My Antenna Tuner, Reconsidered

For many years–17 to be exact–I’ve owned an MFJ 941D VersaTuner II antenna tuner. I never really had any success using it, though, and for most of those years it’s simply sat on a shelf. Not being an antenna genius, I could never figure out whether my lack of success was due to my poor antenna skills or the less-than-rugged construction of the device.

Well, about a week and a half ago, I decided to give it another whack. Connecting my Autek VA1 antenna analyzer to the transmitter connector and a precision 150-ohm resistor that came with the analyzer to the antenna input, I first set the selector switch so that the resistor was connected directly to the analyzer. I measured an SWR of approximately 3:1. So far, so good.

I then switched in the tuner circuit and fiddled with the controls. Voila! I was able to set it so that the SWR I measure was close to 1:1. Setting the antenna analyzer to frequencies in the various ham bands, I was able to get a good match on all bands except one. (I forget which one, exactly). So, I’m guessing that the tuner does indeed work as advertised.

Last weekend, I put up a 30m dipole. Unfortunately, due to my fear of getting up on the roof and the odd shape of my backyard (the land slopes upward such that when you’re at the back of the yard, you can almost see over the two-story house), the highest point of the dipole is only about 20 feet off the ground and one leg is only about 15 feet off the ground. That being the case, the lowest SWR I could obtain is 1.35:1.

That’s not too bad, and I did make some nice contacts with it, but I always feel better when that SWR meter on the rig barely budges off the peg than when it swings up further. Having a little more confidence in my antenna tuner, I thought I’d hook the thing up and see if I could drop that SWR. So, I connected the antenna and analyzer and again fiddled with the knobs. Well, what do you know? I was able to adjust the tuner to that I measured an SWR of almost 1:1.

I then connected the tuner to the rig, and sure enough, the SWR meter barely moved when I transmitted. The proof of the pudding is in the eating, though, so I tuned around trying to find a QSO. As luck would have it, the band conditions were quite poor, and the band was very quiet. I did hear a W7 calling, but he didn’t come back to me. It was getting late, so I called it a night.

I wasn’t able to get back to the shack until last night. I fired up the rig again, and tuned around the band. Again, conditions seemed kind of poor and the band was very quiet. I sent out a couple of test transmissions and verified that the SWR reading was still low. I then set the meter to read forward power, just to make sure I was putting out a signal, and sent out another test transmission. Everything seemed to be working fine.

Tuning around some more, I happened upon J8DX on 10.107, operating from St. Vincent in the Caribbean. After a couple of tries, he came back to me. Cool! My first QSO using the tuner was DX. I’ve since made several more contacts all with good reports.

While the tuner section seems to be working, it looks like the meter circuit is shot. No matter how I set the switches, I can’t get the meter to move. I’ll be contacting MFJ to see if there’s a simple way to test and adjust the SWR/power meter.

I’m also going to try to get the antenna to tune up on other bands. It would be nice to work some 20 meters, or maybe 15 meters, and the ARRL 10-meter contest is this weekend. If I can get the antenna to tune up on 10, I could do a little contesting.

Comments

  1. Bill WB2AIV says:

    I just came across the entry about the MFJ-941D tuner SWR circuit problem. I am encountering the same problem. Did you find an answer? Can you point me to it?

    Thanks.

    Bill
    WB2AIV

  2. Hi, Bill–

    I ended up pulling the thing apart to try and fix it myself. What I found was just atrocious. There were numerous cold solder joints, which I had to touch up. The thing now does indicate forward power and SWR, but the calibration is way off.

    I didn’t think much of MFJ to begin with, and this experience didn’t do anything to enhance their reputation. I’ll give you another example. They attached the meter to the front panel with a gob of hot glue! Of course, this came loose with all the gyrations that I had to make to get the board out of the case so that I could get at the solder joints. Uggggh.

  3. Luther Nallie says:

    I had the problem of meter movement and solved that by replacing the two diodes (1N34A germanium). I got the meter moving by replacing the shot diodes, that I screwed up tuning at full power (big mistake).
    I didn’t take the board out, too many connections, just snipped the diodes close to the body and soldered to the ends sticking up out of the circuit board. Be sure to make note of which way they were facing.
    I now have forward power on 300 and 30 watts but my reflected power is not working. There is a variable resistor located between the diodes which is labeled 500K RL on the schematic. That has to do with the reflected power. I have no idea what the power rating is of that component or where to find it. If anyone can help I would appreciate it.

    Thanks,
    Luther, KQ5O

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