Two different circuits to make your own crystal radio without batteries.
We were going to bring you part 2 of the circuit protection series this month, but we got side-tracked. What started as a little reminiscing turned into a full research project on crystal radios. We didn't realise how many variations there are, and even some that pick up FM! These staples of the hobby electronics world for decades deserve a revival, and they may surprise you.
Crystal radios were the first mainstream, household-accessible radio receiver ever invented. Everything before that had been either a piece of scientific equipment, or it had been large, expensive, and power-consuming, making it suitable only for commercial applications like radio telegraphs.
Batteries for power were nothing like we know today, either, and so were not a practical power source. The 'crystal set' changed all that.
While they became obsolete as a commercially-made product in the 1920s, over one hundred years ago now, they lived on because commercially-made radios were still expensive, bulky, and used large, expensive batteries.
Working class families continued to build crystal sets for decades. They became a favourite of hobbyists, and of teenagers wanting to listen to music that their parents didn't like through to the late 80s. They remain a popular hobbyist circuit today within analog electronics enthusiasts but have fallen from prominence among the rest of the maker community.
We are going to cover several versions of the crystal receiver. Most are variations of the classic design which receive AM signals, while some receive FM signals. These are harder to get right and will not work in many areas, but will get a wider range of programming if you can get it to work and live close enough to an FM transmitter. That's why we are building both: So you will have at least one working.
There is still strong debate over FM and AM radio and which is better. The reality is, there is no right or wrong answer because both have advantages and disadvantages. Which one is 'better' depends on which of the advantages and disadvantages matter or are relevant to you. This is another reason we're building both types.
Unfortunately, many of the advantages of FM do not apply to a crystal receiver anyway, but there are still a couple of good reasons left to build one. We'll cover all of that in the relevant sections.
WHAT IS RADIO?
Radio waves are not magic, it's just that most of the goings-on are invisible. Radio waves are electromagnetic radiation, a form of energy that has properties of both waves and particles and moves with both electrical and magnetic components. As such, scientific debate has raged for a long time about exactly how to characterise them, mainly regarding the wave/particle issue.
We use the term 'radio waves' but really, there is far more to it than waves. Having said that, treating it as a wave does not really change anything for any of us who are not working formally in physics! The really important points are that there is a magnetic part, and an electrical part at 90° to that, and both have the same amplitude (height on a graph for now) at any given point.
All electromagnetic radiation is on a spectrum, based on wavelength and frequency. Wavelength is the distance between two identical points on a wave, and is measured in some multiple of the Metre. Frequency is the number of full wave cycles per second, measured in Hertz.
The electromagnetic spectrum ranges from as low as 3Hz, which are Extremely Low Frequency (ELF) radio waves, to 300 EHz (exaHertz, which is 300,000,000,000,000,000,000 Hz) for Gamma rays.
Wavelengths are from 100Mm (yes, M for Megametres, or one million metres or one thousand kilometres) for 3Hz, to 1pm (picoMetre, one trillionth of a metre) for gamma rays. So, the higher the frequency, the shorter the wavelength.
This spectrum includes visible light, infrared and ultraviolet radiation, and x-rays, which are all things not considered radio waves. Radio waves themselves are often quoted as being between 20kHz, the upper limit of audio; and 300GHz, the lower limit of Far Infrared and upper limit of microwaves. Sources vary with the definitions and limits, and some navies have built ELF radio transmitters to communicate with deeply submerged submarines. This is uncommon, however, and generally, the upper limit of audio frequency is a good place to start talking about radio waves.
These waves are used to carry information. There are different ways of doing this, from just turning on and off pulses of radio waves, as in the early radiotelegraph days, to varying the amplitude of the wave to carry information. Also, a carrier wave can be varied in frequency around a centre point. There are other versions, too, but the point is the same: Some form of modifying the basic, continuous wave gives the ability to transmit information. We haven't discussed amplitude yet, but this is just the height of the wave. We measure the amplitude of ocean waves, for example, in metres. Anyone interested in surfing, boating, water sports, or the weather, in general, is likely familiar with hearing about the height of the waves on a regular basis.
