[BC] Wave lengths

[Cowboy]

On Tuesday 21 February 2006 21:58, JYRussell at academicplanet.com wrote:

> sure it seems obvious... you don't use radio shack stuff to hook you tx up 
> to a 3 bay jampro.   But, why?

 You *could* but only to about 1 KW or so.
 More than that, and likely the line will fail, probably due
 to too much current.

>    Seems like as long as things are close to reasonable, it's fine.  But, 
> when you push the limits, what happens?

 Depends which limit is pushed over the line first.
 Any given conductor size will handle so much current, before turning
 into a space heater, and melting the insulation.

 Any given insulation will handle so much voltage before arcing through.

>   Which lets go first when the coax lets go? The first inductor / capacitor 
> junction? Or the point where the first full quarter wave occurs?

 The point where the limit is exceeded.
 Like anything else made by man, there are tollerances, and these
 limits are not necessarily the same at every milimeter along the lines
 entire length.
 ( although every point should meet or exceed the rating.

> How do you  
> describe when the coax is breaking down...

 FIRE !!   ;-)

> is it a failure of the conductors  
> just due to the amplitude, or, a failure due to the lack of enough 
> dielectric?  (f'rinstance, can you make a stranded RG58 center run down the 
> center of a big enough foam filled line to handle the amplitude?  No... does 
> the center just get hot due to the current?  RF current?

 As above, yes.

> would that mean  
> there's a theoretically some frequency where you COULD do that?

 No.
 50 ohms is 50 ohms ( or 75, or 62, or whatever the line has been
 manufactured to be ) which establishes the voltage to current ratio in
 a matched line.

> So...
> Does that mean that there is a point where there's not sufficient center 
> conductor to allow the RF to be RF... it's actually so lacking in area for 
> that "skin effect" that it become straight out electrical energy... and pops 
> the center conductor?)

 Think space heater, soldering gun tip, etc.

>   Why can't you go the other way... 5 watts on the end of a piece of 1.5 
> inch stuff.  Put the appropriately tuned... though rather small... antenna 
> on the end for the freq. you're at... and it should work, right? 

 REALLY well !

> 5 feet of  
> it ... yeah... but a hundred??  Would the value of all the capacitance just 
> "soak up" all the RF before you get to the end of the line.. ?  DO you just 
> not have enough "oomph" to get the "skin effect" you need to set up a wave? 
> That's what I think would happen... but maybe not.

 No.
 Skin effect has to do with the outer orbitals of the atomic structure transfering
 additional electrons, which do take some space, and stress of materials at
 the atomic level.
 The string of infinitely small inductors and capacitors don't soak up anything.
 Think reactance, not true resistance.

> Done properly, does any transmitter "see" anything beyond the first few feet 
> if coax?  Probably not even the 'first few feet'...
>  unless there's someting wrong.

 The transmitter doesn't really see anything beyond the point of connection,
 like you don't *really* see anything beyond the surface of the retina in your
 eye. You DO see the image formed there by everything beyond.

>  although the tx does have to see the  
> specified 50ohm to be happy... which I guess must be the antenna.  (or, the 
> whole system?  hmmm. )

 The conjugate effect of the whole system as it appears at the point of connection.

>    This is another one of those "printable and bindable" threads.  I ought 
> to be able to understand at least why a particular coax is appropriate for 
> some given job... and which others I can get away with in a pinch... while I 
> wait for the right stuff to get ordered. (Imaginary but possible 
> scenario...)
> ??

 As well as how to grab whatever is handy, and make it work to whatever
 point it can handle before it fails, and stay below that point.

-- 
Cowboy

http://cowboys.homeip.net

System checkpoint complete.