3 ms·

In a 'loopstick' antenna, all that's going on is that one side of the coil is receiving the signal before the other. This is what there's a strong null when the

by zh3 5y ago

In a 'loopstick' antenna, all that's going on is that one side of the coil is receiving the signal before the other. This is what there's a strong null when the coil points towards the source. A little surprised by one point in the article, as I generally find his articles well-informed. In particular, from the purely theoretical point of view there is no difference between a antenna for tranmitting and receiving - however, from a practical point of view if you try and feed 500KW into a little loopstick antenna it (sans safety circuits) will be quite pretty for a second or so (i.e. transmitting antenna have to be large, just to handle the power). The problem is even greater at longwave, where amateurs on 137KHz [0] are well aware of the crazy voltages that develop when trying to feed a 'short' antenna at these frequencies (a 1/4 wave is over 500 metres). [0] https://en.wikipedia.org/wiki/2200-meter_band https://en.wikipedia.org/wiki/2200-meter_band

aj7 5y ago
Yes.
wrycoder 5y ago
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_...
teraflop 5y ago
The other way I've seen this phrased is that unlike a transmitter, it's OK for a receiving antenna to be incredibly thermodynamically inefficient, because a weak signal can always be amplified -- as long as it's not swamped by noise generated within the receiver itself. Let's say an AM radio station transmits a 1kW signal at 1MHz, with a bandwidth of 10kHz. If you have a receiver 100 miles away, and ideal isotropic antennas at both ends, then by plugging some numbers into a link budget calculator, it looks like the received power is about 12 orders of magnitude above the thermal noise floor. That means it's plausible that you could still get a clear signal even if your antenna only manages to capture a millionth or a billionth of the incoming energy.
- hilbert42 5y ago
  Ferrite antennas are remarkably efficient considering their size. If one were to examine the magnetic flux around a ferrite antenna one would find that the flux is channeled from the surrounding area into the ferrite rod (the rod effectively concentrating the field). In effect this makes the antenna's effective size (its aperture) much bigger than its actual physical size. Also, remember broadcast services are designed to put sufficient field (signal) strength into a 'notional' receiver that's operating within the rated service area of the broadcast. Both the transmitter field strength and the receiver sensitivity are designed in combination to provide a worthwhile listening signal. The designed signal-to-noise ratio is dependent on the type of service and for the AM B/C band it is in the order of 40dB or so for the primary service area (the secondary service area typically 10dB less).

by zh3 5y ago

aj7 5y ago

Yes.

wrycoder 5y ago

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_...

teraflop 5y ago

The other way I've seen this phrased is that unlike a transmitter, it's OK for a receiving antenna to be incredibly thermodynamically inefficient, because a weak signal can always be amplified -- as long as it's not swamped by noise generated within the receiver itself. Let's say an AM radio station transmits a 1kW signal at 1MHz, with a bandwidth of 10kHz. If you have a receiver 100 miles away, and ideal isotropic antennas at both ends, then by plugging some numbers into a link budget calculator, it looks like the received power is about 12 orders of magnitude above the thermal noise floor. That means it's plausible that you could still get a clear signal even if your antenna only manages to capture a millionth or a billionth of the incoming energy.

hilbert42 5y ago
Ferrite antennas are remarkably efficient considering their size. If one were to examine the magnetic flux around a ferrite antenna one would find that the flux is channeled from the surrounding area into the ferrite rod (the rod effectively concentrating the field). In effect this makes the antenna's effective size (its aperture) much bigger than its actual physical size. Also, remember broadcast services are designed to put sufficient field (signal) strength into a 'notional' receiver that's operating within the rated service area of the broadcast. Both the transmitter field strength and the receiver sensitivity are designed in combination to provide a worthwhile listening signal. The designed signal-to-noise ratio is dependent on the type of service and for the AM B/C band it is in the order of 40dB or so for the primary service area (the secondary service area typically 10dB less).