How to Build an RF Choke for 444 Mhz

A choke is an electric circuit component designed to bypass frequencies higher than a nominal value. This is accomplished by creating a filter using an inductor, which is a coil of wire twisted around a core. For radio frequency (RF) chokes, this core is made of a ferrite or iron rod, which helps make the magnetic field uniform. If you want to design your own choke at radio frequencies at around 444 megahertz, you can do so with the right ingredients.

Things You'll Need

• Ferrite toroid core
• 1 Mega ohm resistor
• Kynar magnetic wire, 1-coil
• Online toroid winding calculator

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Instructions

• 1

  Perform the following calculation to determine the inductance your choke requires to pass 444 megahertz signals.

  frequency = 444 MHz = 444,000,000 Hz = 1,000,000/(6.28 * L)

  Solving for the inductance L gives you a value of 3.58 henries.

• 2

  Open a Web browser and navigate to a toroid winding calculator website.

• 3

  Enter the inductance value that was solved in Step 1 into the first field on the website.

• 4

  Enter the toroid core size in the second field on the website. Ferrite toroids are named after their size. For example, if you have a “FT-50” toroid core, enter “FT-50” as the size of the core.

• 5

  Enter the material number for the toroid. This will be advertised prominently when you purchase the toroid. A material number determines the magnetic permeability of the core and therefore affects the magnetic field and resulting frequency choke effect. For example, if you have a toroid with an advertised material number of “F,” enter that in the third field in the website.

• 6

  Click on “Calculate” once all fields are filled. The website will calculate the number of windings required to create the choke. For this example, the website calculated that 34 turns are required.

• 7

  Wrap the Kynar magnetic wire around the toroid core, using a piece of tape to secure one end to the core. The wire must be wrapped around the toroid using the smaller of the two circular diameters. A toroid is a donut shape and therefore has two, circular, cross-sections. Wrap one complete turn with the wire per turn calculated in the previous step.

• 8

  Cut the end of the Kynar wire off of the wrapped toroid core and secure it using a piece of tape. You now have constructed a 444 MHz choke.