Audio SWR Meter
in MFJ Automatic Antenna Tuners

 

   This article is for blind and vision-impaired amateur radio operators who are interested in the audio SWR meter in some MFJ automatic antenna tuners. The models with this feature use a system of beeps to indicate the tuned SWR range, and they give an audio alert in Morse Code when the SWR is high.

   Audio notification is built into some MFJ auto-tuners, such as the MFJ-925, -929, -993B, and -998. Models with this feature are identified by the term "Audio SWR Meter" listed in the specifications section of the instruction manuals (accessible on the MFJ web site). This article is based on the MFJ manuals, as I did not have an opportunity to test a tuner directly.

Audio SWR Notification

   Each time the automatic tuning process is complete, the tuned SWR range is reported using the following codes:

                       Tuned SWR                  Audio Signal

                        1.5 or less                      1 beep
                        1.6 to 2.0                       2 beeps
                        2.1 to 2.5                       3 beeps
                        2.6 to 3.0                       4 beeps
                        3.1 or greater                "SWR" in Morse Code


Other Morse Code Alerts

   MFJ auto-tuners with the SWR reporting feature also use three audible Morse Code "Q signals" as alerts about the input power:

1.  If the input power exceeds the maximum rating at any time, the tuner goes into "bypass mode" and sends "QRT" in audible Morse Code.

2.  During tuning, lower maximum power limits apply. If these are exceeded, the tuner goes into "self-protection mode." Tuning stops, and "QRP" is sent in audible Morse Code. For example, the self-protection mode for the MFJ-929 is triggered by either

         a)  forward power above 75 watts and SWR greater than 3.0, or

         b)  forward power above 125 watts independent of SWR.

3.  If the input power is less than two watts during tuning, "QRO" is sent in audible Morse Code as a notification that the power is too low.

Conclusions

   The MFJ audio SWR reporting system is simple and effective. It provides an immediate audible indication of the tuned SWR range when automatic tuning is completed. In addition, audible Morse Code alerts notify you when the input power to the auto-tuner is too high or too low.


Author Information

  Peter DeNeef, AE7PD, is an Extra Class amateur radio operator in the U.S. This web site has no ads or conflicts of interest.
Email:  HamRadioAndVision "at" gmail "dot" com.

rev. 11/5/2011

          Related Article: Large Display RF Power/SWR Meter Speaks

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