EZ-Tuner

Because of its complexity, I designed the EZ-Tuner with ease of servicing in mind. (Wish I could say the same for most of my other homebrew projects!) The above view shows the front panel folded down for access to the stepper motors, rotary solenoid (which turns the inductor switch), and +24V power supply, which mounts on the sub-panel. As shown below, all RF components are housed in a shielded sub-enclosure. Should I need to replace any of these parts (knock on wood!) the rear panel detaches by unsoldering two wires and removing some screws.

From left to right in the above view is the 35-492 pF output capacitor, the dual-coil inductance, the two-pole 11 position inductor switch, and the 19-202 pF (per section) split-stator input capacitor. A 4:1 toroidal transformer is mounted just behind the bandswitch, and behind it is a DPDT Kilovac vacuum relay, used for on-line/off-line switching. Barely visible behind the output capacitor are two 50pF high voltage fixed capacitors.

Below is a view of the chassis underside, showing the EZ-Tuner's three printed circuit boards: a microcontroller board (center) and two identical stepper motor driver boards. The PC boards mount on small tapped metal spacers, and connect via Molex-type connectors and headers. I took the precaution of making a few spare circuit boards, so that boards can be swapped in a few minutes.

The wiring harness uses teflon-insulated wire, which doesn't melt at soldering iron temperatures. Borrowing an idea from my old Collins rigs, I used all stainless steel hardware, with pan-head phillips screws. Also shown in the below photo are the +5V regulated power supply (left front), a small "beep" speaker (right front), and an RFI AC line filter (right rear).

Now, here are a few hints for builders and some more details about the EZ-Tuner's inner workings:

"Measure twice, cut once." It pays to heed the old carpenter's adage when doing metalwork. Drilling the aluminum chassis plate (right) was tedious, even with chassis punches and a drill press. The chassis plate has a total of 64 holes. The twelve threaded metal spacers are for mounting the three printed circuit boards.
.	Setting the inductor taps is easy when using an MFJ-259B analyzer and the geometric resistance box (GRB) designed by Frank Witt, AI1H. Frank's neat little gizmo is basically a series of switched precision resistors, which simulate low or high resistance loads, up to a 32:1 VSWR. To set the taps, one attaches the GRB to the output of the tuner and the MFJ analyzer to the input. Then, after setting the variable caps to the calculated values for matching a 50 ohm load (allowing a few pF for stray capacitance), one simply moves the coil tap around until a 1:1 match is found. B&W coil taps make this a painless chore, since they mechanically clamp the inductor and are easily moved. When the left photo was taken, I was only trying to ballpark the tap locations. I tweaked the taps to their final positions after installing the back panel and compartment cover. After finding the final positions, I soldered the taps to the coils to assure a good low-resistance bond to the inductor
Sometimes, the parts layout won't let you put a tap just where it's needed. In these cases, you can add a smidgen of inductance to the tap itself, as shown in the right photo.
	A half-century ago, the Tektronix Corporation elevated the humble terminal strip to an art form. The EZ-Tuner uses ceramic and silver Tektronix terminal strips (left), salvaged from old Tek oscilloscopes. You won't see these in your MFJ tuner! Click HERE to see the original 1958 patent on the terminal strips! It's important to use silver-bearing solder with these terminal strips to prevent the silver from unbonding from the ceramic. Tektronix thoughtfully attached little spools of the special solder to the chassis of their scopes, for use by service techs. Ahh, for the good old days!
The right photo shows twin 50 pF ceramic 5 kV capacitors, which provide an extra 100 pF of output capacitance to the 492 pF variable capacitor on 160 meters. The ceramic caps are mounted on a small L-bracket affixed to the rear of the variable capacitor and are automotically switched into the circuit with a Jennings RF-3A vacuum relay.
	In a T-network, both the stator and rotor plates of both variable capacitors must be insulated from ground. I fabricated mounting blocks from teflon scrap I found on EBay and used Millen ceramic shaft couplers from my junque box to isolate the capacitor shafts. I knew I'd find a use for them one day!
Shown at right is a closeup of the 4:1 bifilar-wound toroidal transformer, used to transform the unbalanced output of the tuner to a balanced output. The toroid is wound of #12 wire (insulated with teflon sleeving) on three Amidon T-200-2 (red) cores. The cores are wrapped with high-temperature fiberglass tape. Recent editions of the ARRL Handbook show a nifty antenna tuner with the toroid connected to the tuner's input, where it always sees a 50 ohm impedance. I finally ruled out that option, because it requires the toroid to always be in the circuit, even for unbalanced loads.