Amplitude in radio waves is not usually measured in multiple metres. It is generally measured in Watts, a measure of electrical power combining volts and amps, but it can be in deciBels, a unit that relates one quantity to another rather than being a fixed value. There are other units, too, depending on the application and exactly what is being reported. Note that as the signal radiates through space and objects, it weakens. What does not change is the relationship between high and low: The wave looks the same shape, but with a reduced vertical axis when plotted on a graph.
There is peak amplitude, which measures from the zero centre to the top (or bottom, they should be equal) of the wave; peak-to-peak amplitude, which measures from the top of the positive peak to the bottom of the negative peak; Average amplitude, which is the mathematical average of the sine wave as a straight-out summing of all the data points divided by the number of points; and RMS, or Root-Mean-Square, a mathematical average what is far more accurate than plain average values but harder for mere mortals to calculate. RMS is used all the time to compare AC voltage to the DC voltage that would do the same amount of work, but is less useful for radio. Plain average is virtually useless. In radio, generally peak and peak-to-peak amplitudes are used.
Radio waves work because they are electromagnetic, and so have both a magnetic part and an electrical part like in the diagram above. This means they can move through air and even through a vacuum. They can penetrate some materials like water and wood, though metal and water defeat many frequencies except the lowest. At the transmitting end, an electrical signal is generated at the frequency desired, and then sent to an amplifier. The amplifier sends power into the wire. Whenever a current flows in a conductor, a magnetic
field is generated, too. That magnetic field radiates. However, as we saw above, it has an electrical component, too, so it propagates through the air. When that wave meets another conductor, the magnetic component induces a current to flow in that conductor, which can be connected to a radio receiver.
Have we not always been told that current can only flow in a circuit? Yes, that's still true. However, the wire is never just a wire. There is always something else going on. Generally, the wire is one half of a capacitor. The other half is either the ground, or a ground plane (a big piece of metal nearby and connected to the circuit), and the air between them is the dielectric insulator.
So, current flows because charges are moving between the plates of a capacitor. Just like a regular capacitor, if the antenna was fed DC, the charge would build then stay static or stationary, so no more magnetic field would be generated.
There are other antenna designs, too. A dipole antenna, for example, has two equal halves, one of which acts like a ground plane (but in a more matched way). That provides the capacitance for the antenna to work. In a car, for example, the metal body is the ground plane.
Please note that this explanation is rather simplistic, and we may run a Fundamentals on radio transmission or even a classroom if the depth is required, if there is enough interest in radio transmission. However, today our focus is receiving radio signals, so we only need a cursory understanding of how they're transmitted.
AM VS FM
Now we finally have enough basic radio concepts laid down, we can talk about the radio waves that most of us are familiar with, and what makes them different: AM and FM broadcast radio. AM stands for Amplitude Modulation, meaning that the amplitude of the carrier frequency changes to give the information we want to communicate. The frequency stays the same. In public broadcast radio, that information is audio information, and is often speech or music but can be anything else audible: A series of beeps for a time signal, for example. Before the internet and GPS-based time, this was the most accurate way most people had to set a clock or watch by: Just tune into the frequency used for your region, and wait.
This works because the frequency of the AM radio band is so much faster than any audio we want to transmit. There is little need to transmit anything above 15kHz because that's about as high as most 20+ year olds can even hear, and there are few sounds needed in music or speech broadcasts that pass that. The AM radio band is between 530kHz and 1700kHz. Even at 15kHz, on the lowest frequency band, there would be 35 full radio wave cycles for one audio cycle. However, it gets more complicated because sound is rarely one frequency, it's many. So, there reaches a point where there are not enough radio waves to carry all the information we want.
FM radio is different. There is a centre frequency, and the frequency is varied either side of this to encode the information we want to transmit. Throughout most of the world, the FM broadcast band for public use is 87.5MHz to 108MHz. The difference between the nominal centre frequency and the actual received frequency is the amplitude of the information signal. Because the information usually contains lots of frequencies, like music or even human speech with its rich harmonics, the higher frequencies of the FM band are necessary to allow all the information to be transmitted quickly with lots of waves in a given time. Again, this is drastically simplified but it makes the necessary point for this article's purpose.
There has long been a debate about AM versus FM and which is better. It would be stereotyping to say that old people think AM is better and younger people think FM is better, but statistically, that's not too far from the truth. It's human nature to like what we're used to and find that alternatives are just not as appealing. However, there is far more to it.
