Proper Sealing of Outdoor Coax Connectors [or Waveguide Splice]
seems to have a different idea how to properly seal outdoor coax
connectors, such as where the antenna cable connects to the rigid Heliax
feedline, or short jumper cable. The fact is over time, most cables actually allow small
amounts of air and moisture to pass through the jacket and find its
way inside your connectors. This moist air is not a problem until a
rapid temperature shift occurs and condensation forms inside a
connector, typically every evening. Since most commercial installations use type-N connectors,
it takes only 1 drop to alter the SWR and began to reduce the overall
antenna performance. For example, even with just 1 layer of a good
moisture-proof vinyl tape such as Scotch 33, over a connector on a
VHF antenna, within a year there will be at least 3 drops of water
inside the connector. This will reduce the antenna performance by at
least 65% and usually much worse. Therefore, the objective is twofold - seal the connector
from weather and provide thermal resistance to prevent condensation
for occurring inside the connector. Below are several well proven ways to provide excellent
protection from the problem above.
Packing with silicon gel or grease - This concept we learned from the Air Force. It seems that whenever an airplane takes off & reaches altitude, and then eventually lands, the temperature of some of the electrical & electronics equipment [outside the passenger area] can vary more than 50 degrees F, and sometimes over 100 degrees. This thermal shock with moist outside air cause small amounts of condensation in electrical connectors, which leads to erratic performance of fire control RADAR and other Radio equipment. It had long been standard practice to fill the AN style electrical connectors with silicone grease, to prevent arcing between pins from moisture. So it also became a standard practice to pack the inside of all type-N coax connectors with silicone gel. It has been my experience this works very well, but is very messy to work with, in particular at the top of a tower. Andrew has long supplied a tiny unmarked tube of this with each Heliax connector to weatherproof the inside of the front and rear of each connector. If the silicon grease gets on the outside of the cable or connectors, due to mis-handling, vinyl tape will not stick. However, if you are assembling things on the ground, and you have clean rags and solvent to cleanup with, go for it. This product is sold as Dow Corning 4 Electrical Insulating Compound and is widely available from $12 to $18 per 5.3 Oz. tube. I buy from Skygeek.com for under $15 per tube. One tube will last for many years if used just for coax connectors. You may also find new military surplus tubes that are out-of-date, but will work just fine. Be sure to wrap the outside of the connectors with at least two layers of Scotch 33 [and 2242] tape when complete. Be advised, this is not the same product as silicon sealant, this is a gel that remains a gel and does not become solid. Do not use silicone sealant inside a connector.
Glue filled heat-shrink tubing - This concept came from the cable TV industry. These guys also use silicon grease in some aspects of their network, but to get a water-tight seal in underground connectors, they use a heat-shrink tubing, with a heat-activated glue inside. it is best to place a single wrap of vinyl tape over the connectors before placing the tubing in place, otherwise you will have a sticky glue mess on the connectors. Then proceed to heat the tubing with a propane torch, evenly all around. The tubing should extend beyond the connectors at least 2 inches, so there is a cable jacket to cable jacket bond. To remove, simply cut the plastic end to end and peel. This product works great, but using a propane torch at the top of a tower is sometimes a problem, and it sometimes drips hot glue on the workers below. Again, this is great on the ground, but tricky in the air. This product is widely available for around $10 per 1 inch x 48 inch lengths. Be advised some glues may crystallize from sun & weather, therefore radio manufacturers like Codan recommend placing a single wrap of self-amalgamating rubber tape over the tubing.
Wrap with real rubber tape - This is my preference when working on top of a tower, or otherwise. It is the cleanest to use and does not rely on torches or solvents to do a clean job. Scotch 130C can be used in place of Scotch 2242, but is more expensive. The price ranges from $6 to $12 per roll of 3/4 inch x 15 ft. The tape is 0.030 [1/32] inches thick and easily stretches, but do not over stretch it. It is self-amalgamating, so it cannot be unpeeled after a few days. Do not apply the rubber tape directly to the connectors, as it bonds to itself and will not peel off after a few days. To remove, it must be cut off and will leave a black sticky mess on the connectors unless a single wrap of vinyl tape is used first.
Be sure to stretch the tape as you apply it. Fresh tape [within a year] works better than old tape.
- layer 1 is a thin vinyl tape which should be 15% overwrap and is to protect the finish of the connectors, use Scotch 33 ;
- layer 2 is a thick natural rubber tape which should be 15% over wrapped, and extended one inch, on both ends, beyond the prior wrap, use Scotch 2242
- layer 3 is a thick natural rubber tape which should be 15% over wrapped, and extended one inch, on both ends, beyond the prior wrap, use Scotch 2242
- optional layer 4 is a thick natural rubber tape which should be 15% over wrapped, and extended one inch, on both ends, beyond the prior wrap, use Scotch 2242
- Final layer 5 is a thin vinyl tape which should be 15% overwrap and is to protect the prior natural rubber finish, use Scotch 33
When you are done, you will probably have used a full roll of Scotch 2242 rubber tape. These 5 layers can be easily removed with a pen knife, slicing length wise, and peeling off. Without layer 1, you will have a tough time ending up with clean set of connectors as the self-amalgamating rubber tape will leave a sticky and gooey finish.
CoaxSeal.com makes a fine reusable product that remains soft and pliable. It is easily removed and rarely leaves any residue. However, if the splice can be stepped on or is in a traffic or climbing area, the sealant can be disturbed or scrapped off, thereby exposing the connector to weather. I would suggest placing a wrap of Scotch 33 vinyl tape over the CoaxSeal, for protection from scrapes or dislodging. A typical roll is 1/2 inch wide and 60" [5 ft] long. A roll costs around $6 and is widely available at master distributors or on the Internet. Coax-Seal is easily wrapped and then molded by hand to your desired finish.
There may be other practical methods to moisture proof and thermally insulate the connectors, but these above are the best we have seen. For example, there are snap-on covers that provide thermal protection, but are not always waterproof, even filled with silicone gel, as shown to the left. [and what a mess when you open to perform service]
As with all products, some are better than others. It all depends upon on how important it is doing the job right the first time, and adopting a procedure for your crew, so even a not-so-bright employee, cannot do it wrong.
Revised May 19, 2018 by Fred Daniel