Bifilar pancake coil overunity experiment

[F6FLT]

I'm sorry, reading things here, get some new ideas. Or just to explain. TinselKoala thought that a coil, and a capacitor in parallel with it or something, is an equivalent of a bifilar coil that has capacitance. This is certainly not the equivalent, one thing it has not twice the inductance of the ordinary coil, as a bifilar coil has.
...

I think this TinselKoala's view of the bifilar coil is right in the quasi-stationary states approximation. We only have a resonant classical LC circuit.
But when the length of the coil if of the same order as the wavelength of the signal frequencies, stranger things begin to appear.
It doesn't matter the coil is monofilar or bifