# What determines radio frequency for a diode?

I have a very simple working radio as shown in this schematic:

This radio receives one radio station.

My question is, what determines the radio frequency that this simple radio is tuned in to?

• Have your actually tried this? That circuit doesn't look like it would receive a single station. Dec 27, 2016 at 22:19
• It needs an antenna? But it might receive a strong station nearby.
– user137289
Dec 27, 2016 at 22:46
• It's probably just receiving the most powerful AM signal in your area. Dec 27, 2016 at 22:58
• Which radio station? To able to receive "a" radio station it needs to be resonating on that frequency. What will make it frequency selective? And where is the integrator for AM rectification, the headset's inductance? Dec 27, 2016 at 23:27
• ANY wires (or conductor) can act as antenna. You have an inductor in the form of the headphones, giving you a natural resonance with a time constant in seconds of L/R. It will still resonate even if it is not amplified, if exposed to em waves around the same wavelength as the time constant. If the EM signal is powerful enough, you wll get audio. Oh, I guess that should be an answer Dec 28, 2016 at 0:11

## 2 Answers

ANY wires (or conductor) can act as antenna. You have an inductor in the form of the headphones, giving you a natural resonance with a time constant in seconds of L/R. It will still resonate even if it is not amplified, if exposed to em waves around the same wavelength as the time constant. If the EM signal is powerful enough, you wll get audio.

• In the absence of a tuned circuit, is it typical that one particular station is enough stronger than the others than you hear only that station? Dec 28, 2016 at 0:23
• @DanielSank There are not that many powerful am broadcast stations left. Unless they use the same transmitter location, one of them is likely to be much stronger (closer) than any of the others.
– user137289
Dec 28, 2016 at 0:27
• You have a non-adjustable "tuning" circuit. A resonant circuit is what does the tuning. It selects its particular resonant/harmonic frequency over other noise. All circuits with inductance or capacitance (or both) in addition to reistance will have a resonant frequency. The resonant frequency is identified by the time constant. Well walky-talky or the neighbour's intercom can do it too. Dec 28, 2016 at 0:30
• what you don't have is adjustable tuning, or an amplifier. Dec 28, 2016 at 0:35

Years ago (1960s) when I worked for the BBC, part of my job was to answer calls from members of the public. One man rang to tell me that he could hear a BBC radio programme from his bed! I asked if he had a metal bedstead, which he did. He lived in Droitwich where the BBC Long Wave transmitting station was situated. [200kHz Light Programme in those days. Later the LW trnsmitter was used to broadcast Radio 4 on a slightly different frequency.] The contacts between different strands of the bedstead would have acted like the coherer of early radio receivers, which predated the quartz "cats whisker" diode and I presume there was enough received energy to make parts of the bedstead matrix vibrate.