Project History Although I didn't start building the EZ-Tuner until March 2000, the idea had been in the back of my mind for several years before then. I'd long wanted an autotuner that could handle 1500 Watts, because I have several amplifiers in my vintage radio collection, spread over several operating tables, and I was constantly running back-and-forth adjusting my manual tuner. Also, my old homebrew tuner was showing its age, and I knew it was going to have to be replaced or rebuilt before much longer. Unfortunately, there are no commercial legal-power ham autotuners on the market. Most built-in transceiver autotuners are rated at 200 W or less, and most of those can handle VSWRs of only 3:1 or so. If I wanted a high power wide-range auto tuner, it seemed clear I had little choice but to build one from scratch.. This trusty old workhorse dated from 1975 and had melted inductors and arced capacitor plates. Many of the best commercial autotuners, like the SGC-230, use relay-switched toroidal inductors and fixed capacitors. While these work well, it didn't seem practical to me to scale up the concept to the 1500 W level. My best bet seemed to be a conventional variable-capacitor tuner, controlled by stepper motors. Furthermore, I didn't like the idea of a fully automatic 1500 W tuner that "hunted" for a minimum VSWR using a trial-and-error search alogorithm. (And I was pretty sure my homebrew 8877 amp wouldn't like it either!) A manually tuned "memory" tuner with stored settings seemed less likely to lead to inadvertent amplifier damage. About six months before I started on the project, I stumbled across the newly-released Parallax BASIC Stamp BS2sx. With 2000 program steps, 50 MHz clock speed, and 16 I/O ports, it seemed to have enough horsepower for my project. Also, I had used a BASIC Stamp in an earlier QRP project, and really liked its ease of programming, which for me was important. One big decision out of the way! I started the project by designing a stepper motor controller. Although one can buy commercial controller boards, most seemed to be too expensive, had too many unneeded features, or weren't suitable for RF use. For my design, I borrowed ideas on the basic waveform circuitry from a "Nuts and Volts" article, and added an auto-timeout feature and optically isolated motor drivers, which seemed like a good idea for strong RF environments. The 3x5 inch stepper motor controller circuit board was designed using "Circad 98," a great piece of software by Holophase, Inc., Shown here is a "Rev. C" board which has a nice silkscreened component legend and solder mask. Once the stepper motor drivers were built and tested, I started thinking about the features I wanted. I usually begin a homebrew project by sketching out the front panel, since playing with the imaginary knobs and switches on paper helps me focus on the features I really want. Here's the original front panel design, drawn at work during a boring committee meeting. Once I had a general idea of the specifications, I started writing the software code, an effort that took the better part of six months. To test and debug the code as it was being written, I lashed together a breadboard of the control circuitry, as shown below. The BS2sx microcontroller is buried under the rat's nest in the middle. At this point in the project, I planned on using a roller inductor in the tuner (which is why there are three stepper motors in the photo.) Eventually, I decided on a switched fixed inductor to minimize losses and to facilitate rapid bandchanging. I wanted to change bands in seconds, and not wait for a rotary inductor to crank through its turns. Once I had the software more-or-less working, I designed another PC board containing the BS2sx module, frequency counter, and relay drivers. Final testing of the completed controller circuitry was done using a mockup of the RF section, as shown below. For testing, I fabricated a temporary "front panel" out of sheet aluminum to hold the controls and mounted the switch and variable capacitors on a scrap aluminum plate. Finally, it was time to start designing the RF section of the tuner, and this proved to be a much harder job than I'd anticipated. I started by bringing myself up to date on antenna tuner design, with several excellent QST and QEX articles by Bill Sabin, W0IYH, Dean Straw, N6BV, Frank Witt, AI1H, and others. There sure are a lot of subtleties for a device that only consists of two capacitors and an inductor! I eventually settled on the basic T-network because of its versatility, but now I ran into a stumbling block. Virtually all the T-network tuners I saw used roller inductors, with continuously variable inductance. My tuner, by contrast, was going to use a fixed inductor with an 11-position switch. What 11 inductances should I choose to maximize the matching range on all 9 HF bands, while also minimizing power loss (heating) and high peak RF voltages? I didn't even know how to go about answering that question. By now, summer of 2001 had arrived, and I had three weeks to spend at our family vacation cabin in the Montana mountains. When I wasn't hiking in the wilderness or operating W8ZR/7 in the cabin hamshack, I had several hours a day to spend on the problem. Fortunately, I had N6BV's excellent computer simulation