Not built yet. This is my plan and parts list. When it's done I'll add what actually happened.
What to expect
A foxhole radio is a crystal set with a home-made detector. The detector is the part that turns a radio wave into sound, and a razor blade does that job worse than a proper germanium diode. So set your hopes low and you'll enjoy it more.
- Strong medium wave stations only. If there's a powerful AM station near you, or you listen after dark, you should hear something. Shortwave is very unlikely with this design.
- It will be quiet. You listen through an earpiece pressed to your ear, in a quiet room.
- Finding the sweet spot takes patience. Only some points on the blade work. You hunt for one with the pencil tip, and a knock can lose it.
- Tuning is broad. If two strong stations are close together you may hear both at once.
- The aerial and earth matter more than anything else. Same as any crystal set. A short wire indoors will almost certainly give you silence.
Why this way
If you just want a battery-free radio that works well, build the crystal set. This build is about history and about seeing that a radio detector can be made from almost nothing.
A detector only lets current through one way. A thin oxide layer on steel, touched lightly with a point, does that a little. Soldiers had razor blades and pencils, so that's what they used.
Heating carbon steel briefly turns its surface blue. That blue is the oxide layer the detector needs. A shiny new blade has little of it, and modern coated blades have the wrong surface altogether.
Graphite conducts, it's soft so it won't scratch the oxide away, and a fine lead makes a tiny contact. Small contact points are what make this kind of detector work.
The aerial wire itself acts as the capacitor. A sliding contact on the coil changes how many turns are in use, and that tunes the set. One fewer part to find, which was the point in a foxhole.
The originals used toilet-roll tubes. Card soaks up damp and that shifts the tuning. PVC doesn't, and it's the same tube I use for the crystal set.
The signal is tiny. A crystal earpiece needs almost no power to make a sound. Ordinary headphones need far more and would stay silent.
How it works
The same four stages as any crystal set. Only the detector is different.
Catch
The long aerial wire picks up every station at once as tiny currents. The earth rod gives them somewhere to go.
Choose
The coil and the aerial's own capacitance make a tuned circuit. It responds most to one frequency. Moving the slider changes how many turns sit between the aerial and earth, and that moves the frequency.
Detect
Where the pencil lead touches the blue oxide, current flows more easily one way than the other. That chops the radio wave in half and leaves the slow wobble of the audio behind.
Hear
The crystal earpiece turns that wobble into sound. A 100k resistor across it lets the charge drain away so the sound doesn't fade out.
The detector itself is just two bits of metal on a board. The blade is held flat by two drawing pins, which also carry the wire. A safety pin swings on a third drawing pin, with a short piece of pencil lead tied to its end. You move the arm until the lead tip finds a spot on the blue that works.
What to buy
Most of this is cheap. The aerial and earth are the biggest spend, and they're shared with the crystal set, so if you build both you only buy them once. Listings change, so check each one before you order.
These are affiliate links. If you buy through them I make a small commission, at no extra cost to you. It helps cover the cost of running this site. This is a hobby, not a shop.
The detector
Coil and listening
Aerial + earth
You'll also need: a small wooden board (about 20 × 15 cm), a large safety pin, fine sandpaper, a gas hob or lighter and pliers for bluing the blade, thin wire or thread to tie the lead on, and a hacksaw for the tube.
Build it
Seven steps. The coil is the slow part. The detector takes ten minutes to make and as long as you like to get working.
-
Wind the coil
Cut about 120 mm off the PVC tube and sand the ends. Drill two small holes 10 mm from one end, thread the wire in and out to anchor it, and leave a 25 cm tail. Wind about 150 turns, tight and touching, in one direction. That's roughly 70 mm of winding. Anchor the far end the same way and leave another tail.
The exact count doesn't matter much. Roughly 150 turns on a 40 mm tube is about 400 µH, which should reach the bottom of the medium wave band with a typical long-wire aerial. With fewer turns in use, it tunes higher.
Why: there's no tuning capacitor, so the coil has to be big enough to tune low on its own with the aerial's capacitance. Anchoring the ends through the tube stops the winding springing loose.
-
Scrape the tuning stripe
Lay the coil on its side. With fine sandpaper wrapped round a flat stick, rub a narrow stripe along the top of the winding, end to end, until you see bright copper on every turn. Keep the stripe about 3 mm wide and don't press so hard that you thin the wire.
Why: the enamel insulates each turn. The slider needs bare copper to touch, but only along one line, so the turns stay insulated from each other everywhere else.
-
Make the slider
Take about 25 cm of the same 0.4 mm wire and sand all the enamel off it. Fold it in half and twist it to make it stiffer. Wrap it once round the coil so it sits snugly on the stripe but can still be pushed along, and twist the ends together into a short tail. Clip a crocodile lead to the tail.
Why: the ring only makes contact where it crosses the bare stripe, because everywhere else the turns still have their enamel. Pushing it along picks how many turns are in use. It may short a turn or two where it sits, and on a set this simple that doesn't matter.
-
Blue the blade
Hold a blade by its edge in pliers. Pass it through a gas flame for a second or two at a time until the surface turns straw, then blue. Stop as soon as you see blue: if it goes grey, it's overdone. Let it cool on a heatproof surface. Make three or four, as some will work better than others.
