GBPPR Tech Bulletin #4 - How to Choose Inductors at RF

Choosing the right inductor is a concept amateur radio operators fail to recognize.  When you actually want your radio to work correctly, that Radio Shack education just doesn't cut it.

Besides the Q factor, there are several other specifications you'll need to consider when choosing an inductor to isolate RF and DC.  Some of these are:

  1. The value of inductance which determines the reactance that it presents to the RF energy.  Clearly, you want this reactance to be high compared to the impedance in the RF circuit (normally 50 ohms, but may be different depending on where you insert the inductor).
  2. The DC resistance must be low enough that the voltage dropped across the inductor is negligible compared to the operating voltage of your power amplifier's device.
  3. The current rating of the inductor must be adequate to carry the quiescent operating current of your circuit and then some, without overheating.
  4. The self-resonant frequency of the inductor must be higher than the operating frequency of your circuit, otherwise you may end up having a capacitor instead of an inductor, or even a complex reactance function that may interact with your amplifier to produce spurious modulation or oscillation.
  5. The mechanical aspects of the inductor, size, shape, shielding, mount, and orientation will affect stray interaction in the circuit and must be given some consideration.
  6. You must give consideration to thermal aspects of the design as well.  For example, if inductor dissipation elevates its temperature and the inductor has a magnetic core, your circuit will experience poor isolation as the core temperature approaches its Curie temperature.
  7. Methods of Increasing Inductor Q

    1. Use a larger diameter wire.  This decreases the AC and DC resistance of the windings.
    2. Spread the windings apart.  Air has a lower dielectric constant than most insulators.  Thus, an air gap between the windings decreases the interwinding capacitance.
    3. Increase the permeability of the flux linkage path.  This is most often done by winding the inductor around a magnetic core material, such as iron or ferrite.  A coil made in this manner will also consist of fewer turns for a given inductance.

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