Building a 7 MHz small transmitting loop antenna, part 5

Part 5 of this series describes some mechanical design changes and first disappointing tests with the loop.

First design change: From copper pipe to copper strap

As described in parts 1 and 2 of this series, the original design for the 3m x 2m loop radiator was to construct it using several 1m-length segments of 6mm-diameter thin-wall copper pipe. However, such a structure, once built, is impossible to move or alter without tedious disassembly.

Since I wasn't completely confident that the loop would work as expected, I decided to go with a more mechanically flexible solution using copper strap instead of copper pipe. The flexible strap cannot support itself and thus requires a supporting frame to be built, on which the strap is draped. I built a supporting PVC frame, 3 meters by 2 meters, as follows (apologies for the blurry photograph):

The bottom half of the support frame is obscured by lower balcony wall, which has been cropped from the bottom of the photograph.

The above photograph shows a top supporting bar spanning the three vertical support pipes.  In the end I ended up removing this top supporting bar because its weight tended to make the whole structure sway excessively in the wind. Removing the top supporting bar means that the uppermost horizontal run of the copper strap is supported only at the tips of the three vertical support pipes. The strap then runs down the two end-most vertical support pipes and is temporarily affixed to the frame with velcro cable ties.

The above setup allows repeated and easy setup and tear down of the antenna as needed for various changes, especially around the capacitor section. As it turns out, this ability to easily tear down the antenna turned out to be vitally important, as the antenna didn't work as expected and required major changes.

Cutting and splicing the copper strap

I used copper flashing with 0.2mm thickness as the loop radiator. The skin depth of copper at 7 MHz is 24.605 micrometers, so the flashing with 200 micrometers thickness is greater than 8 skin depths thick for RF currents at 7 MHz.

The flashing is available in a single sheet of dimensions 365mm x 600mm. I decided to cut this large sheet into 18 individual strips of 20mm width, then to solder all of these strips together to form a ~10m-long radiator.

According to W9CF's strip-to-round-conductor equivalency calculator at http://fermi.la.asu.edu/w9cf/equiv/, a 20mm-wide strip should have RF resistance equivalent to that of a ~6.2mm-diameter conductor, i.e., equivalently low resistance to the 6mm copper pipes I originally planned on using. Note that the calculator assumes that the strip "is much thicker than a skin depth", which is satisfied by our ~8-skin-depths-thick copper flashing.

The following photographs show the time-consuming process to cut and splice the copper flashing by hand. This took a few hours, and my fingers ached for a few days afterwards; cutting this thickness of copper flashing with small shears is tiring on the fingers. 

After joining all the strips into a ~10m roll, the next step was to wrap the copper strap in vinyl electrical tape, to avoid risk of injury on the sharp copper edges when handling the strap.

The last step was to solder the copper strap to the butterfly capacitor and to mount the capacitor in the box.

Failure #1: Not enough capacitance

I draped the loop over the PVC support frame with the remotely-controlled capacitor at the top center of the support frame. I then created a ~2m-circumference auxiliary coupling loop and placed it at the bottom of the large loop radiator.

Unfortunately, my worst fear materialised: the butterfly capacitor did not have enough capacitance to resonate the loop at 7 MHz, the intended frequency of operation. I could resonate it from about 9165 kHz to 15270 kHz, listening for the noise peak in my receiver.

I did a quick tune up test on 10 MHz and 14 MHz and was able to get the SWR down to better than 2:1, but not much better.

To allow operation at 7 MHz, I investigated soldering another smaller butterfly capacitor inside the box, in parallel with the existing butterfly capacitor, in order to reduce the resonant frequency. However, this plan was mechanically infeasible.

As a stopgap measure to allow operation at 7 MHz I tried attaching an additional 150 pF variable capacitor (with a wiping contact, which is not recommended due to ohmic losses) to the butterfly capacitor using alligator clips (again, not recommended due to ohmic losses), and was able to resonate the loop at 7 MHz with the 150 pF variable capacitor almost completely unmeshed. However, I was unable to obtain a low SWR at 7 MHz.

My inability to reduce the SWR to 1:1 on any band prevented me from measuring the SWR bandwidth of the antenna, which might have been an indication of the antenna's efficiency. I did various tests squashing and stretching the coupling loop but was never able to obtain a good SWR. This is one disadvantage of using a coupling loop: it can be difficult to adjust the coupling loop for a good SWR, especially if the loop losses are excessive and unknown due to environmental or other ohmic losses.

Second design change: Capacitive instead of inductive matching 

One alternative to the difficult-to-adjust inductive coupling loop is to use a capacitive network to match the loop impedance to 50 ohms. There are a number of schemes for doing this, and I chose the scheme shown at http://homepage.ntlworld.com/g4kki.william/army_loop_tuner.htm, which resonates the loop with a split-stator capacitor, and feeds the loop through a matching capacitor into one side of the loop conductor (with the coax shield going to the neutral rotor of the split-stator capacitor). Note the above link refers to this as an "Army loop tuner", though it is not the same capacitive network used by the original "Army loop". G8ODE describes the commonly-seen capacitive matching schemes here: https://rsars.files.wordpress.com/2013/01/qrp-loop-tuner-80-20m-g8ode-iss-1-32.pdf.

I built a prototype capacitive matching network on a polypropylene cutting board, and soldered it to my loop conductor. For the loop resonating capacitor, because the 50pF butterfly capacitor had insufficient capacity, I used a dual-gang 2x440pF variable capacitor in a split-stator configuration.

The loop tuning and matching circuit construction looks as follows:

Note that the capacitive matching scheme is not balanced, and so there is a danger of common-mode currents on the outside of the shield of the coaxial cable. To suppress these, I wound a ferrite-cored common-mode choke on a FT250-43 core, using 12 turns of RG58A/U. According to G3TXQ's page at http://www.karinya.net/g3txq/chokes/, this choke should exhibit >4k ohms impedance at 7 MHz.

Although it is preferable to mount the capacitor at the top of the loop (to keep it and the intense electric field away from lossy surroundings), the heaviness of the new capacitor assembly and the choke precluded top-mounting with my somewhat fragile PVC support frame. Therefore, I mounted the capacitor, feedline choke, and feedline at the bottom of the loop, which was still about 20cm above the concrete balcony floor.

Partial success: Perfect 1:1 SWR achieved

After hooking up the loop to my rig, I tuned the split-stator capacitor and quickly found the capacitor setting (approximately 25% meshed) that would peak the receiver noise at 7 MHz. Then, I experimented with different settings of the 150pF matching capacitor. It turned out that setting the matching capacitor to fully-meshed (150pF) gave a perfect 1:1 SWR.

This was my first experience with a capacitively-matched loop, and it was quite easy to tune the loop for minimum SWR. Adjusting the matching capacitor is much easier in practice than adjusting the size of an inductive coupling loop.

There is some interaction between the tuning and matching capacitors, due to the matching scheme requiring that the loop be tuned slightly off-frequency to provide the inductive reactance needed to effect the impedance transformation; see here for details: http://www.eham.net/ehamforum/smf/index.php/topic,100081.msg802902.html#msg802902. In practice, this interaction really isn't a problem. You just tune the main capacitor for maximum noise, adjust the matching capacitor to some approximate appropriate value, and retune the main capacitor to again peak the noise.

Failure #2: Ridiculously high bandwidth

My joy at attaining a perfect 1:1 SWR was short-lived, as I quickly discovered that the SWR was still 1:1 even after retuning several tens of kHz. In fact, the SWR was less than 1.5:1 over the entire 200 kHz of the Japanese 40m band!

This is quite a bad sign indeed. A previous vertical dipole I built for 7 MHz (2m long) had a 1.5:1 SWR bandwidth of about 40 kHz, and that was broader than the 4nec2 prediction of 20 kHz. A small transmitting loop should have even narrower bandwidth. Therefore, a 1.5:1 SWR bandwidth of 200 kHz is ridiculously high and indicates significant losses within the system.

I can think of a number of possibilities to explain the loss:

1. My split-stator capacitor is too lossy. 
2. The matching capacitor is too lossy.
3. The copper strap is inherently too lossy and/or the electrical tape is increasing the losses.
4. The environment around the loop is too lossy, with the loop running close to the concrete walls.
5. The environment around the capacitor is too lossy, with the loop capacitor near the concrete floor.
6. One or more solder joints is faulty. The solder joints include those that connect the short copper strap segments together, those that connect the strap to the capacitor, and those around the matching capacitor.

Possibility 1 I think is unlikely, as I used the split stator capacitor in a previous small transmitting loop that exhibited a narrow bandwidth. Possibility 2 also seems unlikely because the matching capacitor is not part of the resonant circuit and isn't carrying that much current; its losses should not adversely affect the loop efficiency. And, possibility 3 also seems unlikely because I am inclined to believe W9CF's calculations on the RF resistance of strip conductors.

I can think of the following ways to proceed:

• To attack possibilities 4 and 5, a smaller-diameter loop (reducing radiation resistance) with a wider conductor (reducing loss resistance) could be used. That could allow comparable efficiency to the larger loop while allowing the loop to be positioned farther away from the concrete walls.
• Alternatively, and still for possibilities 4 and 5, an experiment could be done erecting the loop in an open area away from concrete walls, and observing if the SWR bandwidth decreases. If so, then the environment is a major source of loss. An experiment with top-mounting of the capacitor might also yield insight.
• To address possibility 6, I could re-do all the solder joints. This will be quite tedious on the shorter copper strap segments, as these have all now been wrapped in and sealed with electrical tape.
• Alternatively, to address possibilities 3 and 6, an experiment could be done with a continuous conductor instead of my pieced-together copper strap. The continuous conductor could be a wire, a coaxial cable shield, or a long strip of unbroken copper tape. None of these is an ideal conductor for a low-loss small transmitting loop, but using one or more of these continuous conductors with the existing capacitive loop tuner might reveal if the pieced-together and tape-insulated copper strap is excessively lossy in comparison.
• To address possibility 2, the matching capacitor - currently an air variable capacitor with a wiping contact - might be replaced with the high-Q, no-wiper, butterfly capacitor that proved to be unsuitable as the resonating capacitor.
• Finally to address possibility 1, a vacuum variable capacitor could be used as the loop resonating capacitance. This would require two matching capacitors rather than just one. And, again, the weight of the vacuum variable capacitor would preclude top-mounting on my current support frame, requiring a bottom-mounted capacitor.

Most likely I will proceed next by re-doing the easily re-doable solder joints. If that does not improve (reduce) the bandwidth, then I will investigate using a continuous conductor.

My current hope is to achieve a narrow-bandwidth with a capacitively-matched loop and bottom-mounting of the capacitor. If I can achieve narrow bandwidth (and reasonable efficiency), then I will motorise the variable capacitors for remote control of tuning and matching.

Building a 7 MHz small transmitting loop antenna, part 4

Part 4 of this article series focuses on the PWM circuit for precise and slow control of the motor that drives the capacitor.

Attempt 1: Analog-to-analog motor control circuit 

Before discussing the final PWM circuit I used to control my motor, allow me to discuss another highly interesting circuit: the 7N3WVM "Remote Motor Controller" described here: http://www.qsl.net/7n3wvm/DC_mot_control.html. Please have a look at that page before proceeding.

The 7N3WVM circuit is attractive because it seems to offer an intuitive interface to control the remote motor: a locally-operated DC motor that is turned to generate an analog DC control voltage, which is then amplified by a power amplifier and sent to the geared motor. If clockwise rotation of the control motor generates a positive voltage, then counter-clockwise rotation generates a negative control voltage. Either polarity is properly amplified by the power amplifier and is sent along to the remote motor, allowing bidirectional control by turning the shaft of the local control motor left or right. What's more, faster rotation of the local control motor generates a higher voltage, leading to a higher voltage at the remote motor and faster turning of the remote motor.

The system sounds ideal, and initial tests were quite encouraging. I actually wired up a previous loop antenna using this motor control method. However, there is unfortunately a subtle but crippling flaw in this system: the motion of the remote motor is unreliable (more on that in a moment), so when turning the local control motor shaft indoors, you never know if your local knob rotation had no effect, some small effect, or a too-large effect at the remote motor. So you're operating essentially "blind", randomly turning the local control knob left and right, fast and slow, but never really knowing if the remote capacitor is moving at all, moving slowly, or moving too fast.

So what causes this unreliability in the motion? The problem is that rotating the local control motor's shaft generates a varying analog DC voltage depending on how quickly and how smoothly you rotate the shaft. For very slow rotations of the control knob, the generated voltage, after amplification, is too small to turn the remote motor. So nothing happens, and worse yet, you have no way of knowing that nothing happened (unless you implement some means of sensing the remote motor's current angular setting, via e.g. a potentiometer as mentioned in Part 3).

Then, if your steady hand rotates the control knob at just the right speed, then the remote motor will rotate slowly and smoothly. However, if you rotate the control knob too quickly, the local voltage jumps very high, leading to a very high amplified voltage at the remote motor, leading to a quick and jerky rotation of the remote motor. And again, with no feedback at the local operating position, you have no way of knowing when this happens.

I experimented with the supply voltage of the power amplifier; 7N3WVM recommends 3 volts. But I found that 15 volts worked best (note that the NJM2073D chip is rated for a maximum of 15 volts supply voltage), using a 3V motor for the local control motor, and a 3V geared motor (with 661.2:1 reduction) for the remotely-controlled motor. With 15 volts supply voltage for the NJM2073D power amplifier chip, even relatively small voltages, caused by slow rotation of the local control knob, were amplified enough to cause rotation of the remote motor. But even in this best-case scenario, I still could not completely solve the issue of unreliable, jerky rotation. Sometimes, the remote motor would not rotate at all; sometimes, it would rotate too fast.

I think it is very important that any remote control scheme work reliably: if you take some action locally (push a button, flip a switch, turn a knob), then it is imperative to know that something well-defined definitely did happen at the remotely-controlled device. As elegant as 7N3WVM's scheme is, it unfortunately fails this test, at least in my setup with my choices of motors.

I think the 7N3WVM scheme might be made to work well if some of the following ideas are applied:

• Use a higher-voltage motor for the local control motor, to ensure that even slow rotations of the local control knob generate relatively high DC voltages. This should help ensure that slow rotations more reliably generate enough voltage to drive the motor at the remote end.
• Use a higher gear reduction ratio on the geared motor at the remote end. My motor used a 661.2:1 reduction ratio; a further reduction ratio would physically limit the maximum rotational speed of the remote motor's output shaft, which would make jerky motion more tolerable.
• If possible, add a potentiometer to the remote motor shaft to sense the remote motor shaft's current rotation. Then, display the potentiometer value with an ohmmeter at the local control position. This will allow visualisation at the local control position of the effect (if any) that the control knob rotation had at the remote motor.
• When operating the local control knob, make a series of short rotations, separated by a brief and full stop of the control knob, instead of one smooth slow rotation. The intermittent nature of the series of short rotations is probably generating a pulsed output voltage which seems more reliable in ensuring motion of the remote motor.

Attempt 2: Transistor PWM control circuit

The previous paragraph mentioned that in the 7N3WVM control scheme, a pulsing voltage (generated by a series of short control knob rotations) was more reliable in achieving remote motor motion, compared to a steady low voltage (generated by a continuous, slow rotation of the control knob).

Indeed, a well-known technique for achieving reliable slow rotation of a motor, without stalling, is to use a pulse-width modulated (PWM) voltage. The technique and its motivation are described in many places, so I won't repeat the explanation here; see for example this page for a good introduction: http://www.electronics-tutorials.ws/blog/pulse-width-modulation.html.

