Use dielectric grease on coax or not?

[bgephart]

I have read past threads re dielectric grease and they leave me confused. All agree that dielectric grease is non-conductive. But there is some confusion whether to apply dielectric grease to coax or other 12V boat circuit connections. The confusion above relates to "if dielectric grease is non-conductive, how does it enable current to pass in an electrical connection?" I am renewing my VHF coax connections and would like to know how (or if) to apply dielectric grease to that connection. Put it on the center pin? What about the outer braid? Don't do it? HELP!

Bob Gephart - Savannah

[Bob Lascelles]

Bob
In a connector, the purpose of the non conductive (dielectric) parts such as insulators and non-conductive shells (in addition to mounting) is to electrically isolate the electrical contacts, from each other and from the outside and to provide protection from the environment. Any kind of grease used in and around a connector would need to be dielectric to avoid causing electrical shorting between conductors or to the outside. Smearing a conductive grease in a connector could have very bad electrical consequences. Electrical contacts are designed (within a specification) to have sufficient spring force to displace normal contaminants from the surface of the contacts when they are engaged and thus make reliable electrical contact. I guess dielectric grease that is designed to be used in our connectors would be designed so it would be displaced by the normal force of the contacts and therefore not effect the performance of the contacts.

In a coax connector all of this is still true, but the signal is an electro magnetic wave traveling within the dielectric core between the shield and the center conductor. The electrical performance of a coax connector still requires the two conductors to have electrical continuity, so avoiding corrosion is still goodness. It seems to me, whether the connector would be effected by the addition of more dielectric material inside itself (by adding a glob of dielectric grease) would depend upon the wave length and any difference it might cause in the impedance. I think a little dielectric grease on the threads of the connector won't hurt anything.

All the best
Bob

[David Morton]

Interestingly, this exact question was asked in this month's Practical Sailor. I quote from a portion of the answer provided:

" To apply, coat the contact surfaces and then assemble the connection as usual. The clamping pressure, whether a bolt or simply the pressure between the contact surfaces in most plugs, pushes the grease out of the way, providing a good contact. You should also coat or spray the outside ... The remaining dielectric serves to seal the contact area from water, oxygen, and galvanic corrosion.
There are two exceptions to this practice. Grease is not dielectric at radio frequency, so it should not be used on radio antenna connections. When making antenna connections, simply get them as clean as possible and seal well... "

VHF coaxial cable? hmmm, maybe not.

[Jim Walsh]

Interestingly, this exact question was asked in this month's Practical Sailor. I quote from a portion of the answer provided:

" To apply, coat the contact surfaces and then assemble the connection as usual. The clamping pressure, whether a bolt or simply the pressure between the contact surfaces in most plugs, pushes the grease out of the way, providing a good contact. You should also coat or spray the outside ... The remaining dielectric serves to seal the contact area from water, oxygen, and galvanic corrosion.
There are two exceptions to this practice. Grease is not dielectric at radio frequency, so it should not be used on radio antenna connections. When making antenna connections, simply get them as clean as possible and seal well... "

VHF coaxial cable? hmmm, maybe not.

Uh-oh. I always put a little dab of dielectric grease on electrical connections. Maybe I've been lucky so far.