Hi,
This is weird stuff.
> I agree that is true for a normal matched coax application. I don't
really
> know what happens when the VSWR is high. I am not sure the field is
really
> confined to the dielectric between the shield and center conductor in
that
> case. (I'm not sure it's not either, for that matter!)
Radiation from coax is mostly due to imperfections in current balance at
the cable ends. Surprising as it seems, the inside of the shield ALWAYS has
as much current moving in the opposite direction as current on the center
conductor, as long as the shield is several skin depths thick.
Any current that is not equal and opposite on shield and center results in
the unbalanced amount of current appearing on the outside of the shield.
For practical purposes, not a single time-varying field of any type,
magnetic or electric, goes through the shield once the shield is more than
several skin depths thick. VSWR doesn't make a bit of difference because
radiation is a current phase and ratio (between the shield and center)
problem, and the problem could get better, worse, or stay the same with any
direction of SWR change.
> > SWR at the antenna WILL cause higher currents to flow
> > in the feedline and so will tend to exacerbate whatever
> > leakage there is from the coax. I don't know the attenuation
> > of a signal that leaks from a cable vs. what is radiated from
> > the antenna, but I expect it is a LOT. I would expect poor
> > cable might be as bad as 10 dB down and good cable
> > (like hardline) could reach 50 dB or more.
I measured some crappie Radio Shack cable, and it was over 80 dB down over
the HF spectrum. This was for a length of 25 feet ty-wrapped tightly
against another crappie Radio Shack cable. Of course the longer the run the
better the coupling, and the wider the spacing the less the coupling.
I couldn't even detect the leakage on good RG-8, and I was measuring with a
RF millivoltmeter and with 2500 watts driving the cable under test! The
worse radiation was heat.
When you hear numbers like "leakage -60 dB" bantered about, ask them "under
what test conditions and frequency", since frequency, distance, and spacing
affect crosstalk. It's kinda like asking what's your antenna gain and the
manufacturer saying 7 dB. 7 dB what and where?
Common mode current is the real problem.
73 Tom
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|