CASE HISTORY:
In the mid 1990s, I worked with a company that used business band FM to dispatch trucks. A year or two, after replacing the antenna on their base unit, they once again began experiencing poor communications with their trucks. They tested the base antenna and feedline VSWR, with a standard inline VSWR meter. The readings during dry and wet weather always came back normal. The company replaced their antenna, and communications was still poor. They then changed out the coax feed line, and the radio system began working normally. As a check, they retested the feedline using an inline VSWR meter, and the VSWR was normal. That is when they called me in.
PROBLEM INVESTIGATION
I checked the antenna and feedline that was replaced. The VSWR was normal when using an inline VSWR meter. I knew that something was going on, so I retested the antenna and feedline using about 50 watts, on a dual needle, forward and reverse power meter. The VSWR was over a 3 to 1. That immediately indicated that the antenna or feedline was breaking down when the TX power was in the operating range. I tested the antenna and it checked out as good. That made me believe that the coax was at fault.
The PL-259 connector exterior looked good. After disassemblin the cables PL-259 connectors, the obvious fault was that the cable shield was badly corroded. Aluminum and copper particles were visible under the coax outer jacket. (See PHOTO #3) This caused a great deal of voltage breakdown and RF leakage while running at higher power levels. Since the corrosion was primarily near the connector, it made good sense that moisture was the problem.
PROBLEM SOLUTION
After some testing, it became obvious that the source of the moisture was from liquid leaking around the SO-239 connector flange. This moisture then flowed through the edges of the SO-239 insulator. (See Photo #5) A easy and effective solution may be found in PHOTO #5.
.
PHOTO #1 – This PL-239 connector was installed on an antenna system for about 3 years. It was connected to the cable shown in Photo #3. The connector had the lower section of the connector covered with dual wall FIT 321 adhesive shrink tubing. Over that was a layer of FIT321 covering the complete PL-239 connecter, about 6” of the RG-59x cable, and the threaded portion of the SO-239. All of that was covered with rubberized liquid sealant. With this level of sealant, it is very unlikely that any moisture entered the connector through the sealant. Note that the connector pin and threaded retainer has almost no corrosion. It looks almost new.
.
PHOTO #2 – This section of RG-59 cable was from a mobile installation, approximately 1 year old, and was located a couple inches from the PL-259 connector. The cable and connector were covered with adhesive FIT-321 heat shrink tubing, and rubberize sealant, but nothing was done to prevent moisture intrusion through the SO-259 connector. The copper shield was heavily corroded, with minor corrosion of the center conductor. A small amount of copper oxide powder was found on the center insulation, inside the PL-259 connector. The shield was moist when cut open, and the moisture was a light green color from metallic copper ions. The corrosion was found to continue several feet into the cable.
.
PHOTO #3 – This section of RG-8x dual shield cable was from a base station installation, that was approximately 2 year old. The sample cable section was located a couple inches from the PL-259 connector. The cable and connector were covered with adhesive FIT-321 heat shrink tubing, and rubberize sealant, but nothing was done to prevent moisture intrusion into the SO-239 connector. The copper braid outer shield was heavily corroded. The inner aluminized mylar shield had almost all of the aluminum corroded away, and a large number of copper and aluminum particles were present in the cable and PL-259 connector. Moisture and dissimilar metals caused galvanic corrosion. The metal particles, aluminum and copper oxide, and moisture, caused electrical leakage at higher power levels. Capillary action drew moisture farther into the cable, then what occurred with single shield RG-58.
.
PHOTO #4 – In the past, good quality SO-239 connectors used phenolic resin or PTFE insulators. Both were molded into the connector housing and had a fairly tight seal. Unfortunately, today’s prices for those are between $9 and $20. Today, some connectors use nylon, with some higher quality ones using Teflon. The majority use press fit insulators or slide in fit with crimped edges. None of these have very good sealing quality.
