Here are some photographs of my remote-control balanced tuner, based on a design my Rich Measures, AG6K as published in a QST article in the early 1990s. The tuner has been in daily use for the past 4 years and is used at up to 1500W CW output on 80 and 40 meters. The antenna used is a 135 foot ladder-line fed dipole at 65 feet.
The tuner is constructed on a composite wood board covered with a thin coating of vinyl (an 18" deep shelf board from Lowes). The rotary inductors are 22 microhenry units purchased new from Cardwell Capacitors. The variable capacitor is a Jennings 400 pF unit. The circuit design is a balanced "L" and the two inductors are mechanically coupled so that they turn together in the same direction. The capacitor and inductors are turned by reversible DC gearmotors and the inductors are mechanically coupled via cogged pulleys and belts from Small Parts, Inc.
The position of the C and L components is monitored via 10-turn potentiometers which are coupled to the component shafts via cogged belt reduction drives so that the many turns of the component shaft is spread over the 10 turn range of the pots. The position is then read out in the shack on the PIC-based digital controller via the PIC's analog to digital converter ports. The semi-automatic controller stores up to 30 predetermined frequencies and adjusts the capacitor and inductors to positions previously set. An 8 conductor rotor cable is used to communicate with the tuner, and all 8 wires are used. The 8 wires are bypassed to ground via MOVs and .01 disc capacitors on the terminal strip where the cable connects to the tuner.
A DX Engineering 50 ohm to 50 ohm balun is used at the input to the tuner. This balun is rated at 10 KW continuous power, and it replaced my original ferrite bead balun which tended to overheat when running full power RTTY on 40 meters. The output couples directly to the balanced feedline. Limit switches are used to prevent the operator from accidentally driving the inductors and vacuum capacitor against their mechanical stops. Vacuum latching relays are used to switch the capacitor between the input and output side of the tuner to accomodate a range of antenna impedances.
When setting up the PIC controller, I use an MFJ 259B analyzer to provide the signal and manually tune C and L to the proper values while watching the SWR indicator on the 259B. The values are then stored in the controller for that frequency. Once the controller has been programmed for the desired frequencies, all you have to do to tune the system is select the frequency you want and press the GO button.
This is the original version of the controller, using analog meters as position indicators. Changing bands involved holding the lever switches in the ON position while observing the meters and comparing the readout to a paper chart.
The complete tuner including feedline and original ferrite bead balun. Surge protection MOVs and capacitors were not yet installed when this photo was taken. Because of the feedline length, the heatshrink tubing on the ferrite beads tended to melt with 1500W out on 40M RTTY.
In this photo you can see the limit switch arrangement using Cherry microswitches. The 1/4 inch shafts throughout are coupled via plastic tubing and stainless expansion clamps. The cogged belt drive, position sensing pots and vacuum latching relays are also seen here. Note the adjustment screws on the limit switch mechanism to allow fine adjustment of the end points.
This wider angle shot shows one of the reversible DC drive motors and the method of mounting. All of the hardware brackets are galvanized steel from Lowes. A threaded 1/4" shaft is used to couple the components and to provide a means of converting the rotational motion of the shafts to linear motion for the limit switch arrangement.
The second version of the controller used digital meters for position readout. The precision 10 turn pots used in the tuner in conjunction with the three digit LCD readouts allowed precise repositioning of the tuner's inductors and capacitor when changing bands. Getting to the right position still meant holding the switches while watching the meters.
This is the latest version of the controller, a PIC-based design using a 16F877A MCU. You can store up to 30 frequencies with the associated C and L values. To tune the remote tuner, you first select the frequency using the two index buttons, then press the GO button. The two LEDs to the right of the display indicate that motor drive is being applied and you can monitor the progress on the LCD display. The L and C values on the top line are the stored values for the indicated frequency and those on the bottom line are the current values. When the requested C and L positions are reached, the movement LEDs will be out and the TUNED LED will be lit. If the C and L values are not reached in 30 seconds, the STALL LED comes on, indicating a problem in the remote unit. The C and L readout pot gearing is such that the range of tuning of either component can be from 000 to 999. With my C and L components, that gives a resolution of approximately 1/50 of a turn. When the controller senses that the target position of either C or L has been reached and drops the drive power, there can be an overshoot of 1-3 readout units, but that's such a small rotational value that no correction is necessary.