Building a Bi-Directional Coupler

I'm going to show how to build a 1.8 MHz to 54 MHz (limited use for 144 - 148 MHz) Bidirectional Coupler. This Coupler follows the Tandem Match Coupler (TMC) design, which is a bidirectional Coupler. The Coupler is build using lumped components and is fairly easy to build. See the below Picture, Pic 1.

Part List:

• • • • • 2 x RG213 60mm in length

        • 2x Ferrite Toroid (Jaycar LO1230) (T1 & T2)

        • 2x 950mm CuL 0.5mm

        • 2x UHF Socket (SO239) (you choose) (in & out socket)

        • 2x RCA (you choose) (forward & reflected)

        • 1x small AL Diecast Box 110x50x30mm (Jaycar HB5062)

        • Plumbers Tape (PTFE)

        • Electrical Tape

        • Nesquik Tin Lid

Prepare the case, drill the holes for the connectors and mount the connectors. See Pic 2 for a drilling template.

Next we'll build the transformers T1 and T2. Measure 950 mm of 0.5 mm wire for each Toroid. Stretch the wire and wind 40 turns onto each of the Toroid. Make sure that the windings are spread over 300 degree of the Toroid surface. With the RG213 strip passing through the Toroid we will have build a 40:1 transformer. The single turn is the RG213 strip.

Now prepare the two RG-213 strips. Carefully remove the outer sheath of the strips and tightly wind the PTFE tape around the mesh. Then mark the centre of the strip and, from the centre, mark 21 mm to either side and cut the mesh and insulation through to the center conductor. For the RCA connectors it is slightly less, 19 mm from the centre. Use a sharp craft knife or Stanley knife. (Be careful, fingers!) See Pic 4.

Trim the centre conductor of the RG-213 strips to length and slip the Toroid over the RG-213 strips. Do a test fit and if you are happy with the fit, solder the RG-213 strips into its position as seen at Pic 5. I had to add a few turns of electrical tape around the RG-213 strips before the Toroid fit snugly.

Connect one side of the RG-213 mesh to a ground lug. This becomes a Faraday shield if we connect one, and only ONE, end of the mesh to ground. Peel back a bit of the PTFE and solder the ground lug to the RG-213 mesh. Now seems to be a good time to check for continuity and short. The centre pints of the opposite connectors should test short and centre pin to ground (chassis) should test open. If you are happy with your checks, solder the transformer wire ends to the correct spots. One wire goes to the centre pin and one goes to ground. Pic 6 shows the finished project with the additional shield installed.

Well, that's it. All we need to do is measure/test the performance of the 4PDC, see here Testing a Bidirectional Coupler