Crystal Radio Part 2

Daniel Koch

Issue 69, April 2023

It's not 'free energy', but you can listen to radio with no batteries!

Last month, we covered a lot of the theory for basic Crystal Radio operation, and presented two basic designs. Both have mediocre performance and we could not pick up any AM stations with either of our designs until we took them for a drive to a hilltop lookout. This is because we are a long way from an AM transmitter once the extensive hilly terrain in between is considered. That was no real surprise, as we knew those designs were a bit too simple for our circumstances.

However, they were the simplest place to start and many people will have been able to get reception from them. Anyone who lives in a capital city within the regional city limits, or who lives west of the Great Dividing Range with a country AM transmitter within range, would have had a much greater chance of success. Unfortunately, the DIYODE office is located in a place that sits in neither category. For us and people in other areas, success is possible but less predictable.

This month, we'll look at a series of other designs. They're all harder or more fiddly to make than previous designs, and we'll explore different diodes, too. Along with that, there is some extra theory coming. However, we'll still get plenty hands-on. Before you make anything, please revisit last month's article and read the section about antennas and safety. Two of this month's designs benefit from longer antennas mounted outdoors, so lightning is a potential hazard. First, here are some terms that become important as you explore crystal radios, particularly when doing your own research and building designs beyond the ones we present here.


When dealing with inductors and resonant circuits, the phrase 'Q factor' or even just 'Q' is sometimes used. In many cases, it's thrown around with little explanation, and it can be hard to find a definition or understand its context. It is a measure of damping, which in and of itself can seem an abstract concept to many people who are not already very electronics-minded.

In the most accurate physics terms, Q factor is the ratio of the amount of energy initially stored in the resonator, to the energy lost during one radian of the frequency cycle of the oscillator. The degree describes how far something rotates around a point, while the radian describes how much distance of the arc is covered in the rotation. In other words, it's a unit of radius, rather than of rotation.

For practical purposes, a radian equals around 57 degrees, but while that is not how it's defined, that means there are a little over six radians in one full cycle of a wave. The trigonometric function Sine is also defined by radians, so that's why it is used when dealing with the amount of a sine wave that has taken place. The whole radian concept is a little abstract in this context, and the points above are the main ones you need to know, but for those interested in more, we've linked to an article at the end which explains the maths a bit more.

The take-home point is that Q factor is a ratio of the initial amount of energy stored in a resonator, compared to how much is left after a bit less than a sixth of the frequency cycle. One of the other things many people find difficult is that the Q factor is dimensionless. It has no unit symbol as it is only a ratio, and even then it is not expressed with a colon or fraction line (the vinculum).

The higher the Q factor number, the greater the amount of energy that is left after the first radian of the frequency, compared to what was there to start with. It matters to radios and other resonators because the higher the Q factor, the higher the amplitude of the resonator at its resonant frequency (when driven by a sine wave) but the narrower the range of frequencies that it will resonate at. Q factor, therefore, is related to bandwidth.

If a coil has a low Q-factor, it passes a greater range of frequencies as well as not resonating as highly at those frequencies. In a crystal radio, this translates to lower volume and more interference. However, it is not as simple as just 'make a coil with a high Q factor'. Making coils precisely enough to have a high Q-factor, which is in part determined by the exact diameter, core material, wire size, turn spacing, and so on, is nowhere near as easy as any other part of a crystal radio. In addition, very high Q-factor coils with narrow bandwidths are harder to tune with the available capacitors because of their accuracy and lack of

precision in adjustment. So, in many crystal radios, there is a compromise between volume and frequency range, even to the point of sometimes hearing two stations at once, as a trade-off for being easy to build.


Another concept that is even more relevant to some of these designs than last month's is impedance. In the simplest terms we can think of, impedance is the resistance to the flow of electricity in alternating current circuits. However, unlike DC circuits, it changes and is based on a variety of factors. Impedance can more accurately be thought of as like inertia in physics: It is the tendency to resist change. In physics, the inertia usually deals with kinetic energy. In AC circuits, impedance relates to the current flow.

