You have two copper strips to form an antennae. One must be connected
to the center conductor, the other to the shield. Whether you terminate
the coax with some fancy, expensive solution from an electronics
catalogue or Bob's method, at some point the center conductor has to
split out from the shield. The difference in actual performance in the
flying airplane will be lost in the noise (pun intended). In theory,
there is an advantage in the expensive, complicated solution. In
practice, the advantage ain't worth the headache.
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Fri Mar 07, 2008 7:42 am Post subject: Re: Coax termination and electrical theory explained. |
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| Quote: | The difference in actual performance in the
flying airplane will be lost in the noise (pun intended). In theory,
there is an advantage in the expensive, complicated solution. In
practice, the advantage ain't worth the headache. |
Maybe, maybe not. It depends on the details. But you have not tested it, (nor has Bob). I submit that when the HANDY SHORT-CUT solution is proposed as the GENERAL solution, the best interests are not well served.
The coax terminations in electronics catalogs are neither difficult nor complicated. Furthermore you can make you own in a pinch. So there are two choices:
1) The right way.
2) The other way.
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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