500 Watt Link Coupled Tuner Project (2/10/2010)

Although I have been very happy with my 100 Watt link coupled tuner, I needed a tuner that could handle 500 Watts for my new ALS-600 amplifier and Texas Army MARS.  I had two nice capacitors from a Johnson Kilowatt tuner, but needed a good coil.  I decided that it was best to wind the coils first , measure the coil diameter and then build the coil frame afterwards.

The cheapest way for me to purchase copper is to buy the above and strip out the solid 10 gauge wire. I have a lag bolt situated for such endeavors.  I attached each of the three wires and used a razor blade to cut off the insulation.
Most of the wire has been stripped. To straighten and anneal the wire you must have a stronger connection to the lag bolt, so cut off the wire in the above photo that has the insulation on it, and re-twist it as shown here.  Failure to do this will have you hurt and on your ass later on.  ( I know from experience!)
The form I used is a piece of 3" schedule 40 PVC pipe.  You need to drill 4 holes in it as you will see. You can look into the pipe to get the wire through the first 2 holes.
The second two holes should be at least an inch or more from the first two holes.  If the distance between these holes is too small the wire will rip out of the PVC when you pull on it, and you will end up on your ass again.  (Believe me I know!) Twist this end of the wire really good.  You can now pull on the wire.  You need to pull a lot with 10 gauge wire.  The wire will suddenly yield and stretch about 2 inches and then stop stretching as the copper hardens.  All of the bends in the wire will suddenly disappear.  It is easier to do this with 12 gage wire but you also run the risk of breaking the wire.  Be careful.  You have been warned.
You must immediately proceed to wind the coils. Otherwise Murphy will step in and you will kink the wire.  You must wind the coil with as much tension as you can muster.  You must also wind the coils straight.  When you wind it all up to the lag bolt cut the wire off at the bolt.  You now have a coil made.  Put it in a safe place so it does not get bent up.  I needed 22 turns and I got about 24 with 25 feet of wire.
  I needed 7 turns for the center coil and wound about 9.  Save the remainder of the wire for connecting things up in the tuner.  I only had about 6 feet left when the tuner was complete.
I contact cemented the template I made on the computer to an old piece of 3/8 " Plexiglas.  Before laying out the form, measure the OD of the coils.  The coils will relax after they are removed from the PVC pipe.  My coil was about 8 percent larger (3.88" OD).  Note this coil has the center 7 turn coil and the outer 22 turn coils inline. I trimmed the Plexiglas, and drilled oversized holes in it per the previous 100 Watt tuner coil shown on its page.  I also cut grooves for side support pieces, and also grooves at the holes so epoxy glue could be applied later to set the turns.
  This shows the form and the side pieces.
Here I am gluing with epoxy one side piece to the main form. Here the other side piece is being glued.
I originally tried putting the smaller center coil on horizontally. It was difficult. I finally figured out that by using gravity to assist, it was extremely easy.  Just wind the wire around slowly guiding the wire with your fingers.
Here the center coil and one end coil is installed and I have started to wind on the last coil. Use both hands to nudge the wire.  Here I needed to hold the camera with the other hand.
The new coil next to the older tuner. Straighten each coil  until it is against all four guides.  Don't over do it or the coil will go out of round.  Then add the 5 minute epoxy to set the turns.  The resulting  coil will be very rigid.
It took me 23 hours to build the tuner once all the parts were available.  I used 30 amp Anderson Power Poles for the connection to the tuner.  The old style ceramic insulators from the Johnson Matchbox were a pain.  It took me another 6  months to figure out all the bugs so it worked perfectly from 2.3 MHz to 29 MHz. This was the first trial with RG8X braid and small alligator clips.  This overheated so I switched to RG8 braid and large alligator clips but this was still not satisfactory.  I finally developed my own clamps for the jumpers.
The light alligator clips were not sufficient for 500 Watts and were replaced.  See below. View of the 7 turn center coil.  Having the coils in-line made construction a lot easier.  You can see the link between the outer coils at the bottom of the coil.  To join all wire I used butt connectors with the plastic insulation removed, and then soldered them to the wires without any crimping.
  View of the link between outer coils.