On a data level, FM stations take 80kHz of the band each, with some separation between them. Most FM radio increments in most countries are 100kHz apart, with licences being spread so that in any given area, stations are at least 500kHz apart. Am stations only take 30kHz for each station's bandwidth. All radio signals are vulnerable to interference. Other sources can add to or take away from the amplitude of a radio signal. This means that Am radio is far more susceptible to most types of interference. With an AM radio, getting a petrol engine too close results in the sparks being audible on the radio. On cars, this was accounted for by engineering, but if you get a lawn mower too close to the AM radio on the back veranda table, you'll hear the sparks. This seriously affects listening quality. Far fewer sources affect frequency, so FM is immune to more types of interference. If you have an AM radio switched on during a lightning storm, you can hear the distant strikes. In fact, this is one of the easiest lightning detectors around, although you'll have to tune it to a vacant station and put up with the white noise from the static.
AM radios also take a lot more power to run, because physics. Yes, that's the cop-out explanation, but we simplified the radio transmission explanation above and some of what we dropped is necessary to understand why AM transmission occurs at much higher power. That means more electricity to run them, too. On the flip side, AM radio travels further. With any radio wave, the higher the frequency, the more information that can be carried but the shorter the range of travel and the more easily the signal is blocked. So, AM signals can cover vast distances and avoid obstructions better but carry less information less clearly. Higher frequencies also reflect or get blocked more easily, so FM radio can sometimes be patchy, and harder to receive if indoors.
AM radio can also be easily affected by atmospheric conditions, meaning that a signal that was strong can become weak or even wander a little from frequency quite easily. This is not something that higher-frequency FM signals are susceptible to. The bandwidth of FM transmission is higher, too, meaning more information can be carried and therefore a higher-quality sound. Stereo transmission is much easier with FM radio, too, although it is possible with AM, just with much less bandwidth and therefore quality.
For these reasons, and because of market and audience preference, AM stations are often used for talk stations and news-only programs. They are also the preferred emergency broadcaster. Even when internet, electricity, and mobile coverage are all down in a major disaster, one AM station with backup generators and a transmitter can reach battery-powered radios across a vast area of the country.
RECEIVING A RADIO SIGNAL
So, we know radio waves fly through the air. How do you make use of them? A length of conductor is key here. Any such conductor which is in the path of the electromagnetic radio wave will have a current induced in it by the magnetic component of the radio wave, and this current moves back and forth as the wave moves up and down. Capacitance is needed for this to work, and that can be via a ground plane or the ground itself, depending on design. There is a small current now flowing in the conductor which corresponds to whatever is happening in the radio wave.
For many types of radio, the length of the antenna matters. The antenna forms part of a tuned circuit that resonates to boost the signal. For others, including Am, the longer the antenna the better. There is still some tuning needed to get resonance but for many AM radios, a long length of wire out the window gets better reception, and the length is not critical. In fact, for crystal radios, which have no power source besides the energy from the radio waves received, the longer the better because the longer wire receives more signal and therefore produces more current.
Most antennas feed into a resonant circuit. A resonant circuit is formed by components that allow a current to flow from one side to another. This is usually an inductor and a capacitor. The capacitor may be a capacitor component, or one formed by a conductor and another element like the ground or a ground plane. This is called parasitic capacitance. For crystal radios exploiting the antenna's capacitance, the antenna is generally kept shorter than a quarter wavelength. The inductor is a coil of wire, and as a current flows through it, a magnetic field builds up. That magnetic field creates a current that opposes the one that created it, so eventually, an inductor slows down the current creating the magnetic field at an equilibrium point. If the
current stops, the magnetic field collapses and creates a current that was opposite the original one, flowing back the other way.
A resonant circuit only does this at a certain frequency. At this frequency, any energy that comes in adds to what is there, like small pushes of a swing at just the right time. This amplifies the signal and makes it much more useful. An ideal resonant circuit is shown here as just a capacitor and an inductor. While the current flows from the charged capacitor to the inductor, the inductor's magnetic field builds. If the values are chosen correctly, the capacitor has discharged just as the inductor is reaching its full charge, and current slows down and then stops. Now, the magnetic field collapses and shoots the current back to the capacitor. Of course, in real life there are losses so this process is not infinite and requires a little input to keep it going and a little more to make it bigger. That's where an antenna can help.
