DIY Loading Coil Shortens Antenna Lengths

A newly licensed amateur radio operator’s first foray into radios is likely to be a VHF or UHF radio with a manageable antenna designed for the high frequencies in these radio bands. But these radios aren’t meant for communicating more than a double-digit number of kilometers or miles. The radios meant for long-distance communication use antennas that are anything but manageable, as dipole antennas for the lowest commonly used frequencies can often be on the order of 50 meters in length. There are some tricks to getting antenna size down like folding the dipole in all manner of ways, but the real cheat code for reducing antenna size is to build a loading coil instead.

As [VA5MUD] demonstrates, a loading coil is simply an inductor that is placed somewhere along the length of the antenna which makes a shorter antenna behave as a longer antenna. In general, though, the inductor needs to be robust enough to handle the power outputs from the radio. There are plenty of commercial offerings but since an inductor is not much more than a coil of wire, it’s entirely within the realm of possibility to build them on your own. [VA5MUD]’s design uses a piece of PVC with some plastic spacers to wind some thick wire around, and then a customized end cap with screw terminals attached to affix the antenna and feedline to. Of course you’ll need to do a bit of math to figure out exactly how many turns of wire will be best for your specific situation, but beyond that it’s fairly straightforward.

It’s worth noting that the coil doesn’t have to be attached between the feedline and the antenna. It can be placed anywhere along the antenna, with the best performance typically being at the end of the antenna. Of course this is often impractical, so a center-loaded coil is generally used as a compromise. Coils like these are not too hard to wind by hand, but for smaller, lower-current projects it might be good to pick up a machine to help wind the coils instead.

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All-Mechanical Coil Winder Is A Scrap-Bin Delight

If there’s something more tedious than winding coils, we’re not sure what it is — possibly rolling and wrapping coins; that’s really a bother. But luckily, just like there are mechanical ways to count coins, there are tools to make coil production a little less of a chore, but perhaps none that have as much charm as this all-mechanical coil winder.

We’d say that [Ralph (VK3ZZC)]’s amazing invention firmly falls under the “contraption” category, without a hint of the term being used as a pejorative. The rig was based on the MoReCo Coilmaster, a machine that was once commercially available at a fairly steep price, according to [Ralph], and still seems to command a premium even today. Never being able to afford an original, [Ralph] spun up his own from scrap metal and tooling no more sophisticated than a drill press. It’s a riot of brass and steel, with a hand crank that drives the main winding shaft while powering a cam that guides the wire along the long axis of the coil form. Cams can be changed out for different winding patterns, and various chucks adapt to hold different coil forms to the winding shaft.

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Practical Inductors In LTSpice

LTSpice and the underlying Spice engine does a great job of simulating ideal components. But it is also capable — if you know how — of handling models of real-world devices. Inductors, for example, are one of the most imperfect components. Their constituent wire has resistance, and there is parasitic capacitance between the windings. If there is a core, it also will have many imperfections and losses. [Sam Ben-Yaakov] has a lecture about modeling real inductors in LTSpice, and he covers how you can capture some of these imperfections in the video below.

There is a bit of math in the presentation, but we liked that it relates back to datasheets for actual components. Being able to understand what the parameters on a datasheet mean is crucial, and if you ever wondered what some of these entries mean, you’ll get a lot from this video.

The main feature of the model is the flux equation. The tanh (hyperbolic tangent) function is similar to the curve you want for the flux equation, so it plays a major part. Of course, there are other parts of the inductor you may have to model, too, but this is one of the most difficult parts.

You can also model transformers using LTSpice. You can also create custom components.

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Siphoning Energy From Power Lines

The discovery and implementation of alternating current revolutionized the entire world little more than a century ago. Without it, we’d all have inefficient, small neighborhood power plants sending direct current in short, local circuits. Alternating current switches the direction of current many times a second, causing all kinds of magnetic field interactions that result in being able to send electricity extremely long distances without the resistive losses of a DC circuit. The major downside, though, is that AC circuits tend to have charging losses due to this back-and-forth motion, but this lost energy can actually be harvested with something like this custom-built transformer.