programs AAT.exe and TLA.exe, which could generate T-Network solutions for virtually any impedance. Unfortunately, for each impedance and frequency, there are about a zillion matching combinations of inductance and capacitance, each with different losses and peak voltages. I needed some way to sort through all that information, so I could pick out the eleven best inductances for my tuner. Eventually, I was able to identify patterns in the T-Network solutions, which could be displayed in two frequency-independent graphs. One graph showed curves of the network values for different load impedances, and the other showed the corresponding network losses. Armed with these graphs, it was then a straightforward exercise to select my tuner inductances. (See the figures in April 2002 QST for details.) By then, I could hardly wait to get back to Oxford to begin building the tuner. The actual construction only took about three months of weekends and evenings, followed by a month of software debugging and ironing out wrinkles. By the end of November 2001, the EZ-Tuner was finally completed! Here's the RF sub-assembly, prior to final wiring, ready to be mounted in the cabinet. Stepper Motor Hints If you've been rounding up parts for your EZ-Tuner, you've undoubtedly learned that the Ledex rotary solenoid that turns the inductor switch is pretty scarce. Also, you've probably discovered that dual-shaft stepper motors cost $$$ when purchased new, and don't turn up very frequently on EBay or in surplus electronics catalogs. Well, don't despair. There is a solution. In fact, here's one way to kill both birds with one stone. First, let me state the problems more fully. Problem 1: The EZ-Tuner uses two dual-shaft stepper motors to turn the variable capacitors. The front shaft is connected to the variable capacitor, via an insulated shaft coupling, and the rear shaft holds a slotted disk which is used to detect when the capacitor is full meshed. The challenge is how to adapt a single-shaft stepper to do the job of a dual-shaft stepper. Problem 2: If you've decided to use a stepper motor to turn the inductor switch, and if you've read FAQ #14, then you already know you'll need to gear down your stepper. The challenges are (1) to choose the right gears to turn a 30 degree indexed switch, and (2) to design a convenient easy-to-build gear assembly which doesn't take up much space but which leaves room for your limit switch and shaft coupler. E.T.O/Alpha to the Rescue! Below, is how the designers at E.T.O solved the problem in their commercial autotune MRI amplifiers. In these amplifiers, single-shaft stepper motors were used to turn several rotary switches and a variety of air- and vacuum-variable capacitors.  A single-shaft stepper motor and two non-metal gears make a simple assembly that is perfectly matched to a 30 degree (11 position) rotary switch. The stepper motor is fixed to a 3 in x 3.25 in x 0.063" aluminum plate, which is attached to the panel with three 1" threaded aluminum spacers. The gears and limit sense wheel (the circular disk underneath the smaller gear) fit in the 1" gap between the panel and aluminum plate, so that the entire assembly adds only 1 inch of depth to the stepper motor. The gears are 32 pitch. The small gear is 24 teeth, 0.75" diameter, and the large gear is 72 teeth, 2.25" diameter. The combination results in a reduction ration of 1:3, which reduces each step of a common 1.8 deg/step stepper motor to 0.6 degrees. Fifty steps will turn the rotary switch exactly 30 degrees, or one position. Note that a panel bearing (not shown) is required to support the front shaft of the large gear. . At the left is a closeup of the limit wheel (note the small slot). If you want to duplicate this idea, you should attach the wheel to the large gear, rather than the small gear shown here. In this setup, the wheel is fixed to the hub of the small gear with small screws. However, the simpler (though admittedly less elegant) approach that I used is to fit a rubber grommet into the wheel and simply slide the grommet onto the shaft. Once the wheel is aligned properly, a dab of silicone caulk keeps the grommet from moving. (If you really want to get down and dirty, you can use a soup can lid for a wheel, and drill a small hole for the light beam.) Optical interrupters are available at Jameco, All Electronics, Digikey, etc. and cost only a dollor or two. As shown at the right, a mechanical switch can be used to sense the end of rotation of the switch or variable capacitor. The electrical requirement is merely to ground a wire to the controller when the capacitor or switch is at its "home" position. In an optical detector, the wire is grounded when a phototransistor detects a light beam from the slit in the sense wheel. In the setup at the right, a microswitch is closed by a pin attached to the shaft of the gear. At the left is another view of the E.T.O stepper/gear mechanism. A good source of gears (and almost all other hardware) is McMaster-Carr (http://www.mcmaster.com/). Check page 921 of their catalog, or search for "Spur Gears & Racks." You can buy them in stainless steel, aluminum, or acetal (non-metallic) plastic. New gears are rather expensive, but they pop up frequently in surplus catalogs and can often be scavenged from other equipment.