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Dave's Contra Coil Wave Trap, Front View

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Dave's Adjustable Contra Coil Wave Trap

Summer has ended and now it is time to get busy planning the next DX season. I spent most of the hot summer of 2010 working on my website. I'm still not done, but that is why you haven't seen much in new projects on makearadio.com.

I would like to introduce you to "Dave's Home Made Radios Adjustable Contra Coil Wave Trap" or DHMRACCWT. There are two features on this wave trap that haven't been seen too often before. These are a contra trap coil, and an adjustable coupling coil system.

I've used contra coils for about 4 years in my DX crystal sets. They proved themselves very worthwhile and worth the extra work to make them.

There is a slight twist on this contra coil. That is, there is only one range instead of two. This coil is wired in parallel permanently. This doubles the effective litz strands from 165 to 330. Just for that little extra sharpness.

The other feature is a variable coupling system. This allows for setting the inductive coupling to the best value possible. Sure it takes more work to get this set up, at least at first, but if you remember the settings, you can reset them as needed.

The DHMRACCWT is built using a 12x12 inch (30x30 cm) piece of 1/8 inch (3 mm) thick Garolite®. Other materials may be used. I like Garolite® the best. I cut the front panel to be 5-1/2 inches tall (14cm), leaving the rest for the chassis. Small metal angle brackets made by Keystone Electronics attach the panel to the chassis.

Two 6:1 vernier drives are used in this project. The capacitor vernier tuning is very necessary. The coupling vernier isn't needed so much, but it does provide an excellent mounting for the 1/4 inch rod that connects to the coupling coil. Also the 4 inch (10 cm) "Schmardial" system can be employed. Yes, I decided to call this disc mounted on the vernier the "Schmardial". Please get used to this. :)

The coupling coil is a 1-1/4 inch (1-1/2 inch outside diameter (40 mm)) plastic pipe cap. I found this in the local Home Depot radio, I mean plumbing department. I wound 35 turns of 20/44 litz for a 132µH inductance coil. I first tried a 33µH value, but that was way too low. The best value can be found by experimentation, according to your antenna. The shorter the antenna, the larger the coil inductance should be. But it's not critical. I can tell you that the coil winding part is a little tedious, so put away the booze until after winding.

The contra coil is wound on a Genova Products 3 inch (7,6 cm) pipe coupler, which is actually 3-1/2 (8,9 cm) outside diameter. This coupler is made from styrene, a known rf low loss material. You start by drilling the two mounting holes for a #6 screw thread. A 9/64 drill is used and the holes are 3/16 inch (4 mm) from the edge.

Here is where it gets a little tricky. You need to drill 8 more small holes for the wires. There is not a lot of room on this form. So drill 4 holes very near the center of the coupler. Make the holes as close to the center as you can, with only a small spacing between windings. The holes don't have to be next to each other, but offset a small amount. The windings should be closer than shown in the picture below.

The last 4 holes (2 on each end) will be about 1-3/8 inch from the center holes. This is going to be a tight fit. The wires at the end will almost be touching the two standoffs that connect the coil to the chassis.

Each coil is wound with 42 turns of 165/46 litz wire. Start at the center and wind each coil out to the end. The wire must be wound so it travels in the same direction Please refer to my cylinder coils page for more information. There is a section about placing the coil in a freezer for a few minutes and then tightening the wires for proper tensioning.

If you don't feel like winding a contra coil, a cylinder coil using less litz can be used. The coil should be 150 µH or a little less. Refer again to my cylinder coils page for how to wind this size coil.

When done, connect the inside two wires to the parallel wired stators of the variable capacitor (330pF per section). The outside two wires are connected together and to the frame or rotor of the capacitor. This should allow tuning from 530 kHz to over 1700 kHz, the entire MW band. The schematic shows it all.

The coupling coil is mounted to the vernier via a piece of 1/4 inch diameter styrene rod. The rod fits tightly on the coil, but can be glued if yours is loose.

To use this as a wave trap, connect the antenna to one terminal and the antenna input on the radio. A wave trap is very useful for crystal set reception, but is also great for reducing strong signals on a regenerative receiver.

Tune the capacitor so there is a null in the offending signal. Use the coupling control to adjust for the best rejection of the signal. Many times this is at the maximum setting, but if you find the tuning is dampened, then reduce the coupling by turning the coupling coil away from the main coil.

Another use (I always like dual use projects) for this device is a MW signal booster. Connect the terminals to an antenna and ground. Then place the internal loop of the radio within the field of the wave trap. As you tune the wave trap, you will hear a large increase in the signal. I can hear Toronto which is nearly 500km from me during the day.

I hope you enjoyed seeing and reading about my wave trap. This is simple to build and a useful DX tool.

Good DX to everyone – Dave, N2DS

Inside View of Dave's Wavetrap

Dave's Wavetrap Schematic