The above page presents a PWM circuit using a 555 timer IC. I wanted a discrete transistor solution, and found a simple PWM circuit here: http://www.techpowerup.com/forums/threads/make-your-own-fan-speed-control-under-1-usd.124633/#post-1932968.

After building the above transistor PWM circuit and hooking it up to my geared DC motor, I was very pleased to see that it did allow reliable and very low-speed rotation of the motor. Unfortunately, after a few minutes, the smell of burning electronics drifted through the air, and I realised that the last driver transistor was smoking hot and certainly on the verge of failure. I used 2N3904 transistors throughout, and the motor was obviously pulling too much current through the driver transistor.

An obvious solution would be to use a power transistor capable of handling the motor current, but I had none on hand. And anyway, it seemed rather pointless to think about power transistors, when I already knew that I had a component on-hand capable of handling the motor current: the NJM2073D power amplifier chip from my previous experiments.

Final circuit: BJT PWM + power amplifier to drive motor

So, the logical next step was to use the transistor PWM circuit to generate a pulsed voltage, that would serve as the input to the NJM2073D power amplifier, which would then drive the motor. The final circuit as as follows.

Q1 and Q2 form an astable multivibrator. The values of the base bias resistors, R3 (for Q2) and R4/R10 (for Q1) were determined experimentally to yield a low-frequency pulse train. (An easy way to verify the pulse train frequency is to connect a piezoelectric earphone between ground and the Q2 collector and listen for the clicking or buzzing sound.) Switch S1 allows shorting out one of the Q1 base bias resistors, R4, which reduces the total Q1 base bias resistance from 570k to 100k. This has the effect of increasing the frequency of the pulse train, which leads to faster rotation of the output motor. With S1 open, very slow motion of the output motor is possible; with S1 closed, faster motion is possible.

The Q1/Q2 circuit was designed for operation off of 1.2 volts and was initially powered off of a separate 1.2 volt battery. Later, when reworking the circuit to use my main 12 volt power supply, I originally used a few silicon diodes to form a simple voltage regulator to supply 1.2 volts to the Q1/Q2 circuit. But during testing I empirically found a simpler solution: simply inserting R12 (10k) between the 12 volt positive rail and the Q1/Q2 circuit yielded about 1 volt available for Q1/Q2, and the Q1/Q2 multivibrator functioned properly in this simpler configuration. This is a rather ad-hoc solution, but it works.

Q3 is the driver transistor. Originally, the motor was connected directly between the Q3 collector and the positive rail. Now, instead, the Q3 collector does not drive the motor, but instead only provides input voltage (from the multivibrator pulses) to supply to the NJM2073D power amplifier. In 7N3WVM's original NJM2073D circuit, driven by the voltage generated by a DC motor, the input pins 5 and 8 are floating and connected to the DC motor terminals. 7N3WVM's "floating input" configuration allows both positive and negative polarities to appear at the output pins 1 and 3. However, in my circuit, instead of a floating input at pins 5 and 8, I have permanently tied pin 5 to the positive rail (through R13) and pin 8 to the negative rail (through Q3). This means that the output voltage at pins 1 and 3 will have only one fixed polarity and that the output motor can only be driven in one direction. Therefore, I added a DPDT switch at the output (S2-S5) to allow reversing the motor direction by flipping the DPDT switch to reverse the motor connections to the output pins 5 and 8. Physically, the DPDT switch also has a "neutral" position in the center where it is not connected to any output, which allows stopping the motor once the appropriate capacitor setting has been reached. For operating convenience, I chose a spring-loaded DPDT switch that automatically returns to the central neutral position when pressure is removed (an "auto-return" DPDT switch). This allows momentary pressure on the switch in either direction to rotate the motor in either direction, and automatic stopping of the motor when one's finger is removed from the switch.

Resistor R13, between input pin 5 and the positive rail, was determined empirically to limit the input voltage available at pins 5 and 8 such that the geared motor was just barely able to rotate. If R13 is too small, the input pulses at pins 5 and 8 are so large that they result in a strong, jerky motion of the output motor for each pulse. If R13 is sized appropriately, the input pulses (and output pulses) are reduced in strength such that the motor smoothly and slowly rotates. If R13 is too large, then no rotation of the motor is possible because the input pulses and output pulses are too small.

