Re: Using HV COAX without stripping the shield

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Gerry, all,

I've always liked HV x-ray cable for the transmission
line(s) from my pig to the coil. I realize that there is the
possiblity of run-away resonant voltage due to the Blume-
line (spelling?) effect, but I have never personally noticed
this problem when using my x-ray cables with the shielding
intact. In one of my previous big coils, I simply grounded
one of the HV output bushings of the pig and also grounded
this to the exterior shielding braid of the cable. I connected
the internal conductors to the "HOT" HV output bushing of
the pig. This worked great for "me". However, with my lat-
est big coil "Green Monster":
http://dawntreader.net/hvgroup/david/gm.html,
I left both of the HV output bushings of the pig "floating",
but still used  x-ray cable for EACH of the HV outputs
of the pig and STILL grounded the braided shielding of
EACH cable lead. This has also worked well for me.
BTW, the grounding of the outer braid is connected to
the 60 Hz main ground for safety.

BTW, if you'll notice on my webpage, I DO use overvoltage
protection for the HV output of my pig in the form of an
18 kV rated distribution arrester. A distribution arrester
will let you know when you are starting to exceed it's
cut-off voltage by causing the output of the coil to start
getting really "choppy". Once the voltage is dropped be-
low this setting, the output returns to smooth. My 18 kV
arrester hasn't "clipped" the HV input to Green Monster
yet, but I know that it IS functional because I previously tested it 
by slowly ramping up a test voltage to it from an x-ray transformer 
until it started to conduct.
David Rieben

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, November 03, 2005 9:40 AM
Subject: Using HV COAX without stripping the shield


> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
> Hi All,
> It has been mentioned in the group in the past that there can be a 
> high frequency resonance when using a coax cable such as RG8U to 
> connect the HV from a PIG to the TC where the PIG is remoted from 
> the TC.  This resonance can cause a higher voltage standing wave on 
> the PIG end of the coax.  Dr R has suggested stripping the outer 
> braid of the coax to minimize the transmission line effect,  It 
> seems like to me that doing so changes the characteristic impedance 
> of the cable and doesn't really eliminate the problem.
> I was wondering if anyone has experimented with adding a series RC 
> termination to the pig end of the coax to effectively terminate the 
> coax.  The C could be a string of C's to get adequate HV 
> capabilities and the R would be 50ohms if the shield wasn't removed 
> or say 300 ohms if the shield was removed.  The RC time constant 
> would be set to be higher than the resonant frequency of the TC and 
> yet low enough to effectively terminate the coax for the high 
> frequencies where the coax length would resonant.
> Gerry R.