Radio experts.... Help!?

Question

I am a keen experimenter in electronics. I experimented a AM Transmitter circuit. I kept a radio tuned to the particular frequency near the circuit in an attempt to catch the signal. When I switched on the circuit, the radio made a "crack" sound. Same thing when it is switched off. So, I thought this was the signal coming in. But same thing happened another time. This time I kept the aerial of the radio near the switch of my study-lamp. Each time the lamp was turned on/off, the radio made the sound. Why is this happening? I also found that same phenomenon happened when the aerial of a AM Radio kept near the switch of any circuit. Please help...

Answer 1

The “crack” sound you heard was the radio responding to broadband frequencies generated by the current transient that occurs when the switch is opened or closed. These transient frequencies exist momentarily as a consequence of how rapidly the switching action makes or breaks the current. The faster the transient current occurs the higher the frequency content radiated by the switch. Transient analysis of a finite rise-time step function by Fourier transform mathematics will reveal the extent and relative amplitude of these frequencies.

Significant energy can be radiated into the tens of megahertz and is easily detected by a sensitive AM receiver tuned to any frequency where the transient frequencies exist. To verify this, simply tune your radio anywhere across the band of frequencies it can receive and note that the “crack” occurs anywhere on the dial, unless another, stronger, transmitter is present at a particular frequency.

If your experimental AM transmitter is really working, you should be able to set a nearby AM receiver to a quite place on the AM band and “sweep” tune the transmitter frequency across the selected frequency. You should hear a sharp reduction in the background noise as your transmitter signal is received by the AM radio. You may or may not hear a heterodyne audible tone, varying in frequency as you sweep the transmitter frequency across the tuned radio frequency, but the automatic gain control (AGC) should activate when your transmitter “carrier” frequency enters the passband of the tuned radio frequency, causing a sharp reduction in gain and subsequent quieting of background noise.

Also, AM radios usually do not have or need an external aerial. They use an internal ferrite "loop-stick" antenna. If an external aerial connection is provided, there will be a small screw provided for this purpose and you must provide the wire for the aerial.

If your radio has an external aerial (probably telescoping) it is probably capable of FM reception. FM receivers operate at much higher frequencies than AM receivers (at least here in the USA) and can benefit from an extensible external antenna or aerial.

Answer 2

Depending on the phase of the 60 Hz power at the instant the switch moved, an arc (spark) of ionized air passes between the switch contacts. Uncounted billions of ions each resonate according to their own circumstances. The result is that the spark generates many, many electromagnetic waves of many, many different frequencies. The distributed inductance and distributed capacitance of the lamp cord acts as a resonant circuit and radiates the spark's energy like an antenna. Early radio transmitters and Hertz's original experiments all used spark-gaps to drive resonant antennas tuned to the desired frequency.
Inside the radio there is an antenna (probably a coil of fine wire wound around a ferrite core. When radio frequency energy from the spark passes over the reciever's antenna a voltage is induced in the antenna which causes a very small current to flow. Actually many,many small currents flow, depending on each of the many, many frequencies developed by the spark. Some of that energy happens to be precisely the correct frequency to pass through the reciever's tuning circuitry and is then detected, amplified, and applied to the speaker's voice coil. Impulse signals (like the spark but unlike the transmitter's carrier wave) can often "swamp" the tuner's rejection of off-frequency signals and appear at the reciever's output. This is most likely what you observed.
By the way, Marconi noticed the same effect, figured out the concept of resonance, and was thus the father of modern radio. You're actually repeating his early observations.
Hope this helps.....

Answer 3

AM radio will pick up any disturbances so every time a switch makes a little spark it will come out on the radio. An electric spark makes a broad range of radio frequencies.

Answer 4

Maybe you have a fluorescent light, that causes interference.