I recommend taking some lessons from other gear that’s packaged for outdoor use. Like many outdoor preamps. Have a look at picture of these and you’ll see what I am getting at. Moisture comes from 2 places. From precipitation and from the air (condensation). I’ve use the following with great success.

First the precipitation. Cover it like a “hat”. I use a piece of wide PVC or drainage pipe, though a plastic project box will do as well. I seal the top with a cap and leave the bottom open. All cables and connectors must come from the bottom for zero chance of water tricking in through a cable hole on the top or side or through the connectors. Also seal the connectors using some of the new vinyl coax wrap (love this stuff), electrical tape, or coax seal (though the latter can be a bear to cleanly remove later). What you're basically doing is affixing a “bucket” over the top of your diplexer.

Now for condensation. Resist any temptation to seal up every crack and crevice of the diplexer. As long as there is any air inside, you will eventually get condensation due to temp and humidity changes. And it needs a way to get out when that happens. If you cover the bottom of your PVC pipe be sure to leave a weep hole or a small gap perhaps where your cables come in through the bottom so moisture can escape. The only perfect way around the condensation is to have no air at all inside the diplexer housing. Some companies use this approach for outdoor matchboxes and baluns where the whole thing is embedded in a block of epoxy. But before you try taking yours apart and using filler, epoxy, or silicone sealer to fill it up, let me tell you I’ve tried it and my diplexer just didn’t work well afterwards. I suffered added losses and and increased SWR. Perhaps gear just has to be designed for that from the ground up.

Hope that helps.

On Mon, Jul 27, 2015 at 8:38 PM, kb2m@arrl.net wrote:

I'm trying to setup a remote sat station and need help with a desence filter. I was using a Diamond MX72-n and was experiencing water intrusion. I then checked the archives and consulted the group and went to the recommended Comet CF-416. Guess what, I'm still getting water into the duplexer. I thought that it was more suited to outdoor service on the recommendation of the group. So, before I take apart one of the duplexers to attempt to waterproof, does anyone know of a duplexer that is designed for outdoor use on the tower? I really don't have the room for a 12 foot + crossboom, so I need to resolve the desence with the duplexer. Any suggestions from someone who has solved this?

73 Jeff kb2m