MFJ-986 Differential T-Tuner a Ham's Best Friend in a Shack

If there's one piece of equipment that I've always found necessary in any ham shack it's an antenna tuner, also called a transmatch. Some people will tell you that all the transmatch does is "fool" the radio into think it is working into its proper load (the properties where reactive capacitance and inductance are in balance -- I'll define those at the end of this piece).

And, if you were to accept that explanation at face value, you'd think it's just a device to cheat your way onto the ham bands with a minimum of effort, but that explanation is not entirely correct. If you were to do all of the quadratics involved at proving resonance, you'd see that it really doesn't matter whether the radio is "fooled" or not because the transmatch just brings both halves of the quadratic equation into balance so that you have a proper condition for radio operation.

Once you have achieved resonance or tuning for any part of the system, the system, itself, is tuned. Yes, it could be argued that a lot of that "tuning" is turned into heat that is dissipated in the tuner, rather than in the antenna, but, here's a news flash for people who may think that -- as the old antenna ruminator Kurt N. Sterba, who wrote for WorldRadio Magazine (actually a pseudonym for the publisher, we think) -- it really doesn't matter where the tuning occurs, once you have achieved the balance mathematically, the antenna will radiate correctly and the signal will get to another antenna where it's needed.

And this bring us to a topic I mentioned earlier, but we'll go into a little more deeply today, the proper tuner to get the job done. Let's face it, there are lots of antenna tuners out there. You can find them on Ebay or Yahoo or Amazon or anywhere things are bought and sold like Craigslist or Secondhandradio.com. They all do essentially the same thing, bring a system into tune or resonance.

The one you need, though, is not only one that is easy to hookup and easier to use. It is also one that won't break the bank. We alluded to this in another piece that discussed your station in general and made mention of several tuners, but this time we'll mention it specifically, the MFJ-Differential-T Antenna Tuner.

Here's what makes this tuner so nice -- there are only two controls to set and forget. The first is the huge inductor that goes up to nearly 200 turns so that if you can't find an inductance that starts the right hand blade of the two-blade meter heading downward, then we'd suspect your feedline is open, and then there's the "butterfly" capacitor or split-stator capacitor that pretty much gives you almost full coverage under any situation. Indeed, we can't think of any situation where there is 0 capacitance and 200 turns inductance. If there was, we'd suggest you shutdown until you had a chance to go inch-by-inch through all your jumpers and interconnect lines to find the bad one or bad connector.

The official title of this tuner is the MFJ-986 and it allows you the ability to interface two antennas, a dummy load and/or a balanced feedline antenna. Or, you can bypass the tuner by using the six-detent wafer switch in one of the "direct" settings for coax antennas.

In about eight years of continuous operation with various amplifiers and rigs we didn't find an antenna whose VSWR that we couldn't flatten to 1:1 and, if you agree with the math of the situation which demands that if one half of the system is at resonance so is the other half because all reactances are canceled out, then you will agree with that you've tuned your antenna.

The beauty of this tuner is the fact that it's huge inside. In all the years we used it, there was never a situation we could find that would make it arc across. we can remember one early tiny tuner that we used where the antenna was way too short for the frequency and there just wasn't enough inductance available. It was the first and only time we every saw flame and smoke come out from a tuner. Suffice it to say that the tuner's purchase was dictated by limited finances at the time so we went with it and -- it was the very start of our ham career so we were somewhat easily talked into things -- the salesperson at the former local ham store said the little matchbox (another name for a tuner) would handle things easily. Boy, all it takes is a little smoke and flame and you learn quickly.

Actually, all we did was burn up the phenolic board on a six-position wafer switch, but it was done with flare.

There are lots of reasons to like this tuner:

The current balun, rather than a voltage balun, where instead of trying to make a voltage mismatch into a match and which could result in an arc-over (flash of light, cloud of smoke and tuner lamely hacking), forces the current to be the same one both sides with the voltage differential remaining the same. It's a better way of doing things

There are only two dials to set

There are two settings (200 and 2000 watts)

There are two direct setting (possibly a third if you leave out the dummy load

The split-stator capacitor that can put capacitance into any circuit

We could go on, but, the bottom line is this. If you can find an older MFJ-986 on, say, Ebay, and it's in good shape then we'd bid for it, but keep your bidding below $300 or so, because you can buy one new from the manufacturer directly for just a little more and, come to think of it, if this is your first station and you want things to look great, it might not be a bad way to go.