Electronics > RF, Microwave, Ham Radio

Best SO-239 style for outdoor box?


I'm winding a transformer that will go in a small ABS box outdoors near an antenna's feed point. The box will need an SO-239 socket.

I've seen a couple of different styles of SO-239. Some are a basic barrel, held onto the box or panel by a large-diameter nut, and maybe a washer. The other type is on a square plate, with holes at the corners for four small screws.

From a weatherproofing point of view, what's the better choice? Also, is there a best sealant to use, and a best way to apply it?

Avoid the type with a large nut. I use the square flange type with 4 bolt holes. It helps to buy the more expensive types, as some will literally fall apart. I use these from M0MAT on eBay, see item 153363155392

Seal the inside connections and coax tails with “704 silicon”, or liquid electrical tape (rubber dissolved in strong solvents). Do not use ordinary silicon that is found in hardware stores and smells of vinegar, it corrodes metal. 704 silicon can be sourced from China on eBay, one example is item 175402647347
It is the white stuff that is also used to secure electrolytic caps in transistor radios.

Drill a small (1mm) vent/drain hole in the lowest side of the box.

Job done.

73 SJ

Thanks, good suggestions.

The SO-239's from M0MAT look good, but I'm hoping to get one fairly quickly, and international shipping would be a factor. So I might go with an Amphenol from a U.S. source.

Likewise the 704 silicone from China will take a while to get here. I wonder if 3M marine-grade silicone sealant would be an alternative?

Also, just to clarify, would the square flange mount on the exterior surface of the box? Or inside?

“ I wonder if 3M marine-grade silicone sealant would be an alternative?”

I’m not sure. Liquid Electrical Tape is manufactured in the USA, so should be easy to obtain. It comes in various colors and is in a jar with a brush in the lid. Beware, it dissolves rubber self amalgamating tape.

You want the flange on the outside. Otherwise the plug won’t fully seat.



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