I've decided to try constructing a smaller, more traditional loop of 1-meter diameter. A smaller loop means smaller radiation resistance, and that means that small values of loss resistance become more significant. Great care must be taken in all aspects of construction to minimize the loss resistances.
This series of articles will detail the progress of the project.
The capacitor
The last stage of my previous experiments used a dual-gang 365 pF capacitor, intended for receiving use and connected in split-stator mode. After measuring an unusually high bandwidth of my previous loop antenna, I determined that the capacitor was likely the source of the loss. This was determined by temporarily replacing the capacitor with a homebrew capacitor consisting of two long copper strips each 20 cm long and 5 cm wide, with a large copper sheet laid on top, insulated by a polyethylene freezer bag, to capacitively couple the two strips together. This homebrew capacitor yielded a narrower bandwidth than when using the dual-gang 365 pF capacitor, indicating the dual-gang 365 pF capacitor was overly lossy. Such capacitors often use low-quality insulation and use friction to electrically connect the capacitor plates with the frame and rotor shaft. This construction is therefore prone to both dielectric and metal losses.It is probably possible to homebrew a low-loss capacitor for a small transmitting loop. There are many web pages showing examples of homebrew capacitors for small transmitting loops. However, it becomes difficult to engineer a low-loss variable capacitor as the required capacitance increases. Increased capacitance requires larger and/or more numerous capacitor plates or parallel surfaces, which requires more exacting mechanical construction for the moving parts, and/or larger physical dimensions for the moving parts. Achieving high capacitance and low loss involves a number of tricky issues that are generally not covered in amateur literature. Some of the factors involved in low-loss capacitor design include:
- Minimizing series inductance
- Minimizing physical volume of the capacitor
- Ensuring good current flow through multiple parallel current paths
- Minimizing dielectric loss
These issues become especially more difficult as the loop diameter becomes smaller. For example, consider the issue of the required physical volume of the capacitor. As the loop diameter becomes smaller, the required capacitance, to resonate the loop at a given frequency, increases. Some homebrew capacitors, such as butterfly capacitors or trombone capacitors, achieve such required high capacitances by constructing physically long and narrow structures. However, a loop is supposed to be a balanced radiating structure (although it is never perfectly balanced in practice, as any environmental unbalance, including uncontrollable factors like unevenness in ground composition, will unbalance the loop). Any current flowing in one part of the loop should ideally be matched by an identical current at the diametrically-opposed point on the loop conductor. If we have a long capacitor (such as a trombone capacitor) that extends deep into the interior of the loop, this will disturb the loop symmetry more than a physically compact capacitor would. For example, in Reference 1, W8JI states the following:
Look at how short the path is around the loop.
Now look at the path of current through the capacitor, including conductor sizes in that path and length.
Anything we do to increase path length increases Q while also increasing loss resistance, or even odd radiation directions.
The least effective style of capacitor, other than for feed-through bypassing applications where we might want distributed series inductance and shunt C, is a long (as a fraction of wavelength) coaxial capacitor. The most effective styles are multiple stacked layers in parallel with short heavy solid connections. [...]
The same thing that makes the helical winding wasteful makes a trombone or coaxial capacitor less effective. Unnecessary extra series length that does not contribute to physical area enclosed by the loop is bad.
W8JI mentions that a capacitor with a long current path can even result in "odd radiation directions." This would be due to the current flow in the structure departing from the ideal current flow of an ideal loop.
For my 1-meter-diameter loop, I had sketched out some homebrew capacitor geometries, but the best I could come up with was a fixed capacitor (consisting of a compact stack of soldered, non-moving copper plates) in parallel with a small variable capacitor. Any other homebrew high-value variable capacitor, that was feasible to implement in a rather modest home workshop, would result in capacitor that would be relatively large compared to the loop's size of 1 meter, with the resulting dangers of excessive losses and odd radiation directions.
For a single-band loop, a fixed capacitor plus a small variable capacitor could probably work very well. But for my project, I decided that if I'm going to go through the trouble to build a very low loss 1-meter-diameter loop, I want to be be able to use that loop on as many bands as possible. This requires a physically compact, low-loss, widely-variable capacitance.
The type of variable capacitor that best fulfills these requirements is a vacuum variable capacitor, as these capacitors are specially engineered to have low loss and to be able to carry high currents. The use of a large-diameter and variable-length bellows, the vacuum dielectric, multiple concentric parallel surfaces, and large-area silver-plated contact surfaces all combine to achieve a low-loss, high-capacitance structure capable of withstanding high voltages and high currents.
I purchased the following second-hand 1000 pF vacuum variable capacitor.
To verify the integrity of the vacuum, I used N4SPP's method (Reference 2):
[...] a quick test to verify integrity of the vacuum: put the cap in the refrigerator for about an hour. Should be no formation of condensation on the inside of the glass when in the fridge or after taking it back out (on outside is OK)After performing this test, I observed no condensation on the inside of the glass.
Motorized control of the capacitor
For my previous loop projects, that had only small air variable capacitors, I used a small Tamiya gearbox motor as shown below.
Previously, I had given little thought to the torque that the motor could deliver, instead focusing only on a low RPM (required for fine control of the capacitor). The above gearbox motor had enough torque to turn the shaft of all air variable capacitors that I had on hand.
However, some quick tests with the motor showed that it had insufficient torque to turn the shaft of the vacuum variable capacitor. The plastic gears would skip, as they could not deliver enough power to the load.
A more careful engineering approach was needed. Again referring to N4SPP's detailed page (Reference 2), I used his technique of measuring the torque required to turn the capacitor shaft. A 30-cm ruler was affixed at its midpoint to the capacitor shaft with a C-clamp. This gives a 15-cm arm. It is then only necessary to measure, with a common kitchen scale, the amount of "weight" registered on the scale that is required to turn the shaft when the ruler is pressed against the scale. Multiplying the scale's gram reading by the 15-cm length of the arm gives the gram-cm of torque required.
I determined that the maximum torque required for my variable capacitor was on the order of 3000 g-cm, or 3 kg-cm.
The next step was to find a motor capable of delivering at least this amount of torque. I wanted a reversible DC motor that could be driven from 12 volts, as I have a 12 volt power supply available. A reversible DC motor (as opposed to a servo or a stepper motor) has the advantage of requiring no complex control circuitry. The motor's RPM should also not be too high, to allow a fine adjustment of the capacitance at low motor speeds. After doing much online searching, I decided that the following GW370-8 motor, offered by a number of Chinese manufacturers, seemed the best.
The motor specifications state that it rotates at 8 RPM and can deliver a torque of 8 kg-cm. This torque is enough to turn the shaft of my vacuum variable capacitor. The 8 RPM speed should be low enough to allow fine control of the capacitance. If 8 RPM is still too fast, I can investigate using a PWM approach to reduce the RPM even further.
I am awaiting the arrival of the motor. Next experiments will focus on:
- verifying that the motor can indeed turn the capacitor shaft
- designing the physical mounts for the capacitor and the motor
- designing a limit-switch or position-sensing mechanism to prevent turning the capacitor shaft beyond its safe limits.
References
- Rauch, T. (W8JI). RE: Heliax Loop Antenna. http://www.eham.net/ehamforum/smf/index.php/topic,85149.msg620738.html#msg620738 .
- Doerenberg, F. (N4SPP). Magnetic Loop Antenna for 80-20 mtr. http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
