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Re: Using HV COAX without stripping the shield



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).