3 ms·

The loopstick is much shorter than a wavelength, so the phase of the received EM field is the same over its length. However, the magnetic field can induce an R

The loopstick is much shorter than a wavelength, so the phase of the received EM field is the same over its length. However, the magnetic field can induce an RF current in the loop, which is a rather long wire tightly wound over a ferrite core to increase its inductance. The coil is shorted with a capacitor. Together, they form a resonant circuit at the desired frequency (which is selected by making small changes in the capacitor). Energy is transferred back and forth between the magnetic field of the loop and the electric field between the plates of the capacitor. The EM field reverses around a million times per second, and so does the current in the resonant circuit. The overall effect is like pushing a child on a swing. If the period of the swing is synchronized with the timing of the push, the amplitude of the swing will increase. Because of this resonance effect, it is possible to transfer energy from the EM field into the resonator and develop enough voltage to drive an amplifier chain. It's even enough to drive high impedance earphones in a crystal radio set. It also helps that local radio stations are very powerful - several kilowatts, and they are close.

pxmpxm 5y ago
"The overall effect is like pushing a child on a swing. If the period of the swing is synchronized with the timing of the push, the amplitude of the swing will increase." That's a fantastic analogy!
dr_dshiv 5y ago
In the analogy of pushing the child, the length of the swing determines the resonant frequency. In that case, is the LC (length*capacitance?) circuit the equivalent of swing length? Also, with pushing a child, I don’t need to push every cycle, I can push every other cycle (1:2), every third/4th cycle (1:3,1:4), or 2 out of 3 (2:3), etc. Is there an equivalent with radio? Can the LC circuit be tuned to half the signal frequency and still pick up?
- wrycoder 5y ago
  The swing length, to first order, is independent of the period, as Galileo noticed while watching a swinging lantern during a church service. He timed it with his pulse. It's the period (or its inverse, frequency) which is characteristic of a resonance, not amplitude. Interestingly, the period of the resonance is proportional to the geometric mean of the capacitance and the inductance. The proportional factor is 2pi, which is there because of the "rationalized MKS" units chosen. (If it wasn't for that, one Farad resonating with one Henry would have a period of one second.) You're quite right: pushing every other cycle is analogous to pulsing an LC in phase every other cycle. This is done using a class C amplifier[0], which puts out short pulses at the input frequency. The parallel LC in the amplifier load resonant at twice the input frequency sees in phase current pulses in its inductor every other cycle, which add energy. That's how a frequency doubler works, and the technique works for higher harmonics, also, as you thought. > LC circuit be tuned to half the signal frequency In the case of a swing, that would work, because the child isn't there for half the pushes. But in the case of an LC tank, the "pushes" always inject current into the coil in the same direction, so half if them will be out of phase and cancel the previous one. [0] https://en.wikipedia.org/wiki/Power_amplifier_classes#Class_C https://en.wikipedia.org/wiki/Power_amplifier_classes#Class_...
  - dr_dshiv 5y ago
    Sorry, by “swing length” I meant the length of the rope of the swing, not how far it swings (amplitude). You’d agree that Galileo saw lanterns on longer ropes swing slower? > In the case of a swing, that would work, because the child isn't there for half the pushes. But in the case of an LC tank, the "pushes" always inject current into the coil in the same direction, so half if them will be out of phase and cancel the previous one. Thank you for clarifying that for me! So, no “AC” antennas?
    - wrycoder 5y ago
      I apologize - I don’t know how I misread you that badly! You were perfectly clear. I’m having vague visions of a frequency divider using a push-pull output running in Class C.

pxmpxm 5y ago

"The overall effect is like pushing a child on a swing. If the period of the swing is synchronized with the timing of the push, the amplitude of the swing will increase." That's a fantastic analogy!

dr_dshiv 5y ago

In the analogy of pushing the child, the length of the swing determines the resonant frequency. In that case, is the LC (length*capacitance?) circuit the equivalent of swing length? Also, with pushing a child, I don’t need to push every cycle, I can push every other cycle (1:2), every third/4th cycle (1:3,1:4), or 2 out of 3 (2:3), etc. Is there an equivalent with radio? Can the LC circuit be tuned to half the signal frequency and still pick up?

wrycoder 5y ago
The swing length, to first order, is independent of the period, as Galileo noticed while watching a swinging lantern during a church service. He timed it with his pulse. It's the period (or its inverse, frequency) which is characteristic of a resonance, not amplitude. Interestingly, the period of the resonance is proportional to the geometric mean of the capacitance and the inductance. The proportional factor is 2pi, which is there because of the "rationalized MKS" units chosen. (If it wasn't for that, one Farad resonating with one Henry would have a period of one second.) You're quite right: pushing every other cycle is analogous to pulsing an LC in phase every other cycle. This is done using a class C amplifier[0], which puts out short pulses at the input frequency. The parallel LC in the amplifier load resonant at twice the input frequency sees in phase current pulses in its inductor every other cycle, which add energy. That's how a frequency doubler works, and the technique works for higher harmonics, also, as you thought. > LC circuit be tuned to half the signal frequency In the case of a swing, that would work, because the child isn't there for half the pushes. But in the case of an LC tank, the "pushes" always inject current into the coil in the same direction, so half if them will be out of phase and cancel the previous one. [0] https://en.wikipedia.org/wiki/Power_amplifier_classes#Class_C https://en.wikipedia.org/wiki/Power_amplifier_classes#Class_...
- dr_dshiv 5y ago
  Sorry, by “swing length” I meant the length of the rope of the swing, not how far it swings (amplitude). You’d agree that Galileo saw lanterns on longer ropes swing slower? > In the case of a swing, that would work, because the child isn't there for half the pushes. But in the case of an LC tank, the "pushes" always inject current into the coil in the same direction, so half if them will be out of phase and cancel the previous one. Thank you for clarifying that for me! So, no “AC” antennas?
  - wrycoder 5y ago
    I apologize - I don’t know how I misread you that badly! You were perfectly clear. I’m having vague visions of a frequency divider using a push-pull output running in Class C.