Re: Using HV COAX without stripping the shield

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 06:25 AM 11/5/2005, Tesla list wrote:
> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Jim,
>
> OK this is good to know.  I have never seen 50 ohm coax with foam

So called RG-8X or mini RG-8...  It's the general size of RG-58 (i.e. 
about a quarter inch in diameter) and has foam insulation.

LMR400 (from Times Microwave), a coax beloved of VHF and up, is 50 
ohms with a foam dielectric.



> nor 75 ohm coax without foam, but then again.......

RG-11 is a good example of an RG-8/RG-213 sized (half inch) cable 
with 75 ohm impedance and solid dielectric.  You see it a lot surplus 
because it was part of a standard military dipole antenna kit (the 75 
ohms being a decent match to a 72 ohm dipole in freespace, not 
withstanding that relatively few dipoles actually wind up high enough 
for that to work)


> Gerry R.
>
>
> > Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
> >
> > At 10:50 AM 11/4/2005, Tesla list wrote:
> > > Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
> > >
> > > Hi Mike,
> > >
> > > I'm not necessarily taking issue with Dr R.  I'm suggesting an 
> > > alternative way to fix the problem.  Getting rid of the coax 
> > > shielding doesn't get rid of the transmission line, but it 
> > > certainly does change it and this may be enough to "fix the problem".
> > >
> > > BTW, RG8 is 50 ohm cable and the dialectric is PE not foam.  75 
> > > ohm cable typically uses foam to get the higher Zo.
> >
> > 50 and 75 is independent of foam (in that, you can get solid or 
> > foam dielectric for either).  The foam is used for lighter weight 
> > and lower loss at high frequencies (e.g. for Cable TV at hundreds 
> > of MHz, dielectric loss is important).  Solid has a higher 
> > breakdown voltage. Foam has a higher propagation velocity (because 
> > epsilon is lower).
> >
> > Impedance (Z0) is a function of the ratio of inner and outer 
> > conductor diameters (and dielectric constant).