Foxhole radio
From ZombieSquadWiki
Work in Progress
The traditional foxhole radio is made out of scavenged parts by some soldiers during World War II. This page documents that radio as well as improvements to help it work better. The construction of the radio is not very difficult and most people age 6 and up should be able to make one. This is not only fun project, but one that can dramatically increase your ability to get information when you need it most. This radio also takes no batteries for the basic model, instead it is powered entirely by the radio transmitter.
Contents |
Basic Radio
The basic radio is simply a diode, crystal earpiece (piezo electric), and an antenna. There are a few different ways to connect the parts, however I will only express one such design, one that is slightly more complex to make but yields better performance in that you can more easily tune the radio to a desired station.
- The antenna is the top left triangle looking thing.
- The coil is the spring looking thing
- Ground is the right triangle
- The headphones are the ear muff looking things
- The diode is the triangle with the line on the left side
Making the coil
Items required
- About 50 feet of wire
- bottle or other non-conducting form about 6 inches in length and about 2 inches in diameter
- pen, nail, stick to make taps
Where to scavenge parts from
- Wire
- You can use any wire, it must have a non-conductive insulation. Magnet wire is coated with an enamel and works, but its easier if you use slightly thicker wire (22-18 gauge) rather than really thin wire (30 gauge), but both will work. The thinner the wire the shorter your coil will be. Solid wire works better than stranded wire to keep the coil form, but both can be used.
- Bottle
- This can be any empty shampoo bottle, contact lens cleaner bottle, or even pvc pipe. The biggest thing is that you want it to be about 2 inches in diameter, if it is thinner you will have to make more turns than if its thicker. You want the bottle to allow for the wire to be wrapped snugly around it without compressing too much. Even cardboard tubes from toilet paper or paper towels can work, although they are not as weather proof and have a tendency to get crushed if you wind too tightly or travel with it where it gets bumped.
- Pen
- A disposable bic pen works well for making the taps, but you want something that is smooth enough to slide it out when you are done. A nail or smooth stick (like an arrow) works well in addition.
Wind the coil
Start with your form, I used a bottle, but you can use anything that is semi-rigid (the more rigid the easier it is to wrap the wire) and that is non-conductive. Drill/cut 4 holes into the form as shown in the image, this is where you will attach the wire to.
Stick one end of the wire through one of the holes and loop it around, leaving about 6-8 inches hanging off the end.
Make 5 loops around the bottle and then wrap around your pen to make the first tap for the antenna. Repeat this process until the form is filled with a coil. When you have reached the end, loop the wire through the other two holes you made to secure it.
Now you need to remove the insulation from the loops around the pen (called 'taps'). Do not remove any other insulation from the coil. If you have enamel coated wire you can remove this with some sand paper or possibly a solvent. If you use a solvent be careful not to get any on the bulk of the coil, just on the taps. If you have plastic coated wire a sharp knife will work or you can gently sand off the insulation.
Once you have removed the insulation you may optionally place tape or other adhesive such as hot glue or improvised glue made out of tree sap on the coil on either side of the pen. This can help prevent the coil from coming loose which is a bigger problem with fine wire than thick wire. Then carefully and slowly remove the pen by sliding it out. You may have to wiggle some of the taps to get the pen to come out easily. Make sure that you do not cross the wires of the coil when removing the pen.
Making the Antenna
- 50 feet of wire
- alligator clip (in a pinch you can use a clothes pin or anything else that will securely connect the antenna to the loops on the coil)
Where to scavenge parts from
Any wire is suitable for this, it does not have to be particularly thick wire, it can even be the same type of wire that you used for the coil in a previous step. If the antenna is going to have to support any tension, copper coated steel (copperweld) is best since it will not stretch, however anything is really suitable. Copper is better than steel due to lower losses in transmission.
The antenna is quite simple, you just need a length of wire with a clip on one end. String the wire up in a tree or other suitable location and connect the antenna to the taps on the coil you made in a previous step. The alligator clip should be small enough that you can connect it to one tap of the coil and not have it touch any others.
Making the diode
- oxidized metal (razor blade that has been exposed to heat)
- pencil or other resistive material
- safety pin or paper clip
- some wire to attach the pencil lead to the safety pin or paper clip
Where to scavenge parts from
This type of diode is called a "contact point diode" and you can make it with oxidized metal and graphite, fools gold (iron pyrite), carborundum (silicon carbide) and other minerals. The traditional way is with the lead of a pencil (graphite) and a blued razor blade. You blue the razor blade to force it to oxidize, this is done in a fireplace/camp fire or under the heat of a gas stove or blow torch. If you apply heat to a razor blade, you want to do so until it turns blueish. Any rusty metal should work fine though.
