Building a 7 MHz small transmitting loop antenna, part 3

Part 3 of this article series focuses on the mechanics of the motorised variable capacitor.

Mounting the capacitor and motor

I had originally planned to use hot melt glue to mount the butterfly capacitor and the motor directly inside a small plastic box. However, working within the cramped confines of the box interior would make precise placement and alignment difficult. I therefore decided it would be easier to mount the capacitor and motor on a small flat board, which would allow me easy access to all the parts from all angles. Once the parts placement is finalised, I can simply mount the finished board inside the plastic box.

The board I used for mounting the capacitor and motor is a 1mm-thick polyethylene cutting board. I specifically chose polyethylene because of its low RF loss. The electric field in a small transmitting loop antenna is concentrated around the capacitor area, so it is important that any materials around the capacitor have low dielectric loss. As a counter-example, it would probably be a bad idea to mount the capacitor on a wooden board, because wood is a poor RF dielectric. Even though the capacitor losses, caused by poor dielectrics around the capacitor, will be small and likely in the milliohm range, in a small transmitting loop the radiation resistance is also small, making it important to minimise even seemingly trivial losses like dielectric loss. For more reading on the effects of dielectrics on the capacitor losses, here are a few links. This link describes an improvement in the Q of a variable capacitor by replacing the phenolic insulators with HDPE (high-density polyethylene): http://theradioboard.com/rb/viewtopic.php?t=1525#p12346. And, this link describes in detail various loss mechanisms of air variable capacitors, including dielectric losses: http://g3rbj.co.uk/wp-content/uploads/2013/10/Measurements_of_Loss_in_Variable_Capacitors_issue_2.pdf.

The capacitor is mounted on the polyethylene board as follows.

When mounted vertically, the height of the capacitor shaft was lower than the height of the motor shaft, so I mounted the capacitor on top of a stack of three scrap pieces of the cutting board, raising the height by about 3 mm. The scraps were affixed to the main board with hot melt glue, and the capacitor was also held in place by hot melt glue. A shaft coupler was attached to the variable capacitor shaft to enable interfacing with the motor shaft. The variable capacitor's shaft was slightly too thin for a snug fit into the coupler, so I wrapped some paper a few times around the capacitor shaft to ensure a snug fit. A snug fit is important to ensure that the center of the capacitor shaft's rotation is the same as the center of the shaft coupler's rotation (simply tightening the coupler's screws against a too-thin capacitor shaft would lead to eccentric rotation of the shaft coupler).

The motor looks as follows.

The motor shaft is very thin, much thinner than the capacitor shaft, and does not fit snugly into the shaft coupler. Unlike the solution for the thin capacitor shaft, where paper was wrapped a few times around the capacitor shaft to fatten it, with the motor shaft the thinness is so extreme that wrapping paper is not a viable solution for fattening the motor shaft; the paper would simply slip as the shaft turned. Therefore, to expand the small radius of the very thin motor shaft, I applied hot melt glue liberally to the shaft and allowed it to harden into a bulb. Then I carefully snipped away the hardened glue until the diameter of the glue bulb would just fit into the shaft coupler. The hardened glue, being a form fit for the motor shaft and being adhesive by design, grips the motor shaft well enough that it should not slip and can deliver the rotational power from the motor shaft to the load.

The glue-encased motor shaft was then inserted into the shaft coupler, the coupler's screws were tightened around the hardened glue, and the motor was glued in place to the polyethylene board.

Use of hot melt glue

I found hot melt glue especially suitable for this kind of hardware layout because it allows quick and easy initial positioning of elements, without requiring any drilling of screw holes or the like. Repositioning of elements, if needed, is also easy.

When applying the glue, I carefully apply a neat blob or seam to the key connection points required for strength, taking care not to apply too much glue that would be difficult to remove later if needed.

If repositioning is needed, I can pull away the glue with either my fingers or a pair of pliers.

Possibility of remote sensing of capacitor rotation

The current motor setup allows remote control of the capacitor by applying voltage to the motor, but it is not currently possible to remotely determine the current setting of the variable capacitor.