So, impedance is the pushing back against an increase in current flow, not an outright resistance to current. However, it also means a tendency to resist a fall in current. Mathematically and in absolutely accurate terms, the definition sounds nothing like that. But, it is far more complex and involves a bunch of advanced maths, and it doesn't change anything for us for the purposes of this topic, so we'll stick with that explanation.

For resonating circuits, the best power transfer occurs when any parts of the circuit which relate to each other and have impedance, have the same impedance. There is impedance in any resonant circuit, or any coil even if it's non-resonating. For example, there is some natural resonance on an antenna, between the wire and ground. If the impedance of this matches the impedance of the resonator in the receiver made with a capacitor and inductor, power transfer is greatly improved.

Looking back on last month's circuits, one used a pre-made, matched coil. The other used the slider system so that the one coil was divided into matchable sections, one for the antenna and one for the resonator. However, there are other ways of doing this and some are far easier to get tuned, albeit harder to physically build correctly (hence the system we chose last month).


An alternative is to use completely separate coils to match the antenna and resonant circuit, using inductive coupling. This is the same physical principle used to make power transformers, but those are built differently. When two coils of wire are close together, and a current flows in one, an inverted current is induced in the other. In the case of the crystal radio, one coil is attached with the antenna at one end and ground at the other. It may have a capacitor added or rely on the parasitic capacitance present, but it forms a resonant circuit and therefore has an alternating current flowing in it when energised by incoming radio signals.

The other coil is the secondary, and it is the one that forms our adjustable resonant circuit. Some circuits have an adjustable capacitor in the primary side as well. The energy flowing in the primary side through its resonant circuit is induced in the secondary, which can have a different impedance and still transfer energy. This is used to match the two resonant circuits because the inductors can be made to suit the capacitance needed on each side. The direction of these currents is reversed, but that doesn't change anything at the listening end.

There are some other effects to note about inductive coupling. The distance between the coils affects how much energy is transferred between the coils. The greater the distance, the less energy is transferred. While bad news on the face of it, this does have the effect that only the stronger signals are passed to the secondary. This increases 'selectivity', which is the ability of a radio circuit to pick out the signal it needs and ignore others. It also narrows the bandwidth, which is the range of accepted signals. The two concepts are closely related.

The downside of this is that there is often a tuning capacitor for the antenna, another for the receiver side, and adjustment between coils. Changing one affects the remaining two. So, while performance can be significantly better with this system, it is also significantly harder to use.


One challenge with resonant circuits is loading. This is the drawing of current from a resonant circuit, which dampens it or reduces the oscillation quite quickly. The diodes used as detectors have a very low impedance indeed, and this quickly loads a circuit. For the same reason, regular headphones with 8Ω, 16Ω, or 32Ω impedance stop oscillation completely and do not work.

Many crystal sets used headphones of 600Ω impedance and above, the more the better. Modern versions use piezo earpieces which are very high impedance. This is why many designs of crystal radio use a tapped coil in the resonator section. Only a small amount of the coil is loaded by the detector, with the rest dedicated to the resonator section.


Many designs we analysed while researching for this article use variable capacitors that differ from the one and only style and value readily available today. The common unit is the square plastic one we used last month, with a capacitance varying up to 160pF. However, some designs use values up to 600pF. These are mostly older models not available anymore. Some countries have a vibrant vintage electronics and recycled parts market, but many people will struggle to get these capacitors.

There are two main formats. Trimming capacitors are easier to get, but are smaller packages adjusted with a trimming tool (like a plastic screwdriver) then left and not varied again, like a trimpot. Tuning capacitors are the equivalent of shaft-type potentiometers, which can be continuously varied with a dial. Unfortunately, both are limited in variety. The option for many people will instead be a home-made variable capacitor, and that was our reality, too. They're quite easy to make with aluminium foil, the cardboard tube from the same foil or any other kitchen rolled product like paper towel or cling wrap, some paper, and sticky tape. We made ours more user-friendly with a foam-core board structure, and some string and other scrap parts to make a winder. These make it easier to use but it is perfectly ok just to have the two sliding sections described in the first steps.

