So what is a LCT?
The Link Coupled Tuner, or LCT, is an inductively coupled tuner that uses 2ea tuned Tank Circuits to
inductively couple an RF signal from the Transceiver to the Antenna System, and also in reverse.
The design of this tuner matches the unbalanced 50 ohm impedance of
the Shack Equipment to the balanced feed line impedance of the Antenna System.
This is a truly Balanced Antenna Tuner. It is used with a balanced feed line.
There are many different configurations and styles of the LCT tuner to match different feed line impedances.
One of the best places for detailed information about this type of tuner is at L.B. Cebik's site.
Being that he has accumulated and archived so much theory and information on the LCT Tuner,
I'm not going to get into any of that. I will only detail my build.
I chose a Parallel Secondary type LCT because of the Antenna System I was using.
My system consists of a 288' 160m Doublet fed with 600 ohm Balanced Ladder Line.
I wanted to use this Antenna System on all bands, so the feed line impedance range I needed to match
would probably range from around 50 ohms to over 1200 ohms.
The LCT configuration that was built here, is using a Low Pass, Series Resonant Input Circuit Primary.
This consists of an inductor, with a capacitor in series to ground. The Inductor on the Input is called the 'Link'.
This is identified as L1 in my schematic, and I will tell you more about the inductors later.
The input series capacitor is identified as C1.
The Output uses a Parallel Resonant Circuit Secondary, which consists of
an inductor with a capacitor in parallel across the ends of the inductor.
This inductor and capacitor are designated as L2 and C2 in the schematic.
The feed lines are connected to the secondary via taps on the L2 coil.
There is a ground reference Tapped to the center of L2.
I had searched the world over looking for high voltage capacitors
in both the Split Stator and Dual Differential varieties.
I am talking about in the 9kv range, BIG ones!! These are cherished by those who have them.
Needless to say they are hard to find, and I found NONE.
After some redesigning of the circuit I wanted to build, I decided to use Vacuum Variable Capacitors.
They are small in size, high in Voltage Ratings, and high in Capacitance Value.
I found 2ea Comet Vacuum Caps on ePay,
A 20 - 2000pf range at a 3KV Continuous/5KV Intermittent voltage rating for the input.
The other, a 10 to 500pf range at a 9kv Continuous/15KV Intermittent voltage rating for the output.
These would work nicely for a High Power Tuna!!
The Inductors were designed by WW8J, Richard Flake.
I found his web site while researching my design, and just LOVED the Plug-In style coils he was using on his LCT.
I had to have some of those monsters!! They were Massive, hand wound from 1/4" copper tubing and B - I - G!!
I emailed Richard asking all kinds of questions about his tuner.
He was very friendly, knowledgeable, and speedy with his replies.
This started a correspondence that is still ongoing today. He is a very knowledgeable fellah.
His Tuner design actually came from his uncle and mentor, W8MAS(silent key 1970).
I am very PROUD to be able to use it in my Tuner, Thank you W8MAS and WW8J!
I've got a Legacy in my Tuna!
I had the design for the Tuner, along with the type of inductors and caps I was going to use.
Now I needed a plan for the enclosure, and where I was going to put it.
Being the coils were of the Plug-In variety, access to the unit was a must.
I decided to build it inside my existing shelving, on a slide out drawer, with heavy duty tracks.
I also decided to make the enclosure out of Acrylic Plexiglas.
That would be fun as I have never worked with it before.
Now on to the Construction Phase of this project...
These are the Hardwood coil spacers for the 160m L2 coil made from Red Oak.
I liked this idea from Richard, WW8J web site so I followed suit.
The only difference is that I did not split mine down the middle, I left them intact.
I didn't like the idea of having to have to glue them, then try and get the coil symmetrical.
I left the spacers whole and then 'Wound' the coil into them.
This is a shot of how I wound the Coil into the spacers.
Note: One of your spacers will have to have an extra hole drilled in it to capture all the bottom turns.
If you have an 18 turn coil, then there are 19 at the bottom.
This spacer is threaded on FIRST!
When you get to this point, its getting a little tight.
You have to finesse and wiggle and move the coil around to get them to move.
You will know when you get the sweet spot because they will all give way and slide easy.
DON'T FORCE IT! You will split the spacers!
The Coil mount/disconnect were made next. The Design is from Richard, WW8J. The Pin spacing is 1.25"
These were made from 1/4" Acrylic Plexiglas and solvent welded together.
The Banana Plugs are P/N 3264 made by Pomona, and the Jacks are P/N 108-0740-001 made by Emerson/Johnson.
I liked Pomona Plugs because they used Beryllium for the contact spring.
I liked the Johnson Jacks because they had a real deep chamfer for alignment.
Note: What you don't see is the Ground Reference Jack that was added to the coil mounts in order to allow the
center of L2 to be tapped to ground. This is shown latter.
The Front Panel was installed in this pic. It is another piece of 1/2" piece of Lexan.
