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Hello,
i am trying to understand the functionality of B- and E-field antennas, but there is one thing that i dont understand.
Why do the magnetic antennas that are presented in the project description have a resonance frequency? The loops behave like inductors, but there are no capacitors, so there shouldnt be a resonant circuit?
Greetings,
Julian
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(2015-11-06, 15:17)JuR Wrote: Hello,
i am trying to understand the functionality of B- and E-field antennas, but there is one thing that i dont understand.
Why do the magnetic antennas that are presented in the project description have a resonance frequency? The loops behave like inductors, but there are no capacitors, so there shouldnt be a resonant circuit?
Greetings,
Julian
Julian,
Any circuit will have capacitance, the wires from the coil and the PCB tracks for instance, these are kept as low as possible so that the resonance is higher than the frequencies we are interested in (we are looking at a range of frequencies so don't want the loops to show any resonance in this range)
Ben.
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A coil always has a resonance frequency, a frequency where the electrical resistance becomes very large (signal increases dramatically)
There will always be parasitic capacities - capacity from one turn to another, from connecting cable, environment, etc.
We must ensure that this frequency is far away from our range.
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2015-11-06, 16:43
(This post was last modified: 2015-11-06, 16:44 by JuR.)
(2015-11-06, 16:35)Benedict.Smith Wrote: (2015-11-06, 15:17)JuR Wrote: Hello,
i am trying to understand the functionality of B- and E-field antennas, but there is one thing that i dont understand.
Why do the magnetic antennas that are presented in the project description have a resonance frequency? The loops behave like inductors, but there are no capacitors, so there shouldnt be a resonant circuit?
Greetings,
Julian
Julian,
Any circuit will have capacitance, the wires from the coil and the PCB tracks for instance, these are kept as low as possible so that the resonance is higher than the frequencies we are interested in (we are looking at a range of frequencies so don't want the loops to show any resonance in this range)
Ben. Ok, thanks! Do you have any approximate value of the capacity from the whole system? I want to calculate the resonance frequency with a given induction .
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2015-11-08, 09:17
(This post was last modified: 2015-11-08, 09:21 by Steph.)
On multiturn loops the capacity and thus resonance frequency depends on the antenna design (number of turns, wire diameter, spacing, arrangement).
On single turn loop the resonance frequency is determined by its circumference, which is half the wavelength.
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2015-11-10, 14:20
(This post was last modified: 2015-11-10, 14:23 by JuR.)
(2015-11-08, 09:17)Steph Wrote: On multiturn loops the capacity and thus resonance frequency depends on the antenna design (number of turns, wire diameter, spacing, arrangement).
On single turn loop the resonance frequency is determined by its circumference, which is half the wavelength.
I dont understand why the resonance frequency isnt depending on the inductivity and the capacity, i thought the resonance frequency is calculated by the formula: ![[Image: 4649ce6f1aa676a20ef8e19ba16d4b25.png]](https://upload.wikimedia.org/math/4/6/4/4649ce6f1aa676a20ef8e19ba16d4b25.png)
I have another question: In section 4.2.2. (amplifier 13) is said that specific VLF-signals have to be filtered out, but the VLF-signal is a electromagnetical signal so it should affect magnetical antennas too. Still the amplifier for magnetical antennas only uses 1 filter for frequencies above 50kHz and doesnt offer a filter for frequencies above 49k and 22k. Why the signals only need to be filtered out when a electrical antenna is used? (Or is this just not implemented yet?)
Greetings,
Julian
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Julian,
I think (if I have it right) that a single turn loop is seen electrically as a half wave stub antenna with one end being 0v (ground) and the other end being 'hot' at the resonant frequency, (you may also need to take the connecting wires into account unless they are kept parallel to act as a feeder) if you have more than a single turn then you start to get capacitance between the turns.
However for our purposes the actual frequency is irrelevant provided it is well above the range we are interested in.
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(2015-11-10, 17:58)Benedict.Smith Wrote: I think (if I have it right) that a single turn loop is seen electrically as a half wave stub antenna with one end being 0v (ground) and the other end being 'hot' at the resonant frequency,
That would be a quarter-wave-loop, which has low impedance without imaginary component, but is not in resonance. The resonant half-wave-loop has high impedance at the feeding point and the current antinode is on the opposide side of the feeding point. But in both ways, you're right, for our use it's impossible to build a loop in that dimension, since it would be kilometers 
Feasible are single-turn loops with a transformer or multiturn loops. On the first, the resonance frequency depends on the trafo design, on the second on the loop design. In both cases it is not so easy to calculate the resonance frequency, because of the many variables. I have some wires here and could do a measurement if that helps. But it will work just fine if you follow the examples in the documentation. Better antenna does not neccessarily mean better locating; more important is a noise-free environment.
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