Notice, however, that in the above image, the motor shaft extends not only to the left side towards the capacitor, but also extends to the right side, which is currently unused. In the future, it might be possible to couple the right-hand shaft to a potentiometer, which would then allow remote reading of the potentiometer value to sense the current angular setting of the variable capacitor.

This sensing capability might be exploited to implement an automatic tuner, where for example the transmitter's frequency is automatically read via some interface and a computer activates a control voltage to rotate the motor appropriately, monitoring the potentiometer value to gauge the capacitor's rotation and stopping when the rotation reaches some previously-saved value appropriate for the current frequency. Of course, implementing such a system requires consideration of real-world robotics issues like backlash and overshoot, and would likely require some sort of a PID controller implementation in software.

RF choking, or lack thereof

By choosing to mount the motor directly next to the capacitor, I am placing the motor and its control wires in the vicinity of the capacitor's strong electric field. A more conservative approach would place the motor at or near the loop's zero-voltage point (diametrically opposite the capacitor), using a very long shaft to couple the bottom-mounted motor to the top-mounted capacitor. In a previous loop design I did exactly this, using a one-meter plastic coupling shaft. The problem is that a long coupling shaft will usually introduce extra backlash into the system, making the capacitor slower to respond to the motor rotation, due to time it takes for the long plastic shaft to twist until it has enough tension to drive the capacitor. And in the current loop design, a 2-meter shaft would be necessary, which would introduce even more backlash and would be mechanically challenging as well due to the long shaft length.

For the above reasons I chose to mount the motor directly next to the capacitor, to simplify the mechanics. Also, it's worth noting that the MFJ 1786 loop antenna (generally regarded as a well-performing, properly-constructed small transmitting loop) also places the motor directly at the capacitor.

However, the mounting of the motor and its control wires near the capacitor could conceivably cause RF currents to flow on the motor control cables, which could lead to problems both on transmit and receive: on transmit, the transmitted energy might make its way back into the motor control circuitry causing equipment damage or posing a shock hazard; on receive, the motor control wires might form part of the antenna system and allow induced noise on the control wires to couple into the capacitor and the loop antenna, where it then would be passed back into the receiver as unwanted noise.

The way to solve this problem is to use an RF choke or chokes on the motor control wires to prevent RF currents from flowing on the control wires. The MFJ 1786 loop antenna uses RF chokes for this purpose. Also, N4SPP's page on small transmitting loop construction (http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm) shows how he uses two ferrite chokes on his motor control cable.

I considered using ferrite chokes, but was concerned about how to decide on the specifics of the choke design: the proper core material, core size, number of turns, choke location, etc. Before diving too deeply into the choke design, I decided to run a 4nec2 antenna simulation to see if choking was even necessary or not.

I made a 4nec2 model of the loop and tried running a disconnected wire down from near the capacitor to ground, to represent the motor control wires (which are near, but not connected to, the capacitor). The separation between the capacitor segment and the motor wire segment was 1 cm. Also the motor wire deliberately ran away in an asymmetrical fashion (which makes it easier for unwanted currents to flow) and was running very close to the main loop conductor (1cm spacing). Even in this asymmetrical condition, the current induced on the unchoked wire, with 5 watts transmitting power, was only about 20 milliamps maximum, which I consider to be low enough not to be concerned about.

Although the simulation results lead me to believe that the unchoked motor cables will not carry significant RF current, it is possible however that for certain specific lengths, the control cable might be more prone to coupling into the capacitor and becoming an unwanted part of the antenna system. This will be determined during actual usage of the antenna. I plan to adjust the antenna for minimum SWR, then alter the routing of the motor control cables to see if the SWR changes. If it does, then choking of the motor wires may be needed.

Abrasion of the hardened glue

After testing the ability of the motor to drive the capacitor, some slippage started to occur after a few minutes. This was caused by the coupler's screws not being tightened enough around the hardened glue; the motor shaft and glue bulb were rotating, but the shaft coupler and the capacitor shaft were not rotating.

