Reflected power alarm for the MFJ-993B

This article describes an add-on to a MFJ-993B auto ATU to provide an audible alarm when reflected power exceeds a set threshold. A deficiency of the original design IMHO.

The solution uses the generic heating / cooling controller (hcctl) configured for its alarm function only, including a function to silence the alarm.

screenshot-03_12_16-18_34_50Above is the directional coupler part of the MFJ-993B. The REF test point is designed to present voltages within the range 0-5V when used within the stated power ratings.

Circuit modification

The above circuit will be modified as follows:

  • the REF TP will be connected to the alarm module input; and
  • the alarm module connects to ground and +5V, and an attention switch to ground to silence the alarm.

Note that the alarm feature of hcctl samples the alarm 4 times a second, it is not a continuous monitor, but that should be sufficient for the purpose at hand.

The attention switch is a on-off-mom miniature toggle which can be switched to the on position to permanently silence the alarm, or to the momentary position to silence the current alarm which will reset when the alarm input condition is removed.

The load imposed by the alarm module is trivial and so recalibration of the MFj-993B is not required.

Alarm module configuration

Let's design the nominal alarm threshold for Pref=5W, which at 100W corresponds to VSWR=1.58 and ReturnLoss=13.0dB.

It was determined by measurement that the voltage at REF TP was 500mV for Pref=5W. We want to configure the alarm threshold in hcctl EEPROM for 0.5/1.1*1024=466.


Above is a screen shot of the EEPROM structure of hcctl configured for this alarm only application.

Sensitivity can be changed by changing the EEPROM calibration constants as explained.



Above, the module constructed on a small piece of Veroboard.


Above, the underside of the board.


Above, the alarm module fixed to the rear panel of the MFJ-993B.