Due to the number of empirical adjustments made to the circuit, it is likely that the circuit will need to be adjusted if used with a different motor and/or a different capacitor (whose shaft stiffness will determine how much power the motor needs in order to rotate it).

Video of PWM-controlled motor in action

The following video shows the operation of the PWM-controlled motor. Notice the slow rotation of the motor gears and the extremely slow movement of the capacitor vanes, which are important to allow fine adjustments to the capacitor setting.

The video starts out with the slowest PWM pulse train. Later in the video, I close switch S1 which leads to faster motion. I also switch the DPDT switch to show reversal of the motor motion.

There is some backlash when reversing direction; it takes several seconds for the capacitor shaft to start rotating in the opposite direction. This was expected and is tolerable.

Most importantly, the circuit (after the empirical adjustments described above) is very reliable. When voltage is applied, the motor always rotates, at a slow and reliable speed. This makes remote operation easy; in particular, it is easier than the 7N3WVM analog-motor-to-analog-motor circuit, which was unreliable and prone to both under-responding (motor stalling) and over-responding (jerky rotation). But with the PWM circuit, it is possible to know with certainty that the remote motor will rotate in response to the local switch positions: with the DPDT switch in the upper position, the motor will definitely rotate clockwise with a well-defined maximum speed and no jerkiness (due to the speed limit imposed by the multivibrator pulse train); with the DPDT switch in the lower position, the motor will definitely rotate counter-clockwise, again with no jerkiness; with the DPDT switch in neutral, the motor will stop.

The system of course is not perfect: as with any mechanical system, there will be some unpredictabilities in the operation like slightly varying rotational speed, temperature-dependent expansion of parts or heating of grease, slow startup and overshoot on stopping due to friction and inertia, and gear backlash when reversing directions. Nevertheless, I expect that the system will be sufficiently convenient, accurate, and reliable for tuning the loop antenna.

https://www.youtube.com/watch?v=-gtEbqFzXX4

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. 

Building a 7 MHz small transmitting loop antenna, part 2

I did some more exploratory work on the mechanics of constructing my 7 MHz small transmitting loop.

Fitting copper pipes together

The copper pipes I am using are 6mm in diameter with a wall thickness of 0.5 mm. This means that the inner diameter should be 5mm.

My plan on fitting the copper pipes together was to insert a short length of 5mm copper pipe into the 6mm pipe, as an inner segment to provide some structural stability. Then the adjoining 6mm copper pipe would also be slid snugly over the 5mm copper pipe, and butted against the other 6mm pipe. The butt joint, reinforced by the interior 5mm pipe, would be heated and soldered.

The problem in reality is that the 5mm pipe cannot quite be inserted into 6mm pipe due to manufacturing tolerances. The outer diameter of the 5mm pipe is slightly greater than 5mm and/or the inner diameter of the 6mm pipe is slightly less than 5mm.

Therefore, I needed to swage open the ends of the 6mm copper pipe, creating a flared opening at the pipe end, so that the 5mm pipe could fit inside.

My hand tools are limited, but I found that I had a screwdriver of just the right diameter to swage (somewhat crudely) the 6mm pipe opening wide enough to accommodate the 5mm pipe.

My swaging process is quite crude and requires me to wiggle the screwdriver in the 6mm pipe opening to flare it open. This process is not precisely repeatable, so each time I swage a pipe, the exact depth of the flare (i.e. the maximum depth into which the 5mm pipe can be inserted before it is stopped by friction against the unflared 6mm pipe wall) is different. This means that the required length of the inner 5mm connecting pipe segment will be different for each set of pipes to be connected due to the inexact and differing depths of each flared-open pipe end. This is illustrated in the above diagram by the different lengths of flared end A and flared end B.

