Sunday, Feb. 2, 2020
As those of you who have followed this site may know, I have a fetish for antenna tuners. In addition to some of the various incarnations of the MFJ 941 and 949 tuners stuck in field day boxes, I have several Heathkit SA-2040 and SA-2060 tuners, and a handful of homebrew tuners in my arsenal.
I’ve shed a couple of tuners, deciding last year to simply use the Heathkit tuners I own in both Studio A and B. Those tuners are ruggedly built and bulletproof.
But of course, you know me — I’m a sucker for a deal … particularly on a piece of gear I’ve never owned (and have always wanted to try).
One of the items in that category is a Differential T type antenna tuner. Since I run balanced feedlines 99.9 percent of the time, I have always wanted to give a Differential T tuner a try. The attraction, of course, is the fact that the Differential tuner has only two tuning variables — a differential type capacitor and a roller inductor.
The capacitor is actually split in two halves electrically. The claim is that a Differential T tuner will only give you one setting that provides a proper match — which isn’t true with the traditional “Ultimate” type tuner with separate tuning capacitors on the input and output.
My Heathkit tuners are t-type tuners, but lthey ack the differential capacitor; the SA-2060 and 2040 use separate variable capacitors, one on the input, and one on the output. I never had a problem with poor matches, but on some bands the tuning seemed “iffy” at best, particularly on 15 and 10 meters. The key is to use the lowest inductance you can, and start with the caps at mid-range or less, and you’ll generally come out OK.
ENTER THE MFJ-986. I recently lucked out and found an MFJ-986 Differential T roller inductor tuner on eBay recently worth the money, and I finally got around to putting it in line a week or so back. Would it be as easy as advertised to tune on my inverted vee fed with a balanced feedline?
I wasn’t disappointed!
The tuner was super simple to find a great match on every band. Tune the inductor for the increased band noise on the radio, tune up the cap to peak the incoming signal (or white noise), and the transmit at low power to fine tune the final tuning. It took less time to do it than it did for me to describe in writing here on my keyboard!
I have an MFJ SWR analyzer that I usually use to find the optimum settings when I install a different antenna tuner, but I never needed it with the Differential T tuner. I will admit I was very surprised — and pleased — with how well the tuner works and how easy it is to use.
If you use a balanced feedline on your HF antenna, I highly recommend the MFJ-986 (or other differential T tuner) to replace your traditional pi-network or T-configuration type antenna tuner. The tuner is rated for “3kw”, which generally means the tuner can handle up to somewhere near legal limit.
The version MFJ-986 I have is apparently an older one; the new MFJ 986s has a list price of $419.95. The front panel on my older version is black on aluminum. The new versions feature the vinyl glued-on front panel. Other than that cosmetic difference, the tuners appear to be the same.
I plan to keep my eyes open for another MFJ-986 priced to sell; I’m so impressed with this one that I would pick one up for Studio A in a heartbeat.
ANOTHER TUNER SITUATION. Of course, I’m still waiting on a 4:1 balun to arrive so I can check out my MFJ-939 compact autotuner. The autotuner was part of a package deal I picked up recently on eBay, and after I read some advice, I bought a 1:1 balun when I really needed a 4:1 balun. Naturally, the 4:1 balun was my first choice, but I decided to research the best balun for an autotuner.
The MFJ-939 is compact, but is rated at 200 watts. I would like to get some experience using it if it works well, so I may put it back inline here in Studio B and move the Differential T tuner to Studio A. We’ll see.
That’s my report for today. I’ll update again after my 4:1 balun arrives.
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