If a blade won't colour at all, or the colour is patchy and rainbow-like, it's probably stainless or coated. Try another, or try a rusty old blade instead, which some wartime builders also used.
Why: the blue colour is a thin film of iron oxide. That film, touched by a fine point, is what lets current through more easily one way than the other. Modern stainless and coated blades don't form the right film, which is why many people's foxhole radios stay silent.
-
Make the detector board
Lay a blued blade flat on the board and push a drawing pin through each end hole. Before you push the left pin fully home, wrap a scraped wire end under its head. That wire goes to the earpiece.
Open a large safety pin into an L shape. Tie or wrap about 10 mm of pencil lead to the point end with thin wire, so a few millimetres stick out. Push a drawing pin through the safety pin's coiled spring into the board as a pivot, with a scraped wire from the coil's top end wrapped under it. Bend the arm so the lead rests on the blade with only its own weight, like Fig 2.
Why: drawing pins make every joint without solder, so you can swap blades in seconds. The contact must be light: press too hard and you break through the oxide and lose the effect.
-
Wire it up
- Aerial → top end of the coil. Same point → detector pivot pin.
- Slider lead → earth.
- Bottom end of the coil: leave it free. Don't connect it to anything.
- Blade pin → one earpiece wire. Other earpiece wire → earth.
- 100k resistor across the two earpiece wires.
Why the free end: if both the slider and the bottom end went to earth, the turns between them would form a closed loop that soaks up signal. Left free, they do nothing.
-
Put up the aerial and earth
Run 20–40 m of wire as high and straight as you can, with an insulator at each end, and bring it in through a window. Drive the earth rod into damp ground near the radio and clamp a wire to it. My crystal radio page has the full aerial and earth method, and it's the same here.
Why: this set has no amplifier and a weak detector, so everything you hear comes from what the wire catches. A poor aerial means silence, however well the rest is built.
Using it
Prove the set first
If you have a germanium diode from the crystal set, clip it in place of the blade, with its band (the cathode) towards the earpiece. Tune to a station.
Why: if the diode works and the blade doesn't, you know the coil, aerial and earth are fine and it's only the blade spot you need to hunt for. Without this step you can't tell what's wrong.
Wait for evening
Medium wave stations travel further after dark. Your first try will be easier then.
Find a spot
Earpiece in, slider somewhere in the middle. Lift the arm and set the lead down on a new spot on the blue, again and again, until you hear a station or at least a crackle. Then slide the ring slowly along the coil to tune.
Why: only some points on the oxide behave as a detector, and nobody can tell which by looking. It's trial and error, just as it was for the soldiers.
Keep it still
Once you've found a good spot, don't knock the table. Put the board on something soft if footsteps disturb it.
Try other blades
Swap in another blued blade and compare. Note which one worked best and mark it.
Keep blades dry. Damp turns the blue to rust and changes how it behaves. Lift the lead off the blade when you're not listening so the tip doesn't wear a dull patch.
If it's not working
| What you hear | Likely cause | Fix |
|---|---|---|
| Nothing, even with a diode | Enamel on a joint, or a poor aerial or earth | Scrape every wire end bright. Check the earth clamp. Get the aerial higher. |
| Diode works, blade silent | No sensitive spot found, or the wrong kind of blade | Keep probing. Try lighter pressure. Blue another blade, or try a rusty one. |
| Sound comes and goes | Lead tip moving on the blade | Keep the board still. Bend the arm for slightly more pressure, but only slightly. |
| Fades to nothing after a moment | No resistor across the earpiece | Fit the 100k across the earpiece wires. |
| Two stations at once | Broad tuning; the aerial loads the coil heavily | Try a shorter aerial. You lose some volume but gain separation. |
| Slider changes nothing | Stripe not bare, or slider wire still enamelled | Sand the stripe and the slider ring again until both are bright. |
| Loud hum | Aerial near mains wiring | Move the wire away from the house. Improve the earth. |
Safety
- Blades are sharp. Hold them with pliers, never fingers, while heating and fitting. Keep the finished board away from children, and wrap spent blades in tape before you throw them away.
- Hot metal. A blade stays hot for a while after it stops glowing. Let it cool on something heatproof.
- Lightning. Unclip the aerial and lay the end outside whenever a storm is near. Never listen during thunder.
- Power lines. Keep the aerial well clear of overhead power and phone lines, including if it falls.
- Earth rod. Check for buried cables and pipes first. Never use a gas pipe or the mains earth as your radio earth.
Notes
Foxhole radios were made by Allied soldiers, especially in Italy and the Pacific, when ordinary radios were banned near the front line. A working radio with a local oscillator gives off a weak signal that the enemy could detect, and a crystal set doesn't. The stories of what worked vary a lot, so I've kept to the parts everyone agrees on: a blued or rusty blade, a fine graphite or metal point, a big coil, a long aerial and a good earth.
The coil figure comes from the standard single-layer coil formula (Wheeler) for a 40 mm former. The aerial's capacitance depends on its length and height, so the real tuning range is a rough guess until the set is built.
Related guides: the crystal set this is based on, Morse code without batteries, and earth batteries and lemon cells.