MINI T MATCH TUNER

Recently after building my SW-30+ from Small Wonder Labs, I found out that the little BLT tuner I had been using for my QRP operations would not tune my 40 meter Extended Double Zepp (178' centerfed with 450 ohm balanced line) on 30 meters. Not to fault the BLT, as it is a fine little work of art to say the least, my antenna just happened to be out of limits for it. So what could I do to put the new 30 meter rig on the air ? Well, for that evening, I managed to get on the air using my old MFJ-901B which has no meter, but does have a resistive bridge and LED indicator that I added for SWR indication. With this setup, I easily managed to get a match on 30 meters and was making contacts within minutes. I already had the SW-40, and the SW-20, and the little BLT worked just fine on those 2 bands, but I thought that it would be nice to have a very small, wide range tuner for working several bands. I looked over several designs, and then thought I had better look in my junk box to see what I had on hand before I decided on what to build. Well, I managed to find 2 indentical poly variable capacitors that checked out to range from 15 to 300 pf, and they even had 1/4" shafts for mounting knobs. Don't ask where I got these, as I've had them for a long time (I hear that Emtech carries them) but had forgotten about them even being here ! Then I thought that they would be perfect in a simple T match circuit. The T match isn't the best or most efficient tuner circuit around, but it will tune darn near anything you connect it to ! With that range of capacitance, I figured I should be able to build a tuner that would at least cover 10 through 80 meters. So I started looking around for inductor ideas, and first thought about toroidal inductors, but didn't have one on hand to wind the amount of inductance I was looking for on. So I decided to wind the inductor on an insulating coil form, and in this case I used some thin wall 3/4" pvc pipe which is slightly larger than 1" outside diameter. Then I figured that to cover down to 80 meters I would need somewhere around 25 uH of inductance. So my Hamcalc program (thanks Murph !) made an easy job of designing the coil. It told me to wind 36 turns of #20 enamel wire on a form of 1.09" to achieve 25 uH. So I wound the coil, and placed a tap every 3 turns and wired it to a 12 position rotary switch I picked up at Radio Shack. I then mounted the caps, and prewired switch/inductor combo into a little enclosure I had sitting around that wsn't a lot bigger than the BLT tuner. For SWR indication, I duplicated the bridge circuit in the BLT tuner and used a super bright red/water clear LED as the indicator. The cpas had no mounts of any kind built in, so I mounted them with double sided tape long enough to hold them in place and then anchored them with hot melt glue. After mounting 2 surplus SO-239 chassis connectors, I finnished wiring the thing up, and the labled the front and rear panels with my electronic label maker and clear lable tape. Now for a test drive. I dug out my trust old MFJ-259B antenna analyzer and connected it to the input of the tuner, and connected my 40 meter zepp to the output. I tried various frequencies, and was able to get a good match on basically every one I tried from 80 meters on up to 10 ! So, I then tried it with my SW-30+, and right away got a good match with the LED going completely out, and was on the air making contacts within minutes. I now have a functional tuner that will tune almost anything, and it will fit in my coat pocket with room to spare ! I know that my BLT is a more efficient design, and will tune in the field with the right antenna, but with it and my little homebrew Mini-Tune in my bag, I will be able to match just about anything I decide to use ! I have even used it with my Icom 703 on 80 meters with up to 10 watts and no apparent problems so far, and have also found that it will load my zepp on 160 but I have to reduce power to prevent arching there !
 


SCHEMATIC FOR THE MINI T - GUESS YOU CAN TELL I LIKE TO BUILD TUNERS !




The schematic for the Mini T Match Tuner shows that it is nothing more than a standard T match circuit just like the circuit used in most commercial tuners these days, with the exception that this is a miniature sized version.
 


INTERNAL WORKINGS OF THE MINI T MATCH TUNER




Here's a shot of the internal parts layout in the Mini T Match. As you can see, this is a very compact tuner that will travel along with you almost anywhere and will tune almost anything including the proverbial bedsprings !