The first receivers made use of something called a “coherer”. It’s really just a pile of metal filings that are loosely placed between two electrodes.
It has a remarkable behavior that allows us to detect radio waves. When no radio waves are present, the resistance between the electrodes is quite high. However, when a radio frequency is present, the metal filings microscopically “weld” together and the resistance drops dramatically. This behavior can be used in an electrical circuit to alert us to the presence of radio waves and it will be covered in detail below. The coherer must then be reset, which involves shaking up the filings to cause the coherer to go back to its non-conductive state, in order to detect the next signal.
Before we look at how the first receivers operated, it should be noted that they don’t have a tuner (a tuned capacitor and coil arrangement) like modern receivers.
Instead, it uses the natural length of the antenna for tuning. The antenna can be any length but if it is ¼, ½ or 1 wavelength of the desired reception frequency then it is more effective. With such a crude tuner this receiver isn’t very selective of the signals it receives.
Here is the detailed look at how this primitive receiver operates:
1.Passing radio waves induce a voltage on the antenna. This “turns on” the coherer C because it is connected between these waves in the antenna and the ground.
2.Now that the coherer has a low resistance, electricity from the battery B1 can flow and that closes the relay R.
3.With relay R closed, the electricity from battery B2 can flow and this closes the relay S with an audible click. This click is the signal that a radio wave has been detected.
4.A spring from either of the relays is attached to the coherer so that, when the relay closes, the motion of the relay closing causes a physical jolt that resets the coherer and turns it off.
5. With the coherer reset R and S both open and the entire system is ready for the next signal.
6. The two coils L are needed to isolate the coherer and antenna from the relay coils and the electrical noise that they generate.
This type of receiver is subject to all kinds of interference such as lightning strikes and sparks from the relays within the receiver itself. It is limited to receiving pulses (like Morse code) and it is fairly “deaf” which means that the radio signals must be strong in order for it to detect them.
An upgrade to this type of receiver would be the classic crystal radio. The most difficult part of building a crystal radio from scratch would be building an extremely efficient earphone (e.g. piezoelectric earphone). Other than the earphone, you would need the crystal diode that gives the receiver its name. If you are interested in learning more, check out this page on building a crystal radio from scratch. That should be enough to get you well on your way to reinventing radio.