[BC] Wave lengths

[Cowboy]

On Sunday 19 February 2006 18:01, JYRussell at academicplanet.com wrote:

>   And what's slowing the 'RF Wave' down on it's trip along the center and 
> outside shield of the coax is some kinda weirdness at the point where the 
> insulation and the copper meet.   A ratio of 'where it is' versus 'where it 
> oughta be' gives a funny version of efficiency called "velocity factor'.

 It's not efficiency.
 The explanation of a whole bunch of small inductors in series, with a whole
 bunch of caps across the line between each set of inductors is a good one.

 ( abreviated ) How long it takes to charge the capacitance of the section
 affects the time it takes before the energy appears to begin charging the
 next section. That time constant is affected by the qualities of the dielectric.

 The various qualities and geometry are chosen such that they cancel, but
 the line "looks like" a 50 ohm ( or whatever characteristic Z ) resistor if the
 line were to continue into infinity, such that there is no reflected wave.

> so, you should be able to use the numbers for the 'velocity factor' to know 
> pretty closely where the wave multiples are in any coax you've got the 
> velocity factor for (if you know the f(mhz)...
> 
>    Have I got it so far?

 Pretty much.

>     So, does the negative part (the ground shield) of the coax have other 
> weirdness where it meets the (usually) black outer cover??  Seems like that 
> conductor has two junctions, one inner (foam) and one outer (black stuff).
> ??  How come half the wave isn't slowed down more than the other??

 In short, in an unballanced sheilded line, the inside of the outer conductor
 can be considered to be a seperate and distinct conducter than is the
 outside of the outer conductor.
 The wave is travelling on the inner surface of the conductor, and not in
 the conductor material itself.
 ( not quite correct, but close enough for this discussion at present )
 Thus, the conditions on the outside surface of the outer conductor have
 no bearing at all on what's happening on the inner surface.
 
>    I could see how ladder line (balanced, parallel lines held apart by 
> insulators...) would have the same 'velocity effecting' factors, since 
> they're both out in the air... but in normal 'unbalanced' coax, it would 
> seem the outer conductor has a junction with insulation the inner conductor 
> doesn't have... so is there a difference you can see by comparing those 
> signals, if you know what to look for...?

 No. See above paragraph.

 Likewise, the inside of the inner conductor has no "power" there
 at all, and can be at the same potential as the outside surface of
 the outer conductor !

 The energy is confined to the space between the surfaces, outside
 surface of the inner, and inside surface of the outer.

> Or is the 'velocity factor' sort of a combination of all the above... ?

 Primarily dependent on the dielectric between the conductors, 
 even in parallel conductor ladder line as well.
 ( which is usually "mostly" air )

-- 
Cowboy

http://cowboys.homeip.net

System checkpoint complete.