Link coupled tuner homebrew

[wb3jih]

Hello all -

I've been building a link coupled tuner from the 1964 Handbook.  Once I got it all together, it works fantastic -- tunes both my doublet and loop.

So I want to increase the power handling capability of the design to handle a KW.  I have capacitors that will handle it, but the link inductor is only #14 wire.  It also uses rotary switches to change both the taps on the secondary and the number of turns on the primary.  I feel that's probably the weakest link, so I'm thinking I might just do plug in coils on a plexiglass board (W7FOX has a great design I want to shamelessly copy on his QRZ page...).

Here's the problem I'm having:  the coil I made is 2 inch diameter, about 9.5 inches long, and 66 turns.  8 turns are cut out of the middle for the primary link (divided in two for 4 in series or parallel).  When I put my LCR meter on it, it shows about 45 uH on the secondary, and 2.5/5.5 uH on the primary.  I then rolled a coil of 26 turns of #8 wire on a 4 inch diameter core which gives me 32 uH total.  A separate primary link is about 3 uH.

I cannot get this to function AT ALL on any band with the larger inductor.  I figured I could at least get a match on 40 on up, since the tap points on the smaller coil show to be about 25 uH, but no way it's happening.

I've been looking for days at various sites discussing the design of this.  The 1964 handbook one is the only one I've been able to get to work.  I am using a dual 100 pF capacitor, two 100 pF capacitors tied together, and a 250 pF capacitor on the input (all variable.)  

I've even tried the K1JJ setup, but I just don't have the facilities to manage the size and turns of copper tubing.  I'm looking to replicate the KW matchbox on this design.  I've probably got enough wire purchased and hours of time to just go out and have bought one, but... where is the fun in that.

So... can anybody help me in resizing the inductor and link on the 1964 design to I can up the power capability?

For the record -- here's the design (page 356 of the 1964 handbook), and what I've built so far:

[W1ITT]

I've built a pile of link coupled tuners over the years.  For the primary link, I wind quarter inch copper tubing over a form which is a white plastic 4" pipe coupling.  I've never had problems with smoking the plastic, whatever it is.   Probably 1/8 tubing would be adequate, but I often operate on the engineering principle that if good is enough, then more is even better.  Then I use whatever coil I want for the secondary and it slides through the inside of the link form.  Both the primary and the secondary sit on ceramic standoffs.  The advantage of using separate coils rather than carving the input link out of the middle is that one can swap and change components without rebuilding everything.  One of my favorite tuners runs optimumly on 80 and 40 meters, although I can put it on 30 and 20 with a bit of messing about.  Rather than trying to build a DC to daylight tuner, it makes a certain amount of sense to optimize components for a smaller range.  Having done that, you can then acquire some big honking double pole double throw knife switches to throw when you change bands. 
As to spending lots of money on parts and pieces, you will still be ahead of the game.  I don't know of any currently made amateur tuner that has capacitors the size that you are using.  I sold one like that at Deerfield a year or so ago, just to get it out of my attic and into circulation.
73 de Norm W1ITT

[km6sn]

If you get a Nanovna it will tell you what is going on.

[kb3ouk]

As i found out with a johnson matchbox, just because you can't get it to tune with a setup that should tune doesn't mean it won't work. I have a 80 meter dipole fed with balanced line and i could get it to tune on other bands easily but couldn't get it to work with that antenna on the phone portion of 80 meters at all, the cw portion would tune fine. What i ended up having to do was tune up at the low end of the band where it did work then keep sweeping up and retuning it til i got to the high end of the band where i was finally able to get a match. I've found that sometimes you have to do a lot of fine tuning with a link coupled design to get the match right. Get it as low as you can with a quick twist of the knobs then start going back and forth with small adjustments til you get it down. You will want to make note of dial positions so you can change bands and come back.

[Steve - K4HX]

What is the wire size on that plug-in coil? Most of the ones I've seen have a wire size that is too small for high-power tuner use. They weren't designed for the high-Q/high circulating current scenarios you are likely to get with a tuner application. YMMV.

How did you come up with that coil design? Did you measure the inductance of the coils in your low-power tuner and then calculate the coils (using the HB formulas) using larger wire?

[aa5wg]

Will have to recalculate my numbers.

[wb3jih]

So I've updated my design.  Based on the schematic of the original, and help from W7FOX, I went with a plug in coil version of the tuner.  This will definitely support a KW.

The tuner is built in a wood and lexan container.  I mounted four banana jacks on a lexan sheet on top, which allows me to easily swap coils measured for specific bands.  I got the idea from looking at the old Johnson plug in coils from 1960.  These were sold in various configurations, and I was able to reproduce them with #6 and #8 wire.  I found an old sales sheet from Johnson in 1960 that showed the "recommended capacitance" needed for resonance at the low end of the band.  From there, I could calculate the inductance needed on the coil as a parallel tuned circuit, and was able to verify the inductance of the original Johnson plug in coils.  I used the 1 KW specification for the coils (the 40 is the 1000LCS40 - 238-104.

I cut 2 1/2 inch by 6 inch polycarbonate sheets, then wound the #8 on a 2 inch piece of PVC under some tension.  When I released it, it was almost exactly 2.5 inches in diameter.  Then I just used the handbook to figure out the required number of turns for the secondary.  For 14 MHz and up, I added two turns for the primary, and for 7 MHz I added five.  The picture shows the 40 meter coil installed.

I wound the 80 meter coil with #12 wire, including the link coil.  In each case, the variable capacitors are set pretty close to half meshed, which gives me quite a bit of range.  I ran a number of values of resistors across the antenna tuner, and was able to tune values from about 10 ohms to 10K.

These are all constructed with the same polycarbonate core, and then the coils are epoxied in place with the requisite spacing (I used plastic conduit hole grommet material to maintain the spacing.)

I then salvaged an old MFJ tuner for the dual meter movement and put in a sampling board connected to a Johnson directional coupler I had in the parts box.  I calibrated the board against another watt meter I had so I can at least get an idea of the power output.  It has a high/low function, so I can scale either 300 or 3000 max.

So I eliminated the band switch by using plug in coils.  It scares people out of the shack, and nobody could easily get their hands on it when I was transmitting. 

Had a lot of fun building this -- it seems to work very well with my ladder line doublet and 40 meter loop.  No heat on the coil to speak of even when running high power (900+) into it with the Hurricane.  Pretty happy with the way it turned out.  I've had some issues with the coil base, made of polycarbonate as well, breaking where the banana plugs are screwed in.  The heavy gauge wire doesn't really stray off of the core though, so it hasn't been a big deal.  I think I need to find a heavier duty alternative.


73 -

Jeff

[Steve - K4HX]

Beautiful! That's a tuner!

[N7ZDR]

Awesome build Jeff-- Kudos to you!

I always wanted to put mine in some type of a box or something but they are just so easy to adjust just the way they are hanging on the wall of the shack. I use 3 tuners on the wall and they just work and work. Once you learn when and where the taps are needed I can change bands in the dark.

Check out my QRZ for pics

Easy and cheap!

Larry
N7ZDR

[KA0HCP]

Magnificent!