In a radio, the tuned circuit is adjustable in some way so that its resonant frequency can be changed. This way, the desired frequency can be amplified and all others rejected. Often, this is by way of a variable capacitor, but sometimes a variable inductor is used. Many common DIY inductors are air cored but ferrite, an iron oxide compound with good magnetic properties, is common in commercial inductors depending on the properties desired. There are air-cored commercial ones too. Moving a ferrite mass in and out of a coil varies its inductance.
After the resonator has selected the desired frequency, it needs to be processed, something known as 'demodulation'. This is not just a straight rectifier, cutting the negative half of the ave off. It is a 'square law detector'. The square law in electronics means that the output of a device is related to the square of the input amplitude. This is absolutely not free energy. Rather, it comes from the ability of the diode to convert Ac into DC. We're not going to try to deal with the maths in a Fundamentals article, but here it is for 'fun' and curiosity. You can read up on it yourself but really, knowing will not change anything for our purposes.
Some devices do this very well, and a contact between certain metals and certain minerals do it particularly well. That's where the crystal radio gets its name. Original crystal diodes were made by having a small chunk of the correct crystalline mineral, and touching a thin wire known as a cat's whisker to the surface until the correct place was found. Early crystal radios and some of the improvised 'foxhole' radios made during the Second World War were made this way. In fact, some commercially-produced crystal radios even had a bracket and arm system for adjusting the position of the cat's whisker on the crystal.
Thankfully, things quickly changed and soon, factory-manufactured crystals were available. They had the whisker and crystal inside a case, usually a glass package, with the whisker already adjusted. Terminals or leads on the outside allowed electrical connection. These were the forerunners of modern diodes, but the silicon diode was also produced much earlier than many people realise. However, it was far more expensive as a detector, and though it was first sold by Wireless Specialty Products Co in 1907, it was for commercial and military radio. The humble crystal
detector continued to be the choice for household and hobby use for many years to come.
The first modern diodes were based on boron-doped silicon with a tungsten whisker in a sealed glass case, with no need for adjustment, and a similar device using germanium mineral. These were originally for microwave radar in the military field, but germanium was cheap to produce compared to early fused silicon, and so germanium diodes took over from cat's whisker detectors from the late 1940s for household use.
The detector is the element that makes the radio usable. Out of the other side of the detector, the radio frequency is gone and the audio modulation that was carried on it is left over. Finally, the signal is sent to headphones. Sensitive, high-impedance headphones are needed, meaning that only a low current is needed to drive them. Most modern listening headphones for music are low-impedance, so be careful. Many designs use a 'crystal' earpiece, which is a different crystal to the detector and only coincidentally named when used with a crystal radio. In fact, crystal earphones are piezo devices, named for the quartz crystal of the piezo element. These take very little current indeed.
WAVELENGTH AND ANTENNA DESIGN
In receivers where the antenna is part of, not just feeding into, the tuned circuit, wavelength matters. The antenna needs to be a whole wavelength or some quotient of the wavelength divided by a factor of two: quarter and half wavelength antennas are common, and eight wavelength antennas are used on many WiFi devices for compactness. All of these are divisions of two. A one third wavelength antenna, for example, is unlikely to work. A one sixth antenna doesn't work either. Two goes into six, yes, but you cannot divide one by two and end up with six. If you divide one by two, you get a half. Divide the result by two, you get a quarter. Divide that again by two, you get an eighth. This is the pattern we need: Factors of two, not just a number that two goes into.
While straight wires work, called a monopole, many antenna designs incorporate a counter-element to act as the other half of the capacitor. These do not have to be even. A dipole antenna features two equal parts, but some designs feature uneven elements. They have to be balanced, but the relationship rather than the size makes them matched. The J-pole used in shortwave and some UHF radio is an example. The depths of this is another topic for another time, but be aware of it as we move forward and if you do your own reading on different radio reception.
Antennas tend to work best away from the ground. The closer to the ground, the more surface clutter there is. That's the term for a whole bunch of reflected radio waves and also ground-carried currents which induce noise. Be careful of three things, however, when putting an antenna up high. The first is safety on two fronts. Be careful working at heights, even from a two-step ladder. Make sure any ladder you are using is on a firm, level base and that any surface you climb or walk on is not slippery or two steep. The second front is the safety of others: Do not string the wire at neck height between two trees!
