I'm going to show how to build a 1.8MHz to 54MHz (limited use for 144-148MHz) Bidirectional Coupler. This coupler follows the Tandem Match Coupler (TMC) design (Figure1), which is a bidirectional coupler. The coupler is build using lumped components and is fairly easy to build.

Figure 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


NOTE: It looks like the LO1230 is hard to get in EU, a close relative would be the FT50(A)-43. So if you have issues getting the LO, use the FT. Compare the AL value of the toroids from here LO AL Values with information from here Toroid Info

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



Figure 2

NOTE: Don't forget to mount two or more ground lugs for the Faraday shields.


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.


Photo 1

NOTE: Start the first turn 70mm of the end of the wire, you will need that length to connect the transformer to the sockets.

Now prepare the two RG213 strips. Carefully remove the outer sheath of the strips and tightly wind the PTFE tape around the mesh. Then mark the center of the strip and, from the center, mark 21mm to either side and cut the mesh and insulation through to the center conductor. For the RCA connectors it is slightly less, 19mm from the center. Use a sharp craft knife or Stanley knife. (Be careful, fingers!) See Figure 3.

F
igure 3

Values in bracket are for the link between the RCA connectors. Of course if you choose to use different connectors your mileage will vary.

Trim the center conductor of the RG213 strips to length and slip the Toroid over the RG213 strips. Do a test fit and if you are happy with the fit, solder the RG213 strips into its position as seen at Photo 2. I had to add a few turns of electrical tape around the RG213 strips before the Toroid fit snugly.


Photo 2

Connect one side of the RG213 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 RG213 mesh.

Now seems to be a good time to check for continuity and short. The center pints of the opposite connectors should test short and center 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 center pin and one goes to ground. See Figure 1.


Photo 3 shows the finished project with the additional shield installed.


Photo 3

Well, that's it. All we need to do is measure/test the performance of the 4PDC.

Testing a Bidirectional Coupler



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