Mini Build: Making a Variable Capacitor

Tools and Materials Required, See Text for Details
Kitchen Aluminium Foil
Cardboard Tube, Approximately 40mm (see text)
A4 Paper
Sticky Tape
Polymer String
20mm Conduit, approx. 300mm
Foam-Core Board
Hookup Wire
Small Screw-In Eyelets
Barbeque Skewer
Sharp Knife
Roller Cutter
Drill and Drill bit to suit skewer
Drawing Compass
Hot-Melt Glue Gun and Glue

To make our variable capacitor, we first collected our materials. There are many, many ways to make a variable capacitor but we based our design on materials and methods available to most people.

We started by carefully cutting our two squares of aluminium foil 150mm × 150mm. The best tool for this is a rotary cutter, like the one pictured, but careful use of a very sharp and fresh utility knife will suffice. The blade needs to be very new and you will need to press very firmly with the ruler to stop the foil pulling instead of cutting.

Also, the squares on our cutting mat are actually half-inch, not 10mm, so the photos look as though the squares are not the right size.

We wrapped one of these foil squares around the cardboard inner tube from a roll of cling wrap. This product happened to be a 33cm-wide roll, while some brands (and most baking paper, foil, go-between, and so on) are 30cm widths. This gave us some extra clearance. We wrapped the foil so that there was around 10mm from the end of the foil to the end of the tube. We carefully taped down the long edge of the foil, then wrapped tape around the ends. One end had a gap left in the tape, where a wire will be secured later.

Next up, we cut a piece of plain copy paper to a 170mm × 170mm square, and then taped the remaining foil square to one side so that there was a 10mm border all around.

We wrapped this around the foil already on the tube. It needs to be firm enough to not flop around, but loose enough to not catch or place too much friction on the tape holding the foil to the inner tube. There will also be a wire under there at one end soon. The idea of only taping one edge is that the foil will slide as the paper is rolled, because the inner and outer surfaces are at slightly different radii as the roll is formed. Finally, when rolled all the way, the layers were secured with tape much like the layer underneath.

To finish the main build, we stripped the ends of two lengths of silicone wire, to form the electrical connections to our capacitor. Any hook-up wire will do but this stuff is very flexible and we had some scraps. We taped one wire to the end of the foil on the inner tube at the outside end, and the other to the outer foil roll at the opposite end.

We now had a functional variable capacitor. Sliding the outer roll over the inner roll varies the capacitance, and it was that simple. However, it's far from user-friendly in this configuration. We set out to improve this, starting with a mounting for the capacitor. We cut a strip of foam-core board 5cm wide and as long as the A3 sheet would allow, and four 5cm × 5cm squares. We also cut two lengths of 20mm PVC conduit, one at 5cm (ours was under but it didn't matter) and one at 15cm. A pipe cutter is the best tool to do this neatly with squared ends but many people do not have one. The next best tool is a conduit cutter (which has a habit of cutting with slightly slanted ends and a pronounced lip on one side), then some sort of cut-off or drop saw, followed by a hack saw then a wood-cutting tenon saw.

Before mounting the capacitor, we found the centre line of two of the squares, then cut V-shaped notches at each end, around 1.5cm in and a total of 8mm wide. That was the theory, anyway, but our unsteady hand-cutting went at all angles. The 1.5cm measure stayed, though. After this, we marked the diameter of the cardboard tube onto the pieces but took this photo beforehand and forgot to take another. If you're good at estimating distance, you won't need to mark it anyway.

We then cut the centres out of the other two square pieces. This involved using the diagonals to find the centre, using a drawing compass to draw a circle with the correct diameter to pass the conduit so that it did not wriggle but turned easily, and then cutting out the circle. We used wad punches but again, they're not in the toolbox of every maker. Careful use of a craft knife/scalpel would work for most people.

It was time to start glueing things together, and our fingers to things. Seriously, be careful with hot-melt glue. It doesn't feel terribly hot at first but it stays on where it spills and continues to feed heat into your skin if it does get on you, resulting in much worse burns than you might think. If you rush to get it off, and use fingers instead of a tool, you risk getting it on those fingers and burning them too. It's easy to forget that if you don't use it often. We glued the cardboard tube to the notched end squares so that it is in the middle of them and both sat flat on a surface.

