coax core and braiding

[johnnyjb]

I know that the centre core on a coaxial cable supplies the signal( rg59 rg6) but do you need to have the braiding or is it just for interference.Any info appreciated

 

[breezer]

you require both to be connected

 

[johnnyjb]

is the braiding for interference or is it like a negetive of the circuit

 

[RF Lighting]

both

 

[lvsystems]

What RF said.

Besides if you terminate co-ax correctly the shield will be connected

 

[Space cat]

The signal doesn't travel in the central conductor or the outer braid but in the space between them as an electromagnetic wave. This is why the choice of dielectric (that's the insulation) is so important in low loss cables. Having said that, there are currents flowing in these two conductors. The magnetic wave cannot exist without them.

 

[johnnyjb]

could you post any links for that info Please Space cat

 

[Space cat]

This:

http://www.radio-electronics.com/info/antennas/coax/coaxial_feeder.php

is the best I could find at short notice. I'm sure I have a book somewhere that has a diagram of a coaxial transmission line complete with electric and magnetic fields but I can't remember the title. Maybe this description will help.

The surfaces of the two conductors facing each other across the insulation form a capacitor and the electric field lines are radial. At the same time, the two conductors form a single turn coil, ie an inductor, and the magnetic field lines are circular. The capacitance and inductance are distributed uniformly down the length of the cable and, curiously, their combined effect makes something that behaves like a resistor. (If you can do calculus you can prove this for yourself.)

If you could put an ohm-meter across the end of an infinite length of coax you could measure this virtual resistor. In Noddy language, what happens is that current must flow into the cable to charge the capacitance. At the same time, voltage is required to get this current moving in the inductance. The ratio of voltage to current is the characteristic impedance of the coax. Inside the cable, a wave front is moving at near light speed charging capacitance as it goes. In a finite length it will hit the end and reflect. The reflected wave doubles the voltage but reduces the current to zero and so, when it gets back, your meter shows an open circuit. To see what is happening you need a very fast meter. It can be done. It's called time domain reflectometry and it's great for tracking down faults in long cables.

 

[johnnyjb]

im a spark so know what cap and induction is but i never knew it was so complicated thanks alot you seem to know a fair bit about it

 

[Space cat]

You're welcome.

 

[johnnyjb]

sorry now one more question would any of ye know how to calculate how long through a cable a signal would go without degrading or even totally disappearing for example a sky digital digibox signal on rg 6

Sorry if im not makin sense

Is there any technical formula or data chart for this

 

[plugwash]

when it gets back, your meter shows an open circuit.

That only happens if the meter has an output impedance that matches the cables characteristic impedance, otherwise you get another partial reflection off the meter.

 

[Space cat]

That only happens if the meter has an output impedance that matches the cables characteristic impedance, otherwise you get another partial reflection off the meter.

Plugwash is right again. That wave will make multiple trips up and down the cable until it decays away. I remember seeing this on a scope.

 

[OLD]

This should help to answer your question johnnyjb
http://www.satcure.co.uk/tech/cable.htm#notes
Coax is about screening the braid is at 0 volts (earth) and the centre carries the signal. attenuation is caused by the fact that the cable is a capacitor so a greater diameter therefor larger distance between conductors or insulation with more air reduces capacitance.
The signal has come over the air so if the cable was not self shielding it would pick up lots of other signals via the air which would be seen as noise the screening gives best signal and lowest noise.

 

[Space cat]

attenuation is caused by the fact that the cable is a capacitor so a greater diameter therefor larger distance between conductors or insulation with more air reduces capacitance.

Most of the losses are caused by the fact that the capacitor has dielectric in it. It's not easy making coax with a vacuum between the conductors! Capacitance is a function of two things: the RATIO of the inner and outer diameters and the relative permittivity of the insulator. The capacitance per metre is predetermined by the required impedance of the cable thus:

C = 1/cZ where c is the speed of light through the dielectric.

It follows that C can only be reduced by increasing c. This in turn can only be done by reducing the relative permittivity of the dielectric.