In our radio, the diode and earphones are connected directly to the antenna and ground. This connection gets the loudest signal. However, it also loads the tuning coil, making it less selective. This means that many lower power or distant stations are drowned out by local strong stations.
We can make the radio more selective by decoupling the tuning coil from the antenna and ground. We do this by adding a small coil. The new coil is attached to the antenna and the ground, and then it is placed inside the main tuning coil.
Wind about five or ten turns of wire around a small coil form such as the plastic container use to package 35 mm film (about 1 inch in diameter). Cut a large hole in the bottom of the plastic bottle on which we wound the large tuning coil. Attach the antenna and ground to the small coil, and place it into the large tuning coil using the new hole you just made. By moving the small coil in or out of the large coil, you can vary the coupling between the coils, and thus vary the selectivity and sensitivity of the radio. If you want loud strong local stations, place it all the way in. If you want to hear the fainter distant stations, pull it out a bit.
Help with construction math
Here is a simple little program that can show you how many turns of wire you need on your tuning coil to resonate with any capacitor you choose:
Building your own earphones You can build your own earphones using a tin can, a nail, a small magnet, and some fine wire. Wind a few hundred turns of wire around the nail. Let the magnet stick to the head of the nail . Attach the coil to the radio in place of the earphones. Hold the open end of the tin can to your ear, and hold the nail very close to the bottom of the tin can. The bottom of the can will be attracted to the magnet, but the coil will make it vibrate with the sound from the radio.
A coil from an old relay or solenoid will often also work, and save you the effort of winding the wire on the nail.
A seashell loudspeaker
I got a large conch shell from an aquarium store for a few dollars. Using a concrete drill, I made a 1/4 inch hole in the shell at the small end (where the shell was formed when the conch was very small). I then glued a piezo-electric earphone to the hole. This makes a nice trumpet-like megaphone and makes the sound of the radio clearly audible across a quiet room. It also looks very nice.
Using an LED for a diode.
Because I have a long (150 foot) antenna, a good ground, and a strong station (50,000 watts) less than 20 miles away, my radio receives enough power to light a low current LED. The LED is a 'high brighness' type (which also means that it will light dimly with a very small amount of current). I connect it instead of diode in the radio, and it glows as the radio operates, getting brighter as the sound gets louder.
If you don't have a strong station nearby, you can add a battery in series with the LED (a small 1.5 volt battery works fine). The LED will light up, and the radio will play much louder than without the battery (if the LED doesn't light up, try connecting the battery the other way around). This arrangement is the best detector I have used so far, and is louder than the 1N34A germanium diode.
