Microphone Impedance for Amateur Radio

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Older vacuum tube ham radio transceivers tend to use the high impedance microphone whilst the newer solid state ham radio transceivers tend to use the low impedance microphone. Mobile radios tend to use low the impedance microphone. Slang terms such as Hi-Z and Low-Z come from the music industry and refer to impedance. A Hi-Z microphone is, for example, a high impedance microphone.

  • What is considered high impedance?
  • How many ohms is a typical hi-Z microphone for old tube transceivers?
  • What about low-Z?
  • How about those microphones with the old crystal element, are they "really - really" high impedance?
  • How about the low-Z microphones which have a 9v battery under the base?
  • Can the impedance of the microphone be checked with an ohm meter?

All good questions and each shall be addressed here.

Impedance is the resistance to a non-DC current. It is the equivalence to resistance with a DC current and both are measured in ohms. In an AC current we speak in terms of impedance rather than resistance. Impedance is more involved than DC resistance because it is actually the combined effect of capacitance, inductance, and resistance on a signal.

Audio microphones in general are divided rather loosely into three classes of impedance.

  • Low ... 600 ohms or less
  • Medium ... above 600 ohms to 10K ohms
  • High ... above 10K ohms

Astatic D-104 microphones from back in the tube radio days is considered a high impedance microphone. An old D-104 with a crystal voice element checks in over 1M ohms which is considered pretty high impedance. Later the D-104 went to an updated ceramic microphone element which remained high impedance. Over time there were changes in the ceramic element used in manufacture. When the amplified D-104 was introduced the impedance changed. If you desire using an old hi-Z microphone on a modern solid state transceiver then amplification is necessary. One approach is the use of a Field Effect Transistor. Later Astatic D-104 microphones were rated at _50K ohms_* impedance, still hi-Z but not as high.

The Yaesu MD-1 is an example of a low-Z desktop microphone for a solid state base ham radio transceiver. The impedance of this microphone is 600 ohms which is pretty common for solid state era microphones for the Kenwood, Icom, and Yaesu radios.

There were different microphone element types used over the years during the development of the microphone which changed dramatically going into the modern solid state era. Originally many of the mass produced microphones used a crystal element which was both fragile and very high impedance. Later there were other ceramic element types used. ...more...

Turner produced many different microphone types over the years and specifically produced microphones for ham radio earlier in the company history. The Turner 254HC Microphone was produced by Turner for ham radio and is high impedance, or hi-Z, at _????_ ohms using a high output ceramic microphone element. Later Turner produced many models of Citizen Band microphones including the model ??? desktop microphone which had a 9v battery under the base. This is a powered microphone which is considered low-Z at _600_* ohms.

Microphone impedance cannot be determined with an ohmmeter because the ohmmeter uses a very low DC current, where as impedance is the opposition to the follow of an AC current. You can use the ohmmeter to help determine what some of the microphone pins are for, such as which pair are for PTT, however, you cannot use to tell what impedance the microphone is rated. An impedance measurement would involve the use of a tone generator and oscilloscope and would not be exactly precise.

Older high impedance microphones can be adapted for use on modern solid state amateur radio transceivers. ...more...

 

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