The next thing to be careful of is lightning. Any
conductor outside could be struck by lightning. There is no rule, either.
While lightning usually strikes the tallest thing around, it does not always. It takes the path of least resistance, and sometimes, that's through a bit more air to a better conductor than a taller object may be. You can get dedicated lightning arrestors for all sorts of antenna situations.
The third is reflections. Like the ground, certain surfaces such as metal roof sheeting reflect radio waves and induce clutter. Even on a tiled roof, the foil sarking insulation under it, if present, may be close enough to cause issues.
Calculating the wavelength of a radio wave is relatively basic maths compared to some of the things we avoided describing in this article. Wavelength is represented mathematically by the Greek letter Lambda, or λ. The formula is the Universal Constant, C, divided by the frequency in Hertz. The Universal Constant is the speed of light, currently defined as 299,792,458 m/s. So:
λ = C ÷ f.
For example, to find the wavelength of Sydney's ABC talk radio on 702kHz, the formula is:
299,792,458 ÷ 702,000 = 427.055m. That's quite useful for calculating half- and quarter-wavelength antennas!
THE BASIC CRYSTAL DESIGN
The schematic shown is for the simplest possible crystal radio design. The inductor and capacitor form the resonant circuit, with the capacitor or the inductor being an adjustable type. There is a diode shown, and this is the detector. Most modern crystal radios use a germanium diode as the detector, although some silicon diodes work and there are sometimes Galena detectors around, too. If you live near enough to an AM transmitter, this circuit will work. So, let's build it! Well, soon.
Sourcing a diode for a crystal radio is getting harder and harder. Few germanium diodes are left on the market. Some silicon diodes work well, too, but not many. The challenge is the forward voltage drop and lack of sensitivity involved in silicon diodes. On the retail market, Jaycar has the BAT46 or BAT48 silicon diode, ZR1141, which is a Schottky type with a very low voltage drop and high sensitivity. It's a metal to silicon diode, which is important.
The voltage drop is not the only factor in whether a silicon diode will work, it must be a point-contact and not a PN junction formed from two semiconductors or even doped onto a single wafer. The BAT46 is often considered a replacement for germanium diodes, and is built using similar technology to those older silicon microwave diodes we mentioned, but at a smaller scale.
Altronics still sells a genuine Germanium diode, the 1N60P, catalogue number Z0052. We weren't able to get any of these in time but being real germanium, they would be worth experimenting with. Instead, we were able to source some from Element14, the 1N270 germanium diode. This was one of few germaniums around and was the only one Element14 had Australian stock of for overnight delivery. It is also specifically called out as an AM, FM, and TV detector element in its documentation. Part number is 186294201.
If you do have time, and really get hooked on crystal radio, then look around for new old stock of germanium or even Galena devices from the past. New old stock means unused stock that has been stored a long time, and is therefore an old item. This is typically for devices that have been out of production for a while and is used to differentiate the stock from that which has been recycled or recovered in some way. New old stock should be unused, but may need cleaning due to surface oxidation.
Try to look for stock with a glass package, which shows the point contact. Be aware, however, that the photo may not be of the actual stock. This even happened with our 1N270s! The photo is of a glass case, the stock was plastic and opaque. We found quite a few options around the world and even some here in Australia, but again, time beat us. Australia Post seems to be still recovering from Christmas and significant rates of sick leave among the workforce, which we all hear about on the news.
Build 1: The Handmade Coil Version
|0.5mm Enamelled Copper Wire||WH4016||W0405|
|Diode of choice, see text.||ZR1141||Z0052|
|Choice of mandrel/former, see text.||NA||NA|
|Copper Tape, with Conductive Adhesive||-||T2980A|
|Hookup Wire for Antenna and connections, see text||WH3010||W2250|
|Grounding Wire Termination, see text.||-||-|
Finally, let's build. We're changing the design a bit, to first try a radio that does not use a capacitor. Instead, it relies on a long wire antenna, and the capacitance between that and a variable coil. There is a ground too, which needs to be earthed but our work station is set up with antistatic grounding points so that should be ok. If yours is not, please don't shove things into a power point!! Use a dedicated ground, or a water pipe (bear in mind much modern plumbing is plastic, even if the bit you can see is copper), or take it outside and shove the wire into the ground. Alternatively, use a mains plug and wire the ground to the earth pin. This is not recommended, however. A better bet would be plugging in something with a metal case, and
using that to earth to (Australian law requires the case to be earthed) or, if you have a lab power supply, oscilloscope, or other test equipment, use the earth point on that.
