DIY RF Choke

By Thomas Edward
Building your own radio frequency inductors.

RF chokes or coils are classified as inductors and are an integral part of electronic circuitry. They are wound lengths of wire whose actual inductance is based on the the number of turns of wire, the wire size, the diameter of the coil, its pitch, wire material, insulation and core material. Coils can be paralleled with a capacitor to make a "tank" or resonant circuit. Many DIY projects call for making your own coils and you follow simple directions. You can modify these directions or build your own "homebrew" coil for your own project. By using formulas or calculators designed to execute the formulas you can build custom RF coils.

Calculate the RF choke (coil) by inputting the following parameters: Frequency (MHz), coil diameter (mm), wire diameter (mm), coil pitch (mm) and number of turns (use a whole number). This is for copper wire only. Run the calculator and note the output results: coil length (mm), wire length (mm), low frequency inductance (uH), coil Q, resonant frequency (MHz), total capacitance (pF) reactance at operating frequency (ohms) and the resistance at operating frequency.

Look at the inductance (uH) calculated. Adjust the inputs and recalculate until you have calculated the desired inductance and the size and shape that is acceptable for your project requirements. The secret here is to experiment with the calculator and change the variables like wire size, length, number of turns and coil diameter. Set the pitch to the space in millimeters between coil turns. Check to see if the results fit and make sense for the size in which you have to work. Other calculators are available and may better fit your application.

Build the RF choke (coil) from the set of parameters determined in Step 1 to meet the specifications of your project inductor. These are air core coils unless otherwise specified. Use a coil form that will give you the correct coil diameter.

Wind the proper copper wire size and length to the calculated diameter. Wind the specified length of sized copper wire to give you the exact whole number of turns. Allow an inch or two extra on each end of the coil both facing down.. Cut these "pigtails" to fit in the circuit and strip the ends and tin with solder.

Test your DIY RF coil with a gate dip meter. Connect a known value of a capacitor across your DIY RF coil to form a tank circuit and resonate it. Couple an external oscillator to your RF coil using another coil as a mutual inductor. When your oscillator is at the resonant frequency of your RF coil and capacitor under test, more energy couples across and your gate dip meter senses this, and shows a"dip" in the meter reading. The value of your RF coil in uH can be determined from the known capacitor resonating with the RF coil and the known values of the oscillator circuit. Use the simple proportion L(x)C(1) = L(2)C(2) where L(x) equals the unknown RF coil value in uH. L(2) is in uH and C(1) and C(2) are in pF.

Tip

Determine the approximate size of your RF coil before you begin calculations. If you are working below 10 MHz, the RF coil should have a maximum length of 3 inches.

For higher frequencies, look at the RF coils in transmitters on the Internet. They have good pictorials and will give you an idea of size and shape.

Warning

Pitch is zero when coil turns are side by side and no gap indicated.

Inductance meters can readout directly in uH. Read the inductance of your DIY RF coil and to verify check against a known value coil on the same meter.

Resources

About the Author

Writing from his Cape Cod home alcove, Thomas Edward won American Express' National Humor Contest and wrote "Stern's Reminder," a nautical fiction, in 1999. His first professional publication in 2005, "My Fathers Who Art in Heaven," was followed by short stories in New England One magazine. Edward holds an M.S. in civil (environmental) engineering from the University of Cincinnati.