With graphite and a blued razor blade if you apply too much or too little pressure it will not work. As a result you may have to adjust the pressure for optimal performance. Carborundum works well with strong pressure, pyrite requires a gentle touch, you may want to probe it with the point of a safety pin to get the right spot.
You will know when you have a more optimal spot when the volume on the radio is at its loudest.
If you want to use a store bought diode, you can. A 1N34A germanium diode works better than a silicon one since it requires less voltage to operate. An LED can be used, if you do this you may want to add the battery listed in the Mods section below, and choosing a low voltage high output LED will work better (and it will glow in sync with the volume of the radio). You may be able to use a LED if there is a powerful radio station nearby that you want to listen to, for faint distant stations it is not recommended.
Make the diode
To make the diode obtain some type of springy metal, a safety pin works best, however a paper clip can provide some pressure. You will want to arrange the parts so that you can move the pencil lead to the best spot on the razor blade for the station that you want to pick up. By moving the pencil lead around on the razor you can adjust the radio to some degree, if you are not getting good performance try a different spot of oxidized (blued) metal on the razor.
- insert photo of contact point diode
Making the headset
- piezo-electric speaker (modern telephone handsets) or crystal earpiece
- make one with a coil around a nail, tin can and magnet
Grounding the radio
A good ground is essential for the radio to operate properly. The best grounds are where you have a lot of surface area in direct contact with the earth. Apartment dwellers may need to use a water pipe, however you should try to use an unpainted section of pipe for best results. In the field you can either drive a metal tent stake into the ground, or a series of stakes and chain them together in a line. A screwdriver with a wire firmly attached to the metal portion shoved into the ground may work well enough.
Mods
These are mods to the basic radio which can improve performance in one way or the other. They may decrease performance in some ways, as such they are completely optional.
Variable capacitor for higher frequency tuning and fine tuning
- 2 sheets of aluminum foil
- 1 sheet of paper or other insulator
Where to scavenge parts from
Kitchens, thin sheets of metal such as those found in hardware stores can also be used. Mylar balloons can be used as long as they are conductive and not coated with a plastic insulator.
Making the capacitor
- insert schematic with variable capacitor in series with antenna
The larger the surface area the larger the charge that can be held. The larger the charge the further the plates have to be if air is the insulator, or the thicker the insulator needs to be (ie more sheets of paper). At least one sheet needs to be movable.
The easiest way to construct this variable capacitor is to take a book and 2 sheets of foil. Use one of the pages of the book as the insulator. Be careful that you do not apply varying pressure to the cover of the book as that will force the sheets of aluminum closer together and change the capacitance.
Connect an alligator clip to one sheet and a different clip to the other sheet. Gently pull one sheet out slowly, and you will be able to tune the radio somewhat. By sliding the sheet of aluminum in and out of the book you have changed the surface area of the plates of the capacitor.
- insert photo of aluminum/book variable capacitor
This allows you to tune the radio, see the "Why it works" section below for an explanation of how this tuning is accomplished.
How to tune
First set the capacitor in series with the antenna to a fixed position. You should not have to adjust this much if at all for the band that you are receiving. See the section on tuning to a specific band.
Next connect the antenna to the coil tap that provides the loudest signal. This is your coarse tuning.
Gently pull out and push back in the variable capacitor that is connected to the coil. This provides your fine tuning ability. When you have the station at its loudest and clearest you are done.
Why it works
A capacitor is simply two plates separated by an insulating material. This material can be air, glass, paper, plastics (most are unsuitable though due to static build up), ceramics, and other non-conductive materials.
When you connect a battery to a capacitor one plate will have more electrons than the other. If you disconnect the battery the plate remains charged, like a battery. If you connect a wire to the two plates the electrons will rush from the plate that has more to the plate that has fewer in an attempt to restore balance. In this way you can make the electrons do work for you while they travel from one plate to another.
When they go through the coil, they create a magnetic charge in the coil which increases until the electron flow has stopped. After the electrons have balanced, there is still a magnetic charge in the coil, which then collapses. A collapsing magnetic field around a coil creates a current similar to passing a magnet through a coil. This current is in the same direction that created the magnetic field.
So if the current goes from the top plate of a capacitor to the bottom plate, the coil will create a current in the same direction when the magnetic field collapses. This means that the bottom plate is now charged and those electrons want to go to the top plate. The current then goes through the coil the opposite direction, creating another magnetic field, which when it collapses it will charge the top plate. This will create an oscillation in the coil, which can increase the signal if its resonating with the frequency of the received signal.
This is called a 'tank circuit' which is not uncommon in radio design.
