Full power antenna tuner.
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UPDATE 2017:  The full power "Big Kahuna Tuna" is finally complete!  Yipeee!   ...it took a while, and when I finally got serious with it, it took two iterations of primary coil, because during initial testing and characterization, I was not able to get any dips or even indication of which way to go toward a match...but more on that below.  

Design:  Circuit is again the classic Link Coupled Tuner design, with Unbalanced Input and Balanced Output to the homemade 3 inch spaced open line, as previously constructed, characterized and used.  I decided this was going to be my ultimate tuner, capable of full power, and then some.  Design is nothing fancy, the size alone would be impressive enough!   

Components:  Collecting the major components took a while, and the design is based on the four major components, three from commercial manufacturers, one homemade.

Primary/Input Capacitor (C1) is a Cardwell single section with about 0.250" plate spacing.  A true "bread-slicer"...I don't know the precise capacitance range (it looks like about max 300pF), but with that plate spacing, it will withstand many times any voltages developing in the primary tank circuit from the Drake L-4B. 

Primary/Input Coil (L1) was homemade, by coiling a enough length of 1/4" copper refrigerator water tubing around a 2 1/2" OD bottle to get 16 turns. 

Secondary/Output Capacitor (C2) is a two-section, also Cardwell unit, matching the overall physical dimensions of the Primary Cap, but with even higher plate spacing of about 0.300".  That should withstand even high mismatch voltages, although I will never expect to have a high SWR! 

Secondary/Output Coil (L2) which I thought would do the job (including down to 160M) was a Johnson Type 4346N5.  This  35turn, 7inch OD coil of 1/2" wide silver plated band, edgewound, uses 3 soapstone supports...or should I say: Used!  Unfortunately, the Canadian sender didn't pack it well enough to prevent two of these three supports from hinting the outside of box, breaking the brittle insulators.  I considered what I might replace them with and settled on 3/8" thick G10 (fiberglass reinforced epoxy) stock.  The G10 was cut to size, to replicate the soapstone pieces, and the slots for the coil were cut on a tablesaw.  The saw-kerf of a carbide-tipped blade matched the thickness of the coil band perfectly, and yes, I am aware that glass-filled G10 is highly abrasive and will dull even a carbide tipped blade, but I was prepared to sacrifice one blade for the project.  As it was, when finished cutting all the necessary slots, the carbide blade tips did not exhibit any wear.  

Construction Notes:  Base is a sheet of Alu 18" X 27" with components mounted on Ceramic standoffs well off the panel.  Variable cap frames, and shafts are at ground potential.  Due to the possible high secondary currents, these connections were made using 5/8" wide flat copper stock.  Connections to the primary coil are by way of some large 1/4" ID test plugs originally from a high current test board. 

The final resting place allocated for the completed Tuner, was on the wall next to the operating position, and just below the Feedline entrance insulators.  Marconi would be proud!  A 1" braid ties the base to the KA1-RBP grounding system. 

Completed Version 1, undergoing characterization test (unsuccessfully!),
 and comparison to the 100W version is still in place on the Drake Linear. 
(Mood lighting in the shack!)

Unsuccessful First Characterization Test:  Initial tests conducted at low power on several bands were inconclusive and totally frustrating, as it was not even possible to see any trends...I just didn't get it...I tried several times and just could not figure out "what it wanted" to go towards a dip of the Reflected Power or peak of the Forward Power.  Nothing I tried seemed to help.  Comparison to the previous 100W version showed it was simply not tunable for some reason!  Although I had built and characterized several of these designs before, albeit at a smaller scale, and I thought I was pretty capable of building this full-sized version, it was not to be that simple...I was stumped and frustrated!  I triple-checked the construction...all looked right...circuit was as it was supposed to be, and there were no silly errors...

The one construction detail which I was unsure about:  The Primary Coil version 1 was designed and installed into Secondary, such that Band taps were at one end only, for simplicity.  The end shown below is the input end connected directly to input SO-239 connector.  G10 replacement insulators on Secondary Coil are visible. 

Input side of L1 Version 1.  Nylon cable clamps are used to locate and secure the tubing onto ceramic standoffs. 

I contacted and discussed what might be wrong with both Mike Stricker (WA1-SEO) and Bob Gunnels (N1KW), whose experience and technical expertise I respect greatly.  In my mind, the problem had to be something with my homemade Primary Coil...what else could it be?!?...I thought about it long and hard and focused on three possible parameters with the coil that I suspected might be causing the untunability.  First, I had simply wound it on the form (a 2 1/2" OD adult beverage bottle), in no particular direction, not even thinking or considering right-hand or left-hand...and when I checked, it turned out to be opposite of the Secondary Coil [...could it be?!?...I didn't want to believe it...alternating magnetism is alternating magnetism...so I discounted this outright!].  Second, I designed and constructed the coil with a fixed connection at one end, and taps at 2, 4, 8, and at the full 16 turns, at the other end, intending to have to change only one terminal on the Primary for band selection.  This naturally also meant that (Third), symmetry within the Secondary coil (and therefore also coupling) would change with each Tap/Band...I was willing to accept this, figuring any minor variation could be tuned out.  [I realized after the fact that this was wishful thinking, and wrong!...clearly I had underestimated the effect, it was more major, and kept the combination of Primary and Secondary from coupling sufficiently!] 