CONNECTOR LEAKAGE POINTS:
There are three locations that allow leakage. The first is the space between the connector’s threaded outer wall and the insulator. The second is between the insulator and center pin. A third would be through the pin itself. Filling the pin with solder, when installing the cable, is an easy fix for through the pin leakage.
CONNECTOR MOUNTING LEAKAGE:
When an SO-239 connector is mounted to a device, the space between the connector flange and the device is a real weak spot. Moisture can be drawn into this gap by capillary action. Once it is there, it can easily get into the cable, through the leakage paths noted above. The mounting screw holes are also an issue. Note that some devices have one of the SO-239 holes open to drain any moisture from the device. Sealing the flange around the hole may or may not be a good solution.
.
PHOTO #5 – This is a photo of the top and bottom, of an SO-239 connector, that was removed from service. The connector was mounted on an unsealed RF choke case, which experienced water leakage. It appears that the water pooled on the rear side of the SO-239 connector, originating inside the box.
The water leaked downwards, between the connector’s threaded outer wall and the insulator, and between the insulator and center pin. This allowed the water to enter the PL-259 connector, that was connected to this SO-239 connector. This eliminated any benefit that may have been gained by having the PL-259 connector sealed with water resistant tape and rubber sealant.
The brown and green materials are copper oxide, originating from the corrosion of the coax cable shield. The pointed areas of copper oxide, may show the locations of RF arcing.
Also note the discoloration of the flange on the connector’s bottom photo. Evidence of water leakage between the transformer case, and the SO-239 flange is visible.
.
PHOTO #6 – This is a photo of a two year old SO-239 connector mounted to a wall of a Balun. I cleaned it up, because it was really dirty. It was properly sealed with RTV 732 and silicone grease. Note that the threads and pin housing are still shiny, without corrosion. The photo below the connector is a section of the coax that was connected to this Balun. The aluminum inner shield and copper outer shield has no corrosion due to moisture.
It is fairly easy to prevent moisture from entering a PL-259 connecter from the cable side, and from the SO-239 side. For the cable side, seal the PL-259 connector with a couple layers of heat shrinkable tubing and rubber liquid sealant. Make sure that the outside of the PL 259 connector is free of silicone grease. For the SO-239 side, follow these instructions.
NOTE: Do not use any silicone grease that contains graphite, aluminum or copper metal.
(1) Apply RTV 734 Sealant around the SO-239 flange, where it comes in contact with the device. Give it a few minutes to be drawn into the gap, and then reapply. The RTV should form a fillet between the device and the SO-239 flange.
(2) Apply RTV-734 over the mounting screw caps. If the screws can be removed, apply RTV on the screw threads, and retighten. The RTV should ooze out from around the cap of the screw.
(3) Apply RTV 734 to any unused screw holed on the SO-239. If one is a drain hole, use your discursion to fill it in.
(4) Apply one or two mL of silicone grease to the pin side of the SO-239, and into the pin hole. Rub it into the two gaps of the insulator. Apply another mL around the pin of the PL-259 connector.
(5) Screw the connectors together, then wipe off any excess that oozes out.
.
PHOTO #7 – There are numerous types of silicon sealant, but Dow makes some of the best. One type that is very resilient, and weathers well, is DOWSIL 734. It flows very well, and fills gaps that may allow water intrusion. The only negative property is that it produces a mildly corrosive chemical when curing. It costs approximately $33 for 3 ounces. If the sealant needs to be used in close proximity to copper conductors, DOW makes an RTV that is noncorrosive while curing. The one negative characteristic, is that it does not flow and fill gaps. It is DOWSIL 748. The price is approximately $15 for 3 ounces.
.
PHOTO #8 – It is highly recommended that PL-239 connectors are filled with a dielectric grease, when connected to an SO-239. One of the very best is CRC Technician Grade Dielectric Grease. It is compatible with RF circuits, is nonconductive, and remains as a gel for many years. It is more expensive than the inexpensive dielectric greases on the market. It costs about $18 for 3.3 ounces.
NOTE: Do not use any type of grease that contains graphite, aluminum or copper additives.