Hi Scott,
I think I can help with your questions. I haven't seen any other
posts yet. I hope this hasn't already been beaten to death for
you off the reflector.
>From: AA9VC@aol.com
To: <towertalk@contesting.com>
>Date: Sat, 12 Sep 1998 15:08:23 EDT
>
>I'm looking for help in the use of silicone grease on
>connectors, such as PL259's. I have Dow Corning #4 silicone
>compound.
We routinely use a silicon based grease to enhance the longevity
of our RF connections. But we use a different (also Dow Corning)
type.
We are concerned with some rather extreme environments. The
grease we use is specifically designed to NOT evaporate under
some very stringent conditions (hard vacuum, 300 C temperature,
etc.). It was developed to seal vacuum systems for things like
mass spectrometers without adding contamination to the system.
So once applied, it stays applied. I have examined connectors
protected with this grease that have been exposed to a relatively
corrosive environment for more than 15 years until some other
part of the system failed. The connector mating surfaces were
like new when we inspected them.
The stuff we use is called Dow Corning Silicon High Vacuum
Grease. For $15 to $20, you can get enough in one tube to last
your lifetime.
The stuff you have may be just fine for the same application.
But I don't have any direct experience with it.
The following information applies to application of ANY silicon
grease to an RF connector.
>
>Exactly how does one use this on the connector?
1. Determine which parts of the connector system are the ones
that actually make the conductive connection across the
interface.
For example, on a PL259, the following places are involved in
making the connection.
a) The outer cylindrical surface of the male center pin
b) The inner cylindrical surface of the female connector
center contact
c) The mating surfaces of the ground connector bodies (the
mating faces with the anti-twist detents)
d) The surfaces of the clamp bearing ring on both the
connector body and the clamping shell (nut) This is the
bearing surface that pulls the connectors together when
the clamping threads are tightened.
e) Both sides of the threaded section between the female
connector body and the clamping shell of the male
side
Other connector styles will have a different list of places
that perform the same basic functions as these.
2. If the connector(s) is not shiny new, clean the above areas
up so that they are.
3. Apply the grease very sparingly to only the above mentioned
places on the connectors to be joined.
Use as little as possible but insure that there is a thin but
continuous coating where the mating contact will occur.
Do not apply grease to any dielectric areas of the
connectors. We routinely violate this (not intentionally) at
power levels up to a couple hundred watts and frequencies up
to 900 MHz with no noted problems. But we are not running
high power and we are always in a system operating close to
1:1 SWR. I don't know what it takes to break this stuff down
but it might be possible. If it isn't on the dielectric
parts, it doesn't matter.
I can say that 150W on the lower HF bands at SWRs up to 8:1
does not produce voltages which are high enough to break it
down.
>Will it short out the center conductor to the outside braid
>(does it conduct)?
No.
Its properties appear to be very good when operated below
dielectric breakdown. If it is not applied to the dielectric
parts, this should not even be a consideration.
>Will it prevent a good connection by creating a barrier between
>the center conductor by not allowing a "metal to metal" contact?
No.
This is the "magic" part. Regardless of whether the connector is
greased or not, the connection between metal parts will occur
only at small spots where the surface topology of the parts
permit them to touch.
If the parts are not grease coated, the atmosphere (with its
oxygen, and corrosive gasses and vapors) will have access (yes,
even through your tape) to the remaining metal all around the
small contact points. The metal can oxidize or otherwise surface
corrode to a state that is not very conductive. When thermal
expansion and contraction move the contact point around slightly,
eventually the contact area can also be corroded or corrosion
products can be carried into the contact point and the connector
fails.
If the connector mating surfaces are grease coated, the contact
points push the grease out of the way and the exact same quality
of initial contact is established. But in this case, the grease
seals the area around the contact point(s) against the corrosive
atmosphere. With the right grease properties (doesn't freeze,
doesn't evaporate, doesn't dry out, doesn't easily wash away,
etc.), this seal is maintained even with the minor thermal
induced mechanical movement of the mating parts.
>
>I was reading through the archives and it seems that it's used
>quite a bit by professional installers (ie cable co). But, I'm
>not real sure on it's proper use from what information I was
>able to collect from the archive.
>
>Thank you for your help.
>
>Scott/AA9VC
I hope you find this useful. I don't work for Dow Corning or
have stock holdings with them (outside an index fund possibly).
I just happen to be a fan of this particular product. It saves
me a lot of 4WD mountain climbing time.
73, Eric N7CL
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|