All data is derived from Cebik's (W4RNL) fine articles on this subject. 

I have discovered three details that needed to be known before I made this a successful project.  First, note that I have the center of the antenna side capacitors grounded to the radio side of the circuit (to the braid of the coax coming into the tuner).  This was suggested in the literature as an option. I have found that this works better than having the antenna side of the circuit float.  Also be sure that the rotor of the radio side capacitor is grounded to the coax (braid) side of the circuit.  Otherwise hand capacitance will be a problem sometimes when you are tuning.  The photos above are incorrect in this respect.  All in all this project saved me about $650 because I didn't have to buy a manufactured unit. 

Second, the very small alligator clips and RG8X braid for the taps as shown in some of the photos were sufficient for 100 Watts but not 500 Watts.  I replaced these with RG8 braid and larger alligator clips but the clips still overheated at some settings.  I finally replaced the alligator clips completely with a more robust system described below.  The tuner now runs cool at 500 Watts.   This tuner is now being used with a 200 foot doublet (dipole) from 2.3 MHz to 29 MHz. 

I was finally able to make heavy duty lugs for the coil taps using  a ring lug.  This is one is rated for a 1/4 inch stud and 10 or 12 gauge wire.  The ring part of the lug is bent in a "U" shape, inserted around the coil turn and then crimped and soldered into place.  A 10 gauge copper wire is then crimped and soldered to it as an attachment point for the jumper.

The alligator clips previously used were not good enough for 500 Watts.  I found these two items as possible substitutes.   One is a grounding bus bar intended to be used in a residence electrical service panel ($6).  The other is an inexpensive copper terminal lug meant for about 10 to 14 gauge solid wire.

I cut the grounding bus bar into sections containing two screws with my Dremel tool.  When I cut off the old large alligator clips from the braid jumpers, I left the soldered cylindrical ferrules on the braid.  The ferrules bolted very nicely to the bus bar.  The other screw is used to attach the coil jumper to the 10 gauge wire stud.

At the bottom is the jumper for the center (input) coil which is inline with the output coils on either side and consists of 7 turns.  The jumper for the outer coil is tapped 5 turns from the end.

  This data from Cebik's site is classic.  These are the recommended values for inductance and capacitance for the coils and capacitors for link coupled tuners.  This information is much more discussed by Cebik is actually contained in the copy of the 1960 ARRL Antenna Book that I have, so this info has been around a long time.
The copper terminal lugs work much better than the bus bar connectors.  The ferrule on the braid end is from the old alligator clips.  It fit perfectly in the lug hole and I soldered this connection. I shortened the 10 gauge wire stubs now that I am using the copper terminal lugs.
This is the current configuration of the high power tuner as of February 15, 2010.  I use it from 2.3 MHz to 29 MHz, typically at 400 to 450 Watts and all frequencies tried so far tuned to 1:1 SWR.  It is convenient to use.  At 3.2 MHz I noted arcing of one of the output caps so I run it at 300 Watts only at that frequency.  I could change the length of the window line as recommended by all other tuner manufacturers but feel this is not the objective of such a wide range tuner design.  I am currently very happy with the  utility and performance of this tuner.  It just took me about 4 years to figure all this out.

Note the added capacitors on the terminal block at the top of the front panel are not used in the final design.

Antenna side of tuner.  Note the four sets of stator plates on the differential capacitor. Radio side of tuner showing the output dual capacitor and the radio side capacitor.
Front of unit.
Tuning it up. This is my vote for the QST cover photo.    ;o)
For more information on the design data for making ink coupled tuners go to Cebik's site at:  http://www.cebik.com, Tales and Technicals section, “Link-Coupled Antenna Tuners”, August 8, 2006, and "Link-Coupled Antenna Tuners: A Tutorial: Parts 1 – 5”, November 25, 1997.

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