To fix this, I first tightened the screws even harder against the hardened glue. I cannot see inside the shaft coupler, but I believe the screws are now so tight that they are biting into the surface of the hardened glue. One concern is that the hardened glue is still quite soft compared to the usual metal shafts for which the shaft coupler is intended. Over time, as the motor is repeatedly rotated both clockwise and counterclockwise, I can imagine the shaft coupler's screws slowly grinding away at the hardened glue until the glue is completely stripped away from the motor shaft, leading to slippage and inability of the glue bulb to deliver power to the load. The expected failure mode will not be immediate; as the glue is slowly worn away around the contact points with the screws, I expect a widening hole in the glue to form, which will first lead to excessive backlash when the motor reverses direction, and may eventually lead to the wearing of a groove all around the glue bulb's circumference, at which point the motor-driven rotation of the glue bulb will no longer be able to push against the coupler's screws and will no longer be able to rotate the capacitor shaft.

In an attempt to delay this mechanical abrasion of the glue, I added more hot melt glue at the shaft coupler's opening where the motor shaft enters.

The hope is that the additional hot melt glue will allow the motor shaft to grip the outer surface of the shaft coupler, and that the required torque to turn the capacitor shaft will be partially provided by the grip of the glue on the coupler's surface. This might reduce the pressure, while the motor is running, of the coupler's screws on the inserted glue bulb, which in turn might lengthen the lifetime of the inserted glue bulb. Time will tell.

Addendum 2015-01-18: The glue seems to become harder after a 24 hours; it seems hard enough that moderate pressure from the screws will not significantly wear away the glue. The slippage mentioned above was caused by insufficient insertion depth of the hardened glue bulb into the shaft coupler, meaning that the shaft coupler's screws were only barely grazing the tip of the hardened glue bulb instead of firmly gripping the main body of the hardened glue bulb. With a sufficient insertion depth, 24-hour-hardened glue, and moderately-tightened screws on the shaft coupler, no further slippage has been observed.

Motorised capacitor in operation

The following video shows the operation of the motor when run from 4 AAA batteries.

https://www.youtube.com/watch?v=_CNhot_yUv8

The motor's rotational speed is still several RPM, which is too fast for precise tuning of a narrow-bandwidth small transmitting loop antenna.

The problem is compounded by the fact that a butterfly capacitor covers its complete capacitance swing in only 90 degrees, as opposed to the 180 degrees of a normal air variable capacitor.

The next post in this series will present a pulse-width-modulated circuit that can achieve a much slower rotational speed of 1 RPM or less.

Addendum: A note on motor lifetime

At the following page, some interesting data is presented on the lifetime of the type of motor I am using: https://www.pololu.com/docs/0J11/all#2. It can be seen that the lifetime for continuous operation is, in the best case, on the order of tens of hours, and in the worst case (when running at higher voltages) is less than ten hours.

This has some serious implications for the overall system design:

1. The voltage used to run the motor should be kept within limits to lengthen the motor lifetime.
2. In case of over-voltage, the motor will fail fairly soon. Assume for example that the motor is run at 6 volts. The expected lifetime will be only about 5 hours or 300 minutes. Then, assume the antenna is used such that the total motor on-time per day is 10 minutes (which is realistic if the antenna is frequently re-tuned, for example for short-wave listening purposes on various frequencies). Then, the motor will fail after only 30 days of operation.
3. In the current loop design, it will be quite tedious to replace the motor because:
• The loop dimensions (3m x 2m) are too large to allow transportation of the loop or laying the loop down flat (in the limited space on the balcony) for access to the capacitor box
• The entire loop must be de-soldered in place, piece by piece (see part 1 of this article series), to access and dismantle the top-mounted capacitor box
• After motor replacement, the entire loop must be re-soldered together
4. To better accommodate the eventual necessity of replacing the motor, it may be better to use mechanical connections to connect the capacitor to the main loop. The mechanical connections could then be easily undone to completely disconnect (mechanically and electrically) the capacitor box from the loop, where it could then be taken inside and the motor replaced. Such mechanical connections could be wing nuts, hose clamps, or similar. These connections will introduce additional ohmic loss compared to a solder connection, but with a large contact area the resistance should be able to be kept within manageable bounds. Additionally, the radiation resistance of the loop is relatively high due to the quarter-wavelength circumference, making tolerable the additional ohmic loss through mechanical connections.