After this, we glued the tube with its mounting ends to the long 5cm strip of foam-core, so that it was aligned with one end. We glued on eyelets over the notches so that one edge was slightly above the surface of the cardboard tube. We did this at both ends but at the end that meets the end of the base strip, we added a length of 20mm conduit in the middle, aligned horizontally.

At the other end, we glued the two squares of foam-core that had the holes in them, against the end of the base strip. They were mounted perpendicular to the other two holding the capacitor, and scraps of foam-core we used as bracing. We could have cut these squares as rectangles that went right to the end-caps of the capacitor, rather than leaving this gap that needed bracing, but we wanted the gap to give tool access when manipulating the string in the coming steps. You can see the eyelets here, too.

We slid in the longer piece of conduit and marked where it meets the outer faces of its support frame, before withdrawing it and drilling two holes all the way through. After being slid back into the frame, lengths of bamboo barbeque skewer were forced into the holes to act as stoppers.

The next item is something we would do differently if we were to build another capacitor. We wrapped the conduit in masking tape for increased friction, then glued a piece of string onto the bottom of the outer paper and foil roll, leaving plenty at each end. We took care to make sure it was straight and did not curve around off-axis. Then, the tube was rolled so that the string was underneath.

The string was fed out through the eyelet, over the fixed conduit, and through the top eyelet at one end. At the other end, with the winder, the string was fed through the eyelet and then wrapped eight times around the conduit. It was done loosely at first. Then, angled pliers were used to pull the string tight and wrap it around the winder drum so that the turns are side-by-side and tight. Finally, the string was passed through the top eyelet.

Finally, before tying the string ends together, we had to hold the ends of the string with some tension, and turn the winder and hold the paper and foil tube still to take all slack out of the system before tying a knot in the string on the top of the tubes. This was fiddly and frustrating, and in retrospect, was a poor way of doing things. We should have skipped the glueing step, passed the string around the eyelets and drum, and then tightened the turns on the drum before taping them down. Then, we could pull the string ends tight into a knot with plenty of tension, because there would be no other fixing point yet holding things in different places. After this, we would glue the string to the top of the paper and foil tube, once the system was tight. We're not sure why we didn't think of this before we started, or thought the way we did it would be better, but it was too late. We have a working capacitor, anyway.

Mini Build: Making with Coupled Coils

Parts Required:Jaycar
17m (approx.) 0.8mm Enamelled Copper WireWW4020
15m (approx.) Meduim-Duty Hookup Wire (Antenna)WH3042
5m to 20m Light-Duty Hookup WireWH3012
1 x Speaker TerminalPT3000
8 x 4mm Binding PostsPT4055
2 x 4mm Banana PlugPP0400
1 x Germanium Diode or Equivalent, see text Issue 67We used: Pedal Parts Australia SEMN0003
1 x Crystal EarpieceAS3305
1 x Crocodile Clip for Banana PlugPA3696
Polymer String, see Text-
40mm Cardboard Tube (approx.)We used: A baking paper core
Foam-core Board-
Hot Melt Glue Gun and Glue-
1 x Home-Made Variable Capacitor-

This design is the same as or similar to a great many of the commercially-available DIY crystal kits from the 30s to at least the 70s, and maybe later. Even the antenna recommendations varied only a little between kits and sources, so ours probably looks the same, too. As we mentioned earlier, some designs just worked well and changing anything can ruin that performance. Some images of similar designs, and a very few schematics, show a common ground but we're keeping ours isolated like the majority of sources we found. However, we added a removable connection so that this can be experimented with.

On the one side is an antenna resonating circuit consisting of twenty turns of wire around a former, or mandrel, which has a nominal diameter of 38mm. 40mm PVC pipe works great for this, but one of the cardboard tubes we had was 39mm, so we used that. Just make sure whatever you use is not metallic or made of anything (like ferrite) that could alter a magnetic field. These twenty turns need to be tight and close together. Right beside them are another one hundred and ten turns, tapped at thirty turns with eighty turns left. Check the diagram carefully, as the orientation matters. The schematic is not, because of drawing conventions, drawn the way that the coil is wound. The two side-by-side coils provide the coupling: there is no need for one to be inside the other. Attached to this is our home-made capacitor, and the diode.