The antenna acts like a capacitor on its own, by adding a capacitor between the antenna and the coil you are simulating changing the length of the antenna. This enables you to change the resonant frequency of the antenna, which can allow you to hear higher frequency stations more easily. This capacitor is usually a fixed value and not adjustable, however a variable capacitor that is not changed works just like a fixed capacitor.
The capacitor connected to the coil allows us to tune the coil itself, this provides for better reception of signals by making them louder. You do coarse tuning by connecting to the tap that is loudest on the radio and fine tuning by adjusting the capacitor that is connected to the coil.
This gives you a fine tune adjustment, since the coil has taps which give you coarse tuning ability.
Decoupling coil
- smaller coil that decouples the antenna from the rest of the radio
- improves weak signal reception but decreases loudness
notes for future editing
The diode and earphones are directly connect to the antenna and ground. This loads the coil making it less selective (ability to tune a weaker station near a stronger station). To improve selectivity of the radio allowing you to hear weaker more distant stations that are drowned out by closer more powerful stations you need to add a decoupling coil.
- 5-10 turns of wire around a small coil form (35mm film canister worrks).
- cut large hole in bottom of the plastic form for the larger tuning coil
- attach antenna and ground to the small coil
- place small coil in the larger coil
- slide small coil in/out to adjust selectivity/sensitivity of the radio
- further in for louder local stations
- further out for weaker more distant stations
Making your own earphone
- wire
- nail
- small magnet (neodymium-iron-boron supermagnets are best)
- tin can (must be attracted to the magnet)
Where to scavange for parts
- A coil from an old relay or solenoid will often work, and save you the time of winding the coil into the nail.
- tin can after you consumed the food contained inside
- you might find suitable magnets in speakers, refrigerator magnets, or other household items.
notes for future editing
- wind a few hundred turns of wire onto the nail.
- put magnet on the head of the nail
- connect the coil in place of the earphones
- hold the open end of the can to your ear
- hold the nail very close to the bottom of the tin can
Why this works
As the current flows through the coil the magnetic field changes, this causes a different pull on the tin can which forces it to vibrate in sync with the changing field. That vibration causes the air touching the can to vibrate, which in turn goes into your ear, and you hear the vibrations as sound.
Speakers
An earpiece is fine for single user use, you may want to improve on this to make it louder so a room of people can hear. Just as the home made speaker above vibrates air, the goal of a louder speaker system is to vibrate more air more violently.
This is more of a creative "use what you have" section, designed to get you to think about the various ways that you can create more sound from the same speaker.
Notes for future editing
- spiral sea shell (conch) drill hole in the small end and attach piezoelectric speaker
- cardboard tubes from paper towel rolls or coffee creamer - popular in jails using headphones
- styrofoam bowls, take 2 and glue/tape the open ends together, put the speaker at one end, cut small holes in the top side
Adding a battery for increased volume
A battery can be added for increased volume. This can be helpful if you want to pick up a very faint distant station. You can add a AA style store bought battery or make one of your own.
How to connect the battery
The battery is installed in series with the diodde. This means that you have to disconnect the diode from either the ear piece or the coil and insert the battery there. If you do not know which is the anode and which is the cathode of the diode, which is more likely with a homemade diode, you may have to experiment by reversing the battery. It should only work in one direction.
How to make a battery
A battery can be made by using two dissimilar metals and an electrolyte.
Items required
- Stainless steel bowl
- newspaper or other paper
- aluminum foil
- An Electrolyte, vinegar and salt works well, other potentials are lemonaide (citrus fruits), baking soda, bleach, potatoes, damp soil with a silver knife and a stainless fork, different metal coins and some blotter paper, and similar components. You can even make lye (sodium hydroxide) by leeching water through the ash from hardwood fires which works as an excellent electrolyte.
Battery Construction
Line the inside of the bowl with paper. This will act as an insulator between the stainless steel bowl and the aluminum foil. Next place a layer of the foil in the bowl and make sure that the foil does not touch the bowl. Add in vinegar and connect one wire to the bowl and one wire to the foil. You want to ensure that the paper gets soaked with the electrolyte, as that is what facilitates the transfer of electrons from one metal to the other.
If you use a larger bowl you will have more current (amps) from your battery, by connecting multiple bowls together in series you can create higher voltages. Connect the first bowl to the foil in the second bowl. Then use the first foil and the last bowl as your leads for electricity.
For this radio you should only need to use one bowl to generate sufficient voltage to increase the signal to an appreciable level, if its required at all.
this is a battery made with rice vinegar, adding salt raises the voltage to about 0.78vdc. The aluminum is the negative lead and the bowl is the positive lead.