To proceed, each end of each pipe to be connected must be individually flared open such that the 5mm pipe can be inserted to a depth sufficient to support the joint. Then, for each pair of pipe ends to be connected, a custom length of 5mm pipe must be cut specifically for that pair of pipe ends so that when inserted, the custom length of 5mm pipe allows the 6mm flared pipe ends to exactly butt against each other. 

Then, the pipes will be soldered together.

Copper soldering with a torch

I have no experience soldering copper with a torch. As an experiment, I grabbed some spare copper flashing I had lying around and tried soldering it with my small torch. The results, as you can see, were highly unsatisfactory, though I at least did get the solder to wet and bond in some places.

Clearly, I will need much more practice with the torch before I will have confidence soldering the pipes together.

The above experiments were done without flux. My next try at torch soldering will use flux applied to the copper surface.

Soldering flux to be used in future copper soldering experiments with the torch.

As an alternative, it may turn out to be easier to use a large-wattage soldering iron instead of a torch to solder these small copper pipes.

Attaching copper pipes to the capacitor box

The top-mounted and motorised capacitor will be mounted in a small plastic box for protection from the weather. Another mechanical problem to solve is how to attach the top-mounted capacitor box to the pipes at the top of the loop.

Due to the top-mounting of the capacitor box and the lack of a central spine in my loop design, the points at the top of the loop where the pipes connect to the mounting box may be subject to moderate stress if the loop flexes, as may happen in wind or during manual repositioning of the loop. And, indeed, we want the pipe-to-box connecting point to bear all of the stress caused by loop flexing, because otherwise that stress would be borne by the stator shafts of the delicate butterfly capacitor, which in my case is quite small and might easily be torn apart, cracked, or warped if the stator shafts are subject to too much stress.

Hot melt glue seems appropriate in this case to bond the copper pipes to the capacitor box. It seems strong enough to bear the expected stresses, yet it can be pulled apart for disassembly by a firm tug with a pair of pliers.

In the following image, I am applying hot melt glue to attach one pipe (that will eventually be at the top of the loop on one side) to the left half of the capacitor box. Later, another pipe will need to be connected to the right half of the capacitor box.

Applying hot melt glue to affix the one copper pipe to the plastic capacitor box. The glue forms a large blob completely covering the copper pipe and the surrounding plastic.

After the glue solidified, it formed a moderately strong bond that served to hold the copper pipe in place against the plastic box. In the following image, I am holding the capacitor box only. The full one-meter length of the pipe is unsupported and hanging freely in air, exerting maximum leverage on the hot glue joint. The joint held and showed no signs of breaking; I estimate the pipe would bend before the glue joint would break. 

The capacitor box is supported, and the one-meter length of copper pipe is hanging freely in air. The glue joint does not break.

Also, holding free the end of the pipe and allowing the capacitor box to hang freely, again the glue joint showed no sign of breaking. The joint still held even when the capacitor box was filled with the extra weight of the capacitor and motor.

The end of the pipe is supported, and the capacitor box hangs freely in air. The glue joint does not break.

These tests indicate to me that the hot glue joints at the capacitor box will be able to bear the stress of the loop flexing.

After these tests, I could break the glue joint by pulling it apart with a pair of pliers.

The above describes the solution for the mechanical attachment of the pipes to the capacitor box. For the electrical connection of the pipes to the capacitor, located inside the capacitor box, I will solder a flat strap to each of the two top-most copper pipes. Each flat strap will then be routed from outside the box to inside the box underneath the box lid, where each strap will be soldered to one end of the butterfly capacitor.

As with all aspects of this loop design, the mechanical and electrical connections should be capable of being reasonably easily disassembled for loop maintenance, transportation, or storage. To achieve this objective, the general approach for assembling the capacitor area will be:

1. Solder copper straps to left and right copper pipes.
2. Electrical connection: Solder each strap to one side of the butterfly capacitor.
3. Insert capacitor in box, routing straps and the connected pipes outside the box.
4. Mechanical connection: Glue (with hot melt glue) the pipes to the top of the capacitor box.
5. Place lid on capacitor box.

In particular, note that this assembly sequence is easily reversible and does not require any tricky operations like torch soldering inside the plastic box.