I had to screw these together as the solvent weld (Mythelyne Cloride) just wouldn't take to the Lexan.
I personally don't like to work with Lexan poly carbonate, I like the Acrylic Plexiglas better.
The Lexan has too much moisture in it and will bubble too easy when you Flame Polish it with the torch.
These are the Iron Powder Toroids I will be using to pick the Current Readings off of the Feed lines for the Milli-Ammeters.
They are P/N T-520-2 from Micrometals. I wrapped 2 layers of PTFE Teflon Plumbers Tape on each Toroid.
Yes there Big!! I didn't want any chance of arcing happening. I also wanted minimal reaction to there presence on the feed line.
This idea came from W5JGV, Ralph's web site. He used a -26 core, the yellow one.
I changed that to a -2 core for more broad banded response as his design was for 166.5kc.
Here is the Toroid with the secondary wound.
It consists of 30 turns of #26 enabled wire. A phono plug was soldered on the end.
You must PAY ATTENTION to the phase of your secondary winding, be sure and mark it.
The winding must be wrapped in the same direction as the signal going thru the primary winding (the feed line).
Then I wrapped a layer of Kapton tape over the winding.
This is the backside if the Rear Panel. You can see the 50 ohm Input, and the Balanced Line Output.
Please excuse the scratches as I didn't have the wood touched up yet when I took the pics.
You can see how the Pull-Out shelf connects to the Rear Panel in this pic.
Here is a pic showing a top view of the wiring.
All the white wire is PTFE Teflon Insulated, 6 AWG tinned wire.
If you look closely, you can see the added Ground Reference Jack for the coils in the middle of the Coil Base.
This is to allow the L2 coil center to be tapped to ground.
Key the Transmitter with about 10 watts or so.
You will see 2 waveforms on the screen almost superimposed on each other.
Adjust the Channel 1 waveform vertical amp control to where the sine wave is filling about 50% to full screen.
Adjust the Channel 1 vertical position control to center the trace up and down.
Adjust the Channel 2 vert amp to the same settings.
Now take the VAR out of its detent on the channel that is higher in amplitude then the other,
and adjust it to perfectly superimpose itself over the other trace.
You might have to adjust its vert position control as well.
Now pick a good setting on the Horiz Amp to display a few waveforms in the screen.
Key the Transmitter with about 10 watts or so.
You can see that there is an unbalance in the amplitudes of the sine waves, but the phase is perfect at 180 degrees.
If there was a phase shift on either side of 180 degrees,
you would see the the sine wave peaks closer and farther away from each other horizontally
thereby making the intersection of the waveforms above and below the center line.
This is an Oscilloscope Trace of the output waveform on 80m at 3.7mc.
You can see the Amplitude and Phase are balanced almost perfect.
The 'ADD' function of the O'scope takes the difference of amplitude and phase from both channels and adds a third trace.
The flatter this trace is the better, and you can see that trace is FLAT!
This is an Oscilloscope Trace of the output waveform on 80m at 3.9mc.
You can see the Amplitude and Phase are slightly out of balance.
This amount is acceptable to me, as I am still not getting any RFI or RF Hot Spots in the shack with it.
I also hardly ever Transmit up there.
The 160m coil set was the next to go thru the optimization process.
I was able to get the 160m band balanced almost perfect through the entire band. See the picture above.
I still have to build the 40m coil set, but when that's accomplished it will go through the same procedure.
There are a few things I would like to say about the Ground Reference Tap and the wire lead that goes to it.
It matters a great deal on the route this lead takes going thru or around the inductor to get to the tap point.
The routing thru the magnetic field of the coils changes the Balance of the output.
Lets say you have the Tap Point directly on top of the coil
If you bring the Ground Lead up the left side of the coil and measure the Balanced on the Feeders,
Then remove, and reroute the Ground Lead up the right side of the coil, the Balance on the Feeders will change.
This holds true even coming up thru the center of the coils.
The best routing for me on the 80m coil set was to bring the lead up around the right side of the coil.
The routing on the 160m coil set was best going straight up the middle of the inductors.
One would ask, but what if I want to use and tune more then just one antenna system with this tuner?
In that case I would say to just keep the Ground Reference Tap exactly in the middle of the L2 coil.
Or don't use a Ground Reference Tap at all and 'Float' the Secondary.
This should be fine if your Aerials and Feed Lines are symmetrical.
The ultimate solution would be to make a separate coil set that is optimized for each Antenna System.
Your going to have to play with it in your own application to see what works the best.
Also, according to Richard, WW8J,
He said that all of his antennas can be tuned flat with the Feed Line Taps at the ends of his L2 inductor.
All Stations, and Antenna Systems tend to be unique in there characteristics due to near field influences and what have you.
So the best thing to do, is to experiment, and experiment well!!!!
Take Care Ya'll,
"Tuna X-treme" page designed by PAP