BNC 50 Ohm coaxial replace with 75 Ohm

Thread Starter

Walter_S

Joined Mar 9, 2018
13
Hello All,

I have an equipment where 50 Ohm RG58 coaxial cable is used for transferring up to -2 kV from one side to another one (4 m cables). Actually this is shv bnc cable. It seems that cable is broken and I have spare but RG59 (75 Ohm), 50 Ohm I'll have to buy which is quite expensive as it is SHV not normal BNC.
My question is - can I just replace 50 Ohm with 75 Ohm cable?? It's just DC signal, high potential but almost no current so I guess I can, but want to get your opinon. According to wiki https://en.wikipedia.org/wiki/BNC_connector connectors 75 and 50 Ohm should fit to each other.
Right now I'm not close to equipment, so want to be sure that I have everything when I go onsite.

Thank you very much for feedback.
 

bertus

Joined Apr 5, 2008
22,266
Hello,

The maximum usable voltage for RG59 is 2300 Volts RMS, wich is close to the mentioned 2000 Volts you want to use.
Better use a better type of coax to avoid break through.
The following page will give you a table with the properties of a great variety of coax cables:
http://www.rfcafe.com/references/electrical/coax-chart.htm

The normal BNC is only rated for 500 Volts:
https://www.amphenolrf.com/connectors/bnc.html

Better look at the MHV series:
https://www.amphenolrf.com/connectors/mhv.html

Bertus
 

Thread Starter

Walter_S

Joined Mar 9, 2018
13
Hello,

The maximum usable voltage for RG59 is 2300 Volts RMS, wich is close to the mentioned 2000 Volts you want to use.
Better use a better type of coax to avoid break through.
The following page will give you a table with the properties of a great variety of coax cables:
http://www.rfcafe.com/references/electrical/coax-chart.htm

The normal BNC is only rated for 500 Volts:
https://www.amphenolrf.com/connectors/bnc.html

Better look at the MHV series:
https://www.amphenolrf.com/connectors/mhv.html

Bertus
Hello Bertus,

This spare 75 Ohm cable which I have is SHV, so no problem. Summarazing, I can replace 50 Ohm with 75 Ohm if it's just DC signal using, right?? When would be a problem to do that in case of frequencies??

BR
Walter
 

MrChips

Joined Oct 2, 2009
30,618
Before you go any further, is the signal on the BNC cables DC or AC?
If AC, what is the max frequency?

Secondly, check that the existing BNC cables and connectors are rated for high voltages. Not all RG58 cables are specified for 2kV.
 

Thread Starter

Walter_S

Joined Mar 9, 2018
13
Before you go any further, is the signal on the BNC cables DC or AC?
If AC, what is the max frequency?

Secondly, check that the existing BNC cables and connectors are rated for high voltages. Not all RG58 cables are specified for 2kV.
This is only DC. I asked about frequencies for the future reference - I supposse when frequency increases then could be a problem with replacing 50 Ohm with 75 Ohm, but I'm not sure. They are rated for 5kV I think - https://www.pasternack.com/shv-plug-standard-rg58-connector-pe4194-p.aspx
 

MrChips

Joined Oct 2, 2009
30,618
You have SHV connectors.
BNC, SHV, and MHV are not compatible.
For DC, the impedance of the cable will make no difference.

You really need coax cables with SHV connectors on the ends.
RG58 or RG59 will make no difference as long as the dielectric is solid polyethylene and not foam.
 

RichardO

Joined May 4, 2013
2,270
You have SHV connectors.
BNC, SHV, and MHV are not compatible.
For DC, the impedance of the cable will make no difference.

You really need coax cables with SHV connectors on the ends.
RG58 or RG59 will make no difference as long as the dielectric is solid polyethylene and not foam.
As MrChips says, never try to plug a cable with a BNC connector onto a high voltage connector. They look the same but they are not. If you manage to get the cable connected you may never get it off without damaging something. (This is a mechanical problem having nothing to do with the voltage).
 

Thread Starter

Walter_S

Joined Mar 9, 2018
13
As MrChips says, never try to plug a cable with a BNC connector onto a high voltage connector. They look the same but they are not. If you manage to get the cable connected you may never get it off without damaging something. (This is a mechanical problem having nothing to do with the voltage).
Thank you All for info - don't worry, cause I have SHV connectors on both sides on my spare cable. The only difference between original cable and spare one is impedance - original is 50 Ohm and spare 75 Ohm.
Assuming that signal is not DC, but has few kHz - then still impedance doesn't matter??
 

MrChips

Joined Oct 2, 2009
30,618
Thank you All for info - don't worry, cause I have SHV connectors on both sides on my spare cable. The only difference between original cable and spare one is impedance - original is 50 Ohm and spare 75 Ohm.
Assuming that signal is not DC, but has few kHz - then still impedance doesn't matter??
Now you have mismatch of impedance and the signal will be degraded with reflections in the cable.
 
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