[BC] Wave lengths

[Jeff Johnson]

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

Primarily, a coax is limited by its voltage handling ability - its 
resistance to arcing. Higher power coax can take more voltage. Current is 
of lesser concern. For example, you might see 5000 volts but only one amp 
on a coax carrying 5KW of RF. That is why it is important to keep an air 
dielectric coax full of dry air or nitrogen. Arcing will commonly occur at 
an insulator or an ell. Any burr on the inner surfaces are obviously 
undesirable.

>  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?  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.

There would be no problem with 5 watts on a large coax. There would be less 
distributed capacitance and less loss. It would be akin to a lunchbox in a 
limo, but it would work excellently. Length could be longer due to less 
loss/foot.

>Done properly, does any transmitter "see" anything beyond the first few 
>feet of coax?  Probably not even the 'first few feet'...
>unless there's someting wrong.  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 transmitter 'sees' only the load, the antenna, if the TX line is a 
perfect impedance match to the transmitter and antenna. If the antenna is 
the same, all the transmitter thinks it 'sees' is a 50 ohm resistor with no 
reactance. The power goes out and never comes back (glory, hallelujah, 
amen!). Additionally, the voltage and current peaks and valleys on the line 
will remain 'standing' at the same places down the length of the line. This 
is known as a 'standing wave'. Any anomaly, known as an impedance 'bump' or 
mismatch, will cause some percentage of the RF to 'bounce' back to the 
transmitter as it is not 'absorbed' by the antenna. Since the frequency is 
relativelyconstant, the line stays the same length, and the anomaly does 
not move, the standing wave going out and the standing wave coming back 
will add to and subtract from each other at certain fixed points. Consider 
the points where the voltages add. It is possible that the added voltages 
will exceed the 'breakdown' strength of the insulation, the dielectric, of 
the transmission line or the output circuitry of the transmitter. An arc 
will occur. This is high VSWR (Voltage Standing Wave Ratio) pronounced 
'visswar' and to be avoided for this reason and others. In FM, the 
frequency will fluctuate, moving the points somewhat.

>   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...)
>??
>Jason

Power handling capacity and frequency handling ability are both to be 
considered. The line must be able to withstand the voltage and current and 
not attenuate the desired frequency too much. The 'characteristic 
impedance' must be correct.

Jeff.Johnson at goodnews.net