Dave's Homemade Radios Banner

Dave's Wavetrap

Outside View of Dave's Wavetrap

Font Decrease Font Increase



What's a wavetrap? It isn't very much! A wavetrap is a coil-capacitor circuit that is coupled to a radio to reduce it's sensitivity at a certain frequency. This is quite useful if you are bothered by interference from close by transmitters. The circuit will absorb rf energy, to allow reception of stations on nearby frequencies. Wavetraps are used in conjunction with outside antennas, as this one was meant to do.

The circuit is very simple - a coil and a capacitor, both coupled the antenna circuit of a radio. There are different ways to couple the wavetrap to the receiver. If you have an open style coil, you can hold it near the antenna coil of a radio. Or, in the case of this project, the antenna is coupled to the wavetrap coils and this wire connects to the antenna terminals of the radio.

I made a three section wavetrap, all independently tuned circuits. There are switches to disable individual traps. For a better notch, several wavetraps can be tuned to the same frequency. You can build as many traps as you might find useful. A trap can be tuned to one frequency and left there.

The capacitors I used were found on eBay, and it is likely you won't be able to find any of these. But I will describe them. They are a dual section, 320pF per section. There is a 4:1 vernier drive to give two full turns of the knob to move the capacitor end to end. A vernier is really necessary to precisely tune to the frequency that you are looking for. The capacitors are large enough in value to tune the entire band.

You will probably use the more common 365 or 410pF variables in your wavetrap. Try to find some that are good quality, so a steep notch can be achieved.

The coils are a toroidal style that I purchased from Amidon. They have been making ferrites for years and you will get good quality toroids there. Be careful of the junk box types, or the unknown online sellers. Your project outcome may not be as good as hoped. I purchased their slightly over 1 inch (24mm) diameter toroids. The part number is FT-114A-61. Please don't substitute unless you are sure what you are using.

I used 100/45 litz wire for this project. I thought the size was just right. The popular 165/46 can be used too, as well as silver plated, Teflon coated wire. If it's high Q, it belongs in this project.

Since my capacitors only go to 320pF, I had to wind my coils to a higher inductance value, about 300µH. The larger value capacitors can use a smaller inductance, around the magic 240µH.

I started by measuring 64 inches (1,63m) of wire for each coil. I made a bobbin consisting of a piece of 1/8 inch (3mm) thick plastic about 5 or 6 inches long (15cm) and 3/8 inch (9mm) wide. I cut a notch in each end of the plastic. I tape the wire to one side of the bobbin and then wound the wire around it. This allows me to pass the wire through the bobbin, and then around again for another pass through. It works quite well. As you wind, the wire will twist and you will have to untwist it by rotating the bobbin. It's all pretty easy.

I wound all the wire on the toroid, except for the fairly short end wires. If you are using larger value variable capacitors, the length will be shorter. After the coils are wound, you will be able to tell the frequency range, and trim the coil a couple turns at a time so you will tune the whole band.

There are three optional toggle switches used to put each trap in or out of the circuit. You can leave these off and move the capacitors to one end or the other so they don't do any unintentional frequency trapping.

There is not much more that I can think to tell you. I told you that this one would be short. Have fun with your wavetrap project.

Dave's Wavetrap Schematic
Inside View of Dave's Wavetrap
Red Version of Dave's Wavetrap

Take a look at my newest wave trap!