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MFJ-971 portable tuner (1 reply and 1 comment)

3 weeks ago
mm7dkf 3 weeks ago

Hello all,

I have purchased a mfj portable antenna matching unit. The question is as a foundation licence holder i am limited to 10w,  the unit has jumpers that can be changed  from 30 -300 watts or 6 -300watts.  The question is, if i have it on 30w is it safe to tune this unit from 5w up to 10w.  I am a little confused with it, or can i only use this unit on 6w?    thanks for any advice.

George M0URB
2 weeks ago
George M0URB 2 weeks ago


Welcome to the hobby!

This tuner can be used with transmitter powers of up to 200 Watts.

Both the push button on the back of the case and the jumpers on the circuit board change the sensitivity of the cross needle meter. They do this by switching different resistors in and out of the circuit. (The user manual has a circuit diagram - the switches and jumpers are in the bottom left corner, with the different resistors just above them.)

If the jumpers on the circuit board are in their default position (position B), the push button on the back of the case will switch the measuring range of the meter betwen 0-300 Watts (button not pressed) and 0-30 Watts (button pushed in). Switching to the lower measuring range makes it easier to obtain a meter reading when using low power, because a TX power of just a few watts would barely move the needle if the meter is set to measure up to 300 Watts.

Changing the jumpers on the circuit board to the other position (position A) will increase the sensitivity of the meter further. This makes the tuner more suitable for "QRP" operation (i.e. operating with up to 5 Watts TX power). With the jumpers in this setting, the button on the back will now toggle between measuring ranges of 0-30 Watts (button pressed) and 0-6 Watts (button NOT pressed). Setting the jumpers to this position would make sense if you operate QRP a lot (or exclusively), but you would run the risk of possibly damaging the meter if the switch is set to the "low measuring range" setting and you accidentally transmit at a power level that exceeds the scale on the meter.

So, unless you are absolutely sure you will never exceed 5 Watts TX power and need the very low measuring range of the meter, I would suggest you leave the jumpers inside the unit in their default position (position B). That way, if the output power of your transmitter is set to anything up to the 10 Watts allowed under your Foundation licence privileges, it will be perfectly safe to use the tuner regardless of which measuring range the tuner's meter is set to. The only difference will be that the needles on the meter will show more movement when it is switched to the lower range (0-30 Watts).

Hope this helps!

George M0URB

2 weeks ago

Thank you very much, much appreciated. looking forward to getting up and running.....

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