[Hyperspace Pilot] hand-wound this ferromagnetic-core transformer using almost two kilometers of 28-gauge magnet wire. The more loops of wire, the more the transformer will be able to couple with magnetic fields generated by the current flowing in other circuits. The other thing that it needs to do is resonate at a specific frequency, which is accomplished by using a small capacitor to tune the circuit to the mains frequency. With the tuning done, holding the circuit near his breaker panel with the dryer and air conditioning running generates around five volts. There’s not much that can be done with this other than hook up a small LED, since the current generated is also fairly low, but it’s an impressive proof of concept.

After some more testing, [Hyperspace Pilot] found that the total power draw of his transformer is only on the order of about 50 microwatts in an ideal setting where the neutral or ground wire wasn’t nearby, so it’s not the most economical way to steal electricity. On the other hand, it could still be useful for detecting current flow in a circuit without having to directly interact with it. And, it turns out that there are better ways of saving on your electricity bill provided you have a smart meter and the right kind of energy-saving appliances anyway.

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How To Make A Larger Air-Cored Inductor

Radio amateurs often have a love-hate relationship with home-made inductors, sharing all kinds of tips and tricks as to how the most stable nanohenry inductor can be wound. But there’s another group in the world of electronics with an interest in high-quality inductors, namely the audio enthusiasts. They need good quality inductors with a values in the millihenries, to use in loudspeaker crossover networks. [Homemade Audio] takes us through their manufacturing process for these coils, and the result is a watchable video resulting in some very well-made components.

The adjustable former is a machined aluminium affair of which we’re treated to the full manufacture. It’s likely the same results could be achieved with a 3D printed reel. The free-as-in-beer Coil64 on Windows is used to calculate the dimensions and number of turns, and it’s set up on a jig with a cordless screwdriver doing the winding. The best technique for flat layers of turns is explained, and a coat of varnish is put on each completed layer. We’re guessing this is to stop the coil “singing” at audio frequencies.

With a set of cable ties holding it together the result is a very tidy component. It’s adjusted a few turns to get the right value with an LCR meter, however experience tells us that a tiny percentage either way won’t harm the resulting network too much. If you make your own speakers, the video below the break could be extremely useful.

Need a loudspeaker primer? We have just the article for you.

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Scripting Coils For PCB Motors

PCB inductors are a subject that has appeared here at Hackaday many times, perhaps most notably in the electromagnetic exploits of [Carl Bugeja]. But there is still much to be learned in the creation of the inductors themselves, and [atomic14] has recently been investigating their automatic creation through scripting.

A simple spiral trace is easy enough to create, but when for example creating a circular array of coils for an electric motor there’s a need for more complex shapes. Drawing a trapezoidal spiral is a surprisingly difficult task for a script, and we’re treated to a variety of algorithms in the path to achieving a usable design.

Having perfected the algorithm, how to bring it into KiCAD?  The PCB CAD package has its own Python environment built-in, but it’s not the most flexible in which to develop. The solution is to write a simple JSON interpreter in KiCAD, and leave the spiral generation to an external script that passes a JSON. This also leaves the possibility of using the same code in other PCB packages.

You can watch the whole video below the break. Meanwhile for more PCB electromagnetics, watch [Carl Bugeja]’s 2019 Supercon interview.

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Beautiful Inductors, Now Not Such A Lost Art

As ferrite technology has progressed into a mastery of magnetic permeability, the size of inductors has gone down to the point at which they are now fairly nondescript components. There was a time though when inductors could be beautiful creations of interleaving layers of copper wire in large air-cored inductors, achieved through clever winding techniques. It’s something that’s attracted the attention of [Brett], who’s produced a machine capable of producing something close to the originals.

Part of the write-up is an investigation of the history, these coils were once present even at the consumer level but are now the preserve of only a few highly secretive companies. They are still worth pursuing though because they can deliver the high “Q” factor that is demanded in a high quality tuned circuit. The rest of the write-up dives in detail into the design of the wire feeder, and the Arduino motor control of the project. There should be enough there for any other experimenters to try their hands at layered inductors, so perhaps we’ll see this lost art make a comeback.

Custom coils are a regular requirement for anything from radios, to musical instruments, to switching power supplies, so it’s not surprising that quite a few projects featuring them have made it here. One of the more unusual of late has been one that winds toroids.