The 1N270 diodes we received were sadly not in a glass case. It should be noted before we build that the geographical location of the DIYODE office, along with the building, are both unfavourable to AM reception.
In fact, FM reception for the local stations is not great either, and there are no local AM stations left. So, to test, a broad cross-spectrum transmission is needed to make sure the receiver is picking up at least something! Enter the SparKit Wimshurst machine! The sparks generated from this should be easily heard on any functioning radio detector.
However, not many people have one of these, so you can use a 9V battery and a plug-to-plug jumper wire. Jam, tape, or otherwise attach one end of the wire to one terminal, and quickly touch the other end to the other terminal. This is a dead short so make it fast, but in that moment, a tiny spark is generated.
If you place this near a functioning receiver, you should hear it. Better still, use a battery snap with fly leads. This will be more reliable. To make sure you are getting a spark, use a known functioning radio to test it, tuned to an empty part of the AM band, like we have here. We found the range was around half a metre with the well-built commercial radio, so we know it will need to be right next to the home-made one.
The first challenge was winding a coil. Coil design varies, and diameters of other builders range from 25mm to about 50mm. We 3D printed a 35mm former for ours, but you don't have to. PVC pipe, cardboard tubes from rolled paper products (toilet rolls, paper towel, wrapping paper), or anything non-metallic like that will work.
We just had nothing like that in the office and it was quicker to print than go to the shops! We wound about 100 turns of 0.5mm wire onto the former. We wrapped it in paper first so the lines of the 3D printed layers didn't make it hard to wind the coils neatly.
With the ends taped down, we then filed a line of insulation off the top of the coil, then the bottom. The lines are opposite each other and only scrape the insulation off the very top of each turn. It is important not to go too far and allow the turns to touch, just sand the very top. We made a line with marker both to help us stay straight, and so you can see where we mean because the enamel is clear and not coloured.
Next, the coil was hot-melt glued onto two end supports made from foam-cored board, and then glued down to a baseboard. We used foam-core throughout but used black for the base for contrast so you can see the pieces more easily. The clearance of the bottom of the coil to the base is determined by the next step.
We need to have a contact on either side of the coil. One side changes where the antenna attaches, and so changes the tuning. The underside changes where the coil attaches to the detector and affects volume but also affects tuning because there is a relationship between the two. We have put the less-commonly adjusted connection under the coil, which is to the detector.
The antenna connection goes on top. First, we cut two slices of foam-core, about 150mm long. Next, we taped copper conductive tape to the underside and wrapped a bit around one end. One was fixed down to the baseboard so the copper was in contact with the underside of the coil. A wire was taped to the end, with the wrap-around copper, and a thumb tack used as a pivot.
The same was repeated with the top of the coil, but this one had to have a bracket made from foam-core to elevate it. Also, we glued a fishing sinker on top of the arm for downward pressure, as there is too much flex in the foam-core itself when it's this thin. There was a thumb tack pushed into this one, too, and a wire attached.
Now, the remaining wiring can take place. The detector diode needs to be attached. We chose to wrap the legs of ours around some thumb tacks and push them into the foam-core, fearing the heat of the hot-melt might be bad for the point contact. Incidentally, this makes changing diodes easy for experimentation.
The anode, the unmarked end, goes to the coil while the striped cathode end goes to the earpiece. Then, the wire from the underside of the coil can be wrapped around the thumb tack of the anode of the diode. The other end goes to the earpiece. We could not get the right one in time, again because of stock issues and freight delays, so we instead used a piezo transducer.
This is still a low-current device and should work when held near the ear. Note that it's a transducer, not a buzzer. It is just the piezo element, whereas buzzers in very similar packages have circuitry in them. The other side of the earpiece was wired to a thumb tack which is also common to one end of the coil.