This month, we've made use of binding posts for attaching some components, because they are easy to get, and reliable. We tried making our own spring contacts out of easy to find materials but could not come up with anything reliable. Any good design we came up with used things that fewer people can source.

Making your own air-cored inductors involves using the right wire size. The calculator we like to use, linked to at the end of the article, has coil diameter, coil length, and the number of turns. From the length and number of turns, we can determine the wire size because the length, divided by the number of turns, equals the wire diameter. Inductor design is actually quite an involved topic for a future Classroom, which will be full of maths and very dry theory, but the reality is that for crystal radios, it matters little. Historically, many home-made ones were built with whatever wire was available and worked fine. We tried it with 0.5mm copper wire and it worked too.

Now, it was time to build. Firstly, it was necessary to figure out how much wire we needed and cut some off the roll with enough excess for connections. Using the formula C = 2πr where C is the circumference of the circle, and r is the radius, we can work out that each turn is approximately 120mm of wire. We have twenty turns, so 20 x 120 = 2400mm of wire, plus at least 200mm for connections. We need a 2.44m length of wire, so we'll say 2.5m. The same method is used to find our longer length for the 110 turns, but at 13.5m, this is easier handled while on the roll.

We drilled a small hole in the cardboard, into which we can pass our wire to secure it. We left enough sticking out the end to make connections with. Then, we wrapped twenty turns side by side. At the end of twenty turns, the wire was held firm while a small hole was drilled, and the free end passed through and out the end of the tube. The wire end closest to the open end of the tube is the ground wire, so we coloured it green with a permanent marker. The other was the antenna wire and was coloured red. This colour scheme came from the commercially-produced AM coil. Tape or glue helps keep the wires secure and tightly wrapped.

We pushed the end of the roll of wire through the same hole that the end of the small coil is sitting in, and slid it far enough out of the tube that we could easily make connections with it later. This was the only uncoloured wire of the three and therefore self-identifying. We wound on thirty turns of this new coil, before forming a loop with a tight twist at the base. This loop would be twisted into one form later but for now, we just kept turning for another eighty turns. If you don't like keeping count or just lose track, the fact that the wire is 0.8mm helps estimate the turns if they are tight enough. Thirty turns is 24mm, while eighty is 64mm. Of course, these figures ignore the small gaps that even a tightly wound coil has, and so are a guide. Finally, another hole is drilled at the far end and the wire passed into it and out the end of the tube.

With an inductor ready, it was time to build a mounting base. We used foam-core board to make a base big enough to fit the components on top, and deep enough to mount the binding posts. The sizes are entirely nominal and yours might be very different. We glued the coil onto the back of the board, and used a skewer and a knitting needle to put holes in it for wires to run and binding posts to mount.

All the binding posts were glued in with hot melt glue, after removing the fixing nut and washer from each. As this involved the base being upside down, we didn't realise some of the binding posts bent over slightly and when the glue set, they weren't straight. This is annoyingly obvious in the photos. The coil also misaligned a bit because we turned over the base before the hot melt had set properly. There are binding posts for attaching the capacitor, earpiece, antenna, and ground, plus one to link the ground of the resonant circuit with the antenna ground for experimentation.

With the coil wires poking through the baseboard for neatness, we can connect at will underneath, even if it's messy. We tried to keep it tidy, though. The enamelled copper wire had to be stripped at the ends after being cut to length, but this can be done with a soldering iron if it is at a high enough temperature and the ventilation in the room is good enough. We decided to label the binding posts with a paint maker because they are mirrored from the top view when upside down and that makes it easy to make a mistake. Some people do not cope well with mirrored views! Then, the relevant connections were made.

After that, the base was turned back over and a set of spring speaker terminals installed for attaching the diode. This was the easiest way we had available to be able to easily swap diodes in and out. We would have liked to use crocodile clips but could not find any with flat jaws. The speaker terminals are probably more reliable in the long term, too, as many crocodile clips flex over time at the axle/hinge pin and so become misaligned.

The base was turned back over and the final connections made. This included some hookup wire, which we glued down to keep it from getting caught on anything when the base is moved around. The ends of the longer coil went to the capacitor terminals. The wire from the eighty turn end of the coil was connected to the capacitor post, headphone post, and the ground terminal located over near the antenna ground. This was the longest bit of hookup wire. The wire from the thirty turn end of the coil went to the other capacitor terminal and nowhere else. The twisted centre tap wire went to the red terminal of the speaker connector we're using for the diode, and the black terminal of that connector was wired to the other earpiece binding post.

