Antenna Coupler (SG-237) Weather Protection Box

SGC-237 Antenna Coupler

Note: I have retired the setup described in these pages and replaced this whip and SG-237 configuration with a Hi-Q Screwdriver Antenna.

I chose to use the SGC-237 antenna coupler to match my IC-706 to a ordinary 102 inch whip antenna. The photo at the right (borrowed from the SGC web site) is what the SGC-237 looks like when it comes out of the box. Even though this is a nice weather proof package, the SGC people do not recommend that you leave it out in the sun and rain (and, it does rain here in the northwest).

So, I decided to build a weather protection box to house the SG-237. I designed this box to fit right next to the base of my antenna on my truck since the feed from the coupler to the antenna should be as short as possible -- inches matter. Since SGC corporate offices are located 15 minutes from where I work in Bellevue, Washington, I actually went there to talk to their technical guy on the best design for my box. The result of those conversations are the construction that I outline here.

Coupler Box Parts

Sitting on my workbench are the main parts of this coupler box. The basic construction is a box that is made of two plates of 1/4 inch thick aluminum separated by a frame made of 3/4 inch marine plywood. In the picture you see the plywood frame which is open on one end (the bottom). You also see the aluminum plates underneath.

The other main part of this construction is a mounting frame that is composed of aluminum square tubular stuff. I had all this stuff cut to size and I finished the edges of the aluminum plates and the square tubular on my belt sander to give it a nice finished edge. The total cost of all aluminum and the cost of the cutting was about $12.

And, the aluminum plates are fastened to the plywood frame using bolts and nuts that go from side to side through the frame. You should be able to make out the drilled holes in the material.

Coupler Box Construction, Part 1

In this photo I have bolted the back aluminum plate to the square tubular frame. There is a cross brace on the bottom as you can see and there is nothing on top as that will be fastened to the truck. Near the bottom you see a yellow cable attachment. This is the ground strap that will connect to the SG-237 ground peg. This strap makes the ground of the frame and the SG-237 the same potential (as if it wasn't already with all this aluminum).

Coupler Box Construction, Part 2

This photo merely shows the back side of the construction at this stage.

Coupler Box Construction, Part 3

This shows the tuner box mounted on the truck. The SG-237 is in place sandwiched between the two aluminum plates. The back of the SG-237 is an aluminum base plate and that is mounted by screws to the back aluminum plate of my box. The front plate has about a 1/4 inch clearance between the plate and the front of the SG-237.

This picture just slightly shows the feed line connection on top where the feed goes to the antenna. This is an aluminum bolt that is insulated from the wood on the top as it feeds through. The output feed of the coupler is connected to this bolt on the inside and the antenna feed line attaches on the outside.

The SG-237 is also grounded to the entire aluminum box which itself is grounded to the truck as you can see by the ground strap on the top of the box frame (it is connected to the antenna mounting bracket).

Coupler Box Construction, Part 4

This photo shows the cable connection that feeds the control cable for the coupler as well as the coaxial transmission line. Both cables are wrapped in plastic spiral cable wrap and then fed through a hole that I drilled through the floor of the truck bed. The cables then feed along the main frame rail to where they go up under the floor by the rear seat of the crew cab.

I chose to leave the bottom of the box open. This is for two reasons. One, this allows any water drainage that might get into the box. There is nothing on the bottom for water to collect. Two, it is far easier to hook up this cable assembly with nothing on the bottom to hassle with. It is possible that water can get up into this from underneath but remember that the basic SG-237 itself is water proof to begin with so I am not worried about this opening.

Coupler Box Construction, Part 5

The box is mounted to my truck on the bottom and the top. On the bottom, there is a single bolt that goes through the center of the cross brace between the two vertical frame members. This bolt goes right through the floor of the bed.

The picture to the right shows the top mount. This photo is looking up from the perspective of the bed of the truck. The two top braces are bolted through the lip that is part of the top part of the side panel. You can actually get behind this for mounting bolts but the sheet metal is not that strong -- so, you see the aluminum brace I use behind it to give it some rigid strength. From other photos, you can tell that this is not visible at all unless you are laying on your back on the bed of the truck.

Coupler Box Construction, Part 6

Shiny aluminum is nice in some circles but my truck is black so I painted this box black with a special outdoor waterproofing paint. Several coats in fact and this photo shows it drying as the rear of my truck is backed into my garage because there was a brief summer rain falling outside (this is the northwest).