[TenTec] OT: SWR & Coax

[shristov]

Dr. Gerald N. Johnson wrote:

> > 2. The only way to "prevent" radiation is to counteract it by a radiation
> >     from another AC current of the same magnitude and opposite phase,
> >     flowing in a nearby conductor.
> >     This is how transmission lines work, both "coaxial" and "open".
 
> No. The shield of the coax can have independent currents inside and
> outside. It takes a load like an antenna to cause currents on the
> outside. The shield prevents radiation from the inside current even if
> it doesn't match the magnitude and opposite phase from the center
> conductor.


Ever heard of a coax with the inside current not matching
the center conductor current? Exact matching is the essence of coax.
If it's coaxial, then the currents match.
It's physically impossible for them not to match.


> > 3. If the cancellation mentioned above is not complete (unequal magnitude,
> >     inexact phase, separation too large) the transmission line will radiate.
> >     This is what happens with "unbalanced" lines, carrying "common mode"
> >     currents in addition to normal "differential mode" currents.
 
> Not with coax, because its shielded and the fields are enclosed.


As said above, if it's coaxial, there are always matching currents,
and the radiation is suppressed by cancellation. There is no way for
the inside shield current not to match the center conductor current.  

RF shields work by attenuating the field, but that's another story.
In the case of coax, the attenuation property is useful for separation
of outside and inside currents, but it's not required for the cancellation.
 The cancellation is already done by the time the shield starts to attenuate.


73,

Sinisa  YT1NT, VE3EA