With that done, it was now time to wire external components. The capacitor is not polarised and so either wire could go to either binding post. The earpiece is likewise unpolarised and was connected to the two front binding posts. The diode was placed so that the striped cathode end was in the black terminal and the other end in the red terminal. That just left the antenna connections. The ground was easy, it's just a couple of metres of hookup wire with a banana plug and crocodile clip on one end. This enables it to plug into workbench grounding systems, or clip onto a grounded metal object, or to a nail or metal skewer if manually grounding it.

The antenna was just a 15m long piece of wire in previous designs but this time, the radio would benefit from improved antenna design. 15m is still about right, but the total wire will need to be longer.

The 15m section needs polymer string tied at each end so it can be suspended away from conducting objects and those with water in them. Trees work great but make the string long enough to get the wire out of and away from leaves.

The length of the rest of the wire is determined by the distance to the radio. It can hang vertically and some can go horizontal but keep it off the ground.

In our case, we had nowhere near the office that would suit, so we had to get creative. Our antenna was made with a 15m section of medium-duty hookup wire with small loops of poly string tied to the ends, and glued on. Then, it had a binding post and banana plug attached. This enabled us to use custom lengths of hookup wire between the antenna and radio depending on where we placed both. We used medium duty wire for the antenna because, while light duty wire works, it isn't great at supporting its own weight when strung out like this.

We coiled the antenna and several different lengths of connecting wires and tied them with hook and loop cable ties. This makes things easy to transport, and to photograph

The higher off the ground your antenna is, the better. However, working from heights is dangerous and workplace safety rules in most states tend to kick in for heights above even one metre. A fall at the wrong angle from only a couple of ladder steps off the ground can ruin a spine. Be even more careful if climbing trees is your thing. We do not recommend that, it's an individual decision. We didn't have to deal with either because we had nowhere suitable.

We had to drive our radio somewhere because the concrete buildings of the industrial area we're in shield us from what little signal gets to our hilly region on the absolute fringe of Sydney's AM reception. Because of this, we used photographic lighting tripod stands to suspend our antenna, then extended the stands above head height without climbing. We had to do this on a hill in a national park so everything had to be portable and temporary. The photo is from our carpark because we test everything before we commit to a drive.

Of course, we wanted to test our radio before this, so we used the Wimshurst Machine and 9V battery spark tests.

Mini Build: Making with Direct-coupled Tapped Coil

Parts Required:Jaycar
13m (approx.) 1mm Enamelled Copper WireWW4022
1 x 1nF Greencap Capacitor*RG5010
1 x 100kΩ Resistor*RR0620
1 x Germanium Diode or Equivalent, see text Issue 67We used: Pedal Parts Australia SEMN0003
1 x Crystal EarpieceAS3305
15m approx. Meduim-Duty Hookup Wire (Antenna)WH3042
5m to 20m Light-Duty Hookup WireWH3012
2 x Banana PlugPP0400
1 x Crocodile Clip for Banana PlugPA3696
1 x Binding PostPT4055
1 x Electronics Learning Spring Terminal BaseboardKJ8504

The next design we're going with is a multi-tapped direct-coupled coil. This is a bit harder to manufacture than the single-coil type from last month but much more reliable. It uses a coil with multiple loops passed out, to which the antenna can be connected. With the single coil we had to sand a strip of enamel off the top and bottom of the coil and used less-than-reliable contacts to tap off. It's hard to do this without causing shorts between the coils. The wire is fine and the sandpaper flexible, meaning that too much enamel can easily be removed. In addition to that, the fine wire can roll a bit as it is sanded or filed side-to-side, the combination resulting in bare copper down the sides of each loop that touches the next.

This version is harder to physically make but will likely perform better. And, with our newly-made variable capacitor, we can get closer to traditional designs in terms of coil turn count and so on. Speaking of those, there are very few 'original' designs of crystal radio left to come up with. With the amount of experimentation that has been done over the

last hundred years, we could not come up with any variation or combination that hadn't been tried, except for some that simply didn't work because they were not resonant or selective enough or so on. In fact, those have probably been tried, too, but no one publishes them if they don't work!

This design uses a coil with multiple taps to which the antenna can be connected to, to change the selectivity of the coil and gain a better signal pickup. That's the theory, anyway. This one functions by allowing the resonance of the antenna circuit to change based on the coil tap chosen. This alters both how much of the coil is resonating with the parasitic capacitance of antenna and ground, and the overall relationship of the antenna resonant circuit to the tuning resonant circuit. As an aside, this one also has a filter at the output to improve earpiece performance in some types of earpiece. The filter can be added to any of the previous designs, too.

The coil for this design is wound on a 60mm former. We used a cardboard mailing tube we had, but PVC pipe would be great, too. The coil has a total length of sixty-five turns. It is tapped at six, eighteen, and thirty turns, with the end of the coil at the sixty-five turn mark going to the capacitor. Each tap serves as both an antenna connection point, and the take-off for the diode. That means you will have some adjusting to do, but that also means you can tailor it for conditions in your area.

We started by drilling a hole through the tube to begin our windings, and taping the wire in place inside the tube. Then, we wound on the first six turns. Twisting the wires to make a loop without anything moving is hard, so we glued our sixth turn with hot-melt glue, then twisted the loop.

We continued on that track to make the eighteenth and thirtieth turns. Some coils from similar designs use different counts, some use more counts, and some use a longer coil. So, if you're reading up on crystal radios you may have seen similar designs that use a different turn count. It really isn't critical, but this design is based on a popular commercial kit from the 1950s, so we went with it.

After the coil was finished, we mounted it to a piece of foam-core board to keep it safe and make handling it easier. However, this design is not based on binding posts or a foam-core base. It could be built on a regular prototyping breadboard but we are utilising the base-board from Jaycar's Short Circuits Series One, which is a spring-terminal device. The base-boards from similar electronics learning kits could stand in. We placed the springs so that the wires from the ground, each tap, and the end, had their own spring. The ends of the loops were soldered and cut to put in the spring terminals.

With that complete, the capacitor was connected across the ends of the coil. The ground connection in this circuit is common, and so has a few connections to it. You may find for reliable connection that you have to build a bus of spring clips, using a bare piece of wire to link them. We tried to use the end of the enamelled copper wire but it was too hard to reliably and thoroughly take off all the enamel over the required length, so we went with a scrap of tinned copper wire.

Following this, the diode, filter capacitor and resistor, and earphone were connected. We used a pin-to-pin jumper wire to connect the diode to the taps so the diode itself remains relatively immobile. They are reasonably strong in their glass case but the less manipulation of the legs, the better.

Finally, the antenna and ground connection were added. These are the same as the last build, but the antenna can be like those from last month's build as well, or an even more elaborate design. The antenna was connected to the six-turn tap, as detailed below, and the ground to the beginning of the coil.


Start with your antenna on the lowest-impedance tap: That is, the least turns. Start with the diode on the highest-impedance tap, or even the end of the coil. Then, tune the capacitor (or capacitors, see 'Performance' section) and see if you pick anything up. Then, after the full range of the capacitor is used, reset it to the beginning, move the guide to the next tap down, and try again. Only after the diode has made it to the six-turn tap without decent results, should you move the antenna to the next (eighteen turn) tap, and repeat. In general, the longer the antenna and stronger the signal it brings in, the less impedance there is needed in the coil. So, a longer antenna will work on lower-turn taps, while a shorter antenna will stand a chance on the higher-turn taps.

Mini Build: FM Version

Parts Required:Jaycar
1m (approx.) 1mm Enamelled Copper WireWW4022
7 x Plug-toPlug Jumper WiresWC6024
Scrap of copper or tinned copper wire-
1 x Solderless BreadboardPB8820
1 x 18pF Ceramic CapacitorRC5315
1 x 150kΩ ResistorRR0624
1 x Germanium Diode or Equivalent,We used: Pedal Parts Australia SEMN0003
see text Issue 67RV5728
1 x Tuning CapacitorPS0105
1 x 3.5mm Mono Headphone SocketAS3305
1 x Crystal Earpiece-

It was long said and still commonly is claimed that crystal radios cannot receive FM. However, some designs do exist. We decided to build one to find out. All are very similar, at least the few that we could find. The lack of designs stems from the fact that by the time FM radio was becoming dominant, portable household radios had become transistor-based, smaller, and affordable for the average person. The batteries they run on had also become an everyday thing by that time. Added to that, is the fact that many of the AM designs that are more sophisticated are developed by enthusiasts, many of whom stick to AM radio as a preference. After all, it all started with AM.

The design looks similar at first glance, particularly from the schematic. However, the resonant frequencies are much higher and therefore, the component values change. This means a much shorter antenna, smaller coil in terms of diameter and number of turns.

We're also going back to the commercially-made tuner capacitor. Because the value works and it's easier to adjust. This one is built on a breadboard, because there are a few more components and the coil is smaller and free-standing. The only connection that comes off the board is the ground, as the antenna is a straight and much shorter piece of copper wire sticking out of the breadboard.

We started by winding the coil. This is five turns of 1mm copper wire, wrapped around a 12mm mandrel, and tapped at 2.5 turns. Some sources say four turns, but we got better performance with five (we made both).

Your experience may differ. The tap was soldered on, because the turns are spaced apart after winding, so the twist deforms the turns too much. A jumper wire was soldered onto the ground end, too, as well as two on the antenna end. One is to connect the antenna, the other to connect to the circuit.

Next, the antenna was cut to 180mm and straightened, and the jumper wire from the coil soldered onto the bottom. The thick wire will damage the breadboard, so we used Blu Tack to mount it on top later. Wires were also soldered onto the tuning capacitor. Note carefully in the picture which capacitor connections are used: When viewed from above, the left-hand one is not needed. We also soldered wires onto a 3.5mm mono headphones socket, to plug the earpiece into.

With that, the parts were inserted into the breadboard, and the jumper from the antenna soldered onto the coil. The ground connection was made with the same system as we used on the previous designs, but with a jumper lead on the circuit end so that it fits into the breadboard. The breadboard itself was Blu Tacked onto a slice of foam-core, as was the tuning capacitor so it can be turned easily, and the headphone socket so it can be used without too much handling. That completed the FM radio build.


Performance of any of these radios is hard to gauge because of our location. All perform well when receiving the signal from a spark, but that's almost a given. The tapped coil with the variable capacitor gave us the strongest results but at the other end of the scale, the FM one was the hardest to get right. At the higher frequencies involved, it takes some finesse to get the coil right. We're in a bit of a black spot because all the local FM transmitters are up on the hinterland mountains and we are underneath those with a foothill in the way. However, we got results on all of them when we drove up to a coastal national park and found a hill that faced Sydney, the FM version still being the most troublesome. It will be worth the fiddling if you have the patience, because there are good examples of people getting this to work well in the right area.

If you're using a very long antenna, you may actually pull in enough signal to damage the diode. In that case, a 10kΩ resistor inline is enough to limit the current. These crystal diodes are far more fragile and have far lower current limits that general purpose silicon diodes do. This is one of the reasons, besides trying different models, that we made our designs with easily swappable diode connections.

If you do not get a listenable signal with your crystal radio first up, it probably just means you're in for some experimentation. On the multi-tapped coil design, you can try any of the taps to connect the antenna but you may also need to vary where the taps are and how many turns there are in each section of the coil. That's the advantage the design last month with the continuous coil with sanded strips gave us: Infinite variability. However, it was fussy. We need a much better slider system.

The other main point of variability is the capacitor. You might like to add a commercially-made variable capacitor parallel with your home-made one to add some finer adjustability. Capacitors in parallel add up, so adding the 30pF to 160pF tuning capacitor gives greater control. Set the tuning capacitor to its midpoint, then adjust the home-made one, then use the tuning capacitor to find the exact point you need. Further, you may find you need to reduce the amount of foil used in the plates of the capacitor, to reduce its overall capacitance and therefore create a better match to the coil. While you could shorten the tubes, we found it was better to reduce how far the foil wrapped around. This meant we had the full length for adjustment, but still less foil and therefore less capacitance.