Finally, the external wires were attached. The first was a ground wire and connected to the thumb tack for the earpiece and end of the coil. You can put what you need on the end of this.
We used a wire with a banana plug on it, so it can fit in our bench earthing setup, but also have a crocodile clip attached for clipping to a nail in the ground when outside. The remaining wire was the antenna. This is called a 'long wire' antenna and is just that. It's as long as you can practically make it. We chose a random length of wire from an older lightning detector experiment, length unknown. One end was attached to the thumb tack for the contact on the top of the coil. That's it!
To test the receiver, start with the bottom contact half-way up the coil, and move the top contact from one end to the other, slowly, listening carefully to the earpiece. At some point, you should at least hear something.
If you don't, check the circuit with the battery spark idea described earlier. This should also help you adjust the bottom contact for volume before settling tuning on the top. If you hear the spark but not any radio stations, then the signal is too weak. Try a longer antenna, better ground, or take it outside if you can.
There are lots of things you can change and experiment with in this design. The first is the length and diameter of the coil. Because the coil is tapped, you can make it as long as you want, but choose three or so diameters and make coils for each. Try substituting them into the design, modifying the mounting and top arm contact as you do so, to suit. Once you have one you like, you can also swap diodes in and out easily to see what effect this has on performance.
Build 2: The Ferrite Version
|AM Radio Coil||LF1020||-|
|Diode of Choice, see text.||ZR1141||Z0052|
|15 to 30m Hookup Wire||WH3010||W2250|
This version does involve a tuning capacitor, but we're cheating and using a pre-wound AM radio coil. These are available from electronics retailers and make life a lot easier. Performance may not be as good in some ways, because these really should be tuned with their antenna in a wavelength-based relationship and so do require a bit more fiddling to get the length right. They will also not pull in nearly as much signal without the long-wire antenna. However, they can be simpler to build and they can work with the long-wire, too, depending on location and signal strength. A half-wavelength antenna tends to have the best tuning, but a long-wire gives the strongest signal. Many similar designs use a wire between 15m to 30m if they are not using a wavelength-derived antenna.
This design makes use of a transformer coil, where one small coil is wound around a larger coil. We'll describe why this is further on. We like this design because it makes use of the 30pF to 160pF variable capacitors available at retail stores in Australia. Overseas, variable capacitors of up to 400pF are easy to get and those work with smaller hand-wound coils.
We started by mounting the coil to a baseboard. We used foam-core again. We found the ends of the smaller coil, which should be red and green. The red wire attached to the antenna. The order is important because of the way the coil is wound. If the current passes the wrong way through the coil, it will not induce the correct current in the larger coil. We soldered the antenna wire to the coil wire, then secured the wires with hot melt glue to avoid stress on the joint from movement.
Next, a ground wire was attached to the green wire of the small coil. The length of this will be determined by your situation but the same points apply as to the previous version from earlier. We fixed this down, too. Notice that the colours can be hard to see sometimes so we checked our coil in good light and used coloured permanent markers to make it clearer.
The variable capacitor was glued to the baseboard and the uncoloured wire soldered to the right-hand terminal of the variable capacitor. We also soldered on a diode, and you may have used a different one to us. The anode, or unmarked end goes to the capacitor and the cathode, or striped end, faces away. We wrapped the coil wire around a bit so that we can solder and desolder diodes at will, without having the coil wire pull away all the time.
Next, one wire of the earpiece and the remaining, black-coloured wire from the coil were attached to the middle terminal of the variable capacitor. These can be soldered together as they will not be removed. Finally, the other earpiece wire was soldered to the cathode, or striped end, of the diode. This completed the radio.
To use this one, all you need to do is rotate the variable capacitor. There is one point of tuning rather than two like the previous design. Life will be easier if you attach the plastic dial that should have come with the capacitor. You might be able to slide the ferrite core in and out to explore what this does to the reception, but some are glued in firmly.
NEXT MONTH: Tapped and transformer coils
That's all we have for this instalment. We'll get stuck into a discussion about tapped and transformer coils next month, because there is quite a bit to say about them. We'll also continue with more designs and even cover one that should receive FM signals.
There will also be an amplified version, too, but this breaks from the principles of a crystal radio and will crush the souls of purists. Part 2 of the circuit protection Fundamentals is coming, too, but this subject is a lot more fun!