With the positioning (coupling) of Primary not symmetrical within the Secondary or consistent with bands...it just HAD to be this causing the tuning difficulties (Mike and I spoke of "Coupling" and "Q" in our discussions...)...I decided the Primary Coil needed to be reconfigured!  I removed it, figuring on replacing it with one which addressed both issues of position symmetry within the Secondary, and with taps going out to both ends.  Yes, this would necessitate two connectors, and would add one more step onto Band Switching, but so be it...I wasn't getting anywhere as it was!  I did consider band-switching by way of an extra large format rotary switch which I had, but that would increase the overall size of what was already a monster, even more...I decided Band-switching would remain strictly manual (nobody is going to call KA1-RBP an "Appliance Operator"!).  

 Primary Coil Version 1: 

Primary Coil Version 1, asymmetrical, with taps only departing to the right, being revised on the bench. 

Reconfiguring Primary Coil: Coil is relocated to a wooden board fixture with good access to position of taps for locating and soldering.   1 inch high threaded porcelain standoffs on the fixture are located in the identical positions as in the actual tuner secondary, so transferring over, will be possible with minimum difficulty.  Tubing at taps is flattened, and formed with a half-wrap at Coil interface points, then soldered in place using a mini-torch.   

Bi-symmetrical Primary Coil Version 2 on fixture.  

Replacement of improved Primary Coil into Secondary:  Thanks to the fixturing, reinstallation goes without issue.  In the final installation, tap ends are supported by nylon clamps, extending beyond insulators, and they are simple to connect to, however since a fair amount of force is applied to tubing in the process of pushing on the snug fitting connector, some undesirable movement would occur as the tubing slid back in the clamp.  This movement was cured by removing the clamps from the tubing, one at a time, surrounding the tubing with a single layer of minimum thickness double-sided self adhesive tape and reinstalling the clamp.  After the tubing was essentially made one with the clamp, no further slippage along the clamps is noted. 

Successful Characterization Attempt Two:  C1 and C2 are set to midrange,  Primary Turns (L1) to 4 Secondary Turns (L2) to 5 [semi-random values based on experience and really a SWAG] and Feedline to 3 [totally random, and another SWAG!].  A dead-quiet frequency is chosen in the middle of the 20M band.  Power is set to minimum in the RTTY mode on the ICOM-756, and the PTT Lock on desk microphone is used to apply continuous low power to the system, while monitoring the Daiwa CN-801 Cross-Needle HF SWR and Power Meter, and adjusting Caps.  When adjusting both C1 and C2, a big trend and difference is immediately apparent at C2 minimum.  L2 is adjusted down to 3 turns, and when adjusting C1, the Reflected Needle drops like a stone to ZERO!  SUCCESS!...we have a 1:1 SWR, and a high Q! 

Total Time to find the match: Less than around thirty seconds!  WOW! After the baffling frustration at characterization attempt No 1, and how fast and successfully attempt No 2. has gone, I am giddy with delight!.   

Characterization notes for 20M.  White high current connector, with 1/4" ID matching the tubing perfectly, can be seen. 

Other bands are characterized, taking not much more time. 

Proof of the Pudding:  The 20M Band seems to be open to Europe...it been a while since wrestling with this tuner has put a crimp in my on-air activities, so I return a CQ call of 9A2CD in Croatia, and he comes back to me at the first call and gives me a 5-7...most encouraging.  Subsequent responses after the first or second call bring 5-9s from operators running linears and Beams and Yagis on top of towers...I am quite satisfied with the performance of my Dipole and Tuna...and I haven't even applied Drake L-4B full power yet...watch this space!   

Clean-up:  Where characterization points of earlier, smaller Tuners were marked with colored dabs of Testor's enamel, components of the "Big Kahuna Tuna" are large enough to allow the use of colored electrical tape here...20M gets Red, 40M gets Yellow, etc.    

Big Kahuna Tuna in Service: 

Thanks and Acknowledgments:  To N1KW, Bob Gunnels, and WA1-SEO, Mike Stricker for technical assistance and sanity checks...and alternating magnetism IS alternating magnetism!   

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External material sources are attributed.  Otherwise, this article is Copyright © 2021.  Ronald Kwas, KA1-RBP.   The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion. 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article or diagrams, you must give credit to the author and to this site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment.