Obtain a 10-foot piece of 5/8” soft copper tubing available at many home supply stores. I got mine at Home Depot. Draw a 36-inch diameter circle on your garage floor. Lay a yard stick across the circle with one edge on the center. Put a cross mark on the circle on both sides where the yard stick crosses. Form the tubing to the circle. Have a friend stand on the tubing to maintain the curve as you form it. This is quick and easy. Put a mark on the copper on the side opposite the open end to indicate the center of the antenna-to-be.

Remote Tuning by Radio-Controlled DC Motor

Shaping The Tubing - Simple And Easy

The huge capacitor was connected two-gangs-in-parallel in series with the other two gangs in parallel to avoid any lossy moving contacts.   

The RC receiver and other circuitry and components shown in the diagram below was entirely contained in the black box. The interior shot on the right shows the receiver in the upper left, the motor servo in the upper right, the relay (blue object) in the center, the microswitch actuators and the top of the microswitch servo at the lower right. The four AA cells are shown in a clip below the black box in the center photo. Even with a one-half rpm motor, tuning was touchy, as the tiniest movement of the motor shaft made a relatively large change in capacitance. A smaller capacitor would have alleviated that problem, but backlash, the time and rotational distance required to take out the slack in the gear train in the motor gearbox, would have remained a negative factor. Even so, this RC approach could be made to work very well with suitable parts and it has the important advantage that if a bias-T were to be used for power instead of batteries, no wires other than the coax would go out to the loop.

Remote Tuning by Arduino-Controlled Stepping Motor

This capacitor, 7pF to 55pF, stator to stator, tunes from 13mHz to 33mHz, which is perfect for 20 through 10 meters. To make tuning even easier than the smaller capacitor alone would have provided, I installed a 10:1 gear reducer that can be seen in the first picture on the left. To avoid any negative factors that might result from anything other than the tuning capacitor near the top of the loop I extended the shaft with a 1/4-inch wood dowel down to the stepping motor. Although the capacitor and motor shafts are far enough apart that slight misalignment probably wouldn’t hurt anything, I installed a helically-cut coupler on the motor shaft. It has enough flexibility to compensate for any possible problems caused by misalignment. The combination works perfectly in this application.

As part of the change to stepping motor drive I also prepared a new feed loop as shown in the photos below. The red material is Sugru, a moldable silicone-like product that remains slightly flexible when cured. This feed loop is made from a piece of RG58. Note that the shield is used as the conductor. The inner conduct-or is not connected. I used coax because it is easy to shape as the feed loop might require. A vertically elongated shape provides a SWR between 1.0 (28mHz) and 1.3 (14mHz) to 1 over the 5-ham band range except at 24mHz it is 1.6 to 1.

Above, see the control box at the operating position. It contains the Arduino, the stepping motor shield, a shaft encoder and two pushbuttons. Power is from an external A123 Battery Pack. The pushbuttons provide rapid rotation in both directions for fast slewing. The knob is for finer tuning and when it is depressed a switch on the encoder activates a super-slow tune rate. It is very easy to tune precisely to the minimum SWR with this setup.

Switch debounce caps and resistors are seen above.

Google Sugru to see a list of suppliers. I bought my stepping motor, motor shield and helical shaft coupler from Adafruit Industries: adafruit.com

To see the Arduino code for this stepping motor control, which any and all are welcome to use, click HERE or on the link at the top of this page.

The diagram below shows details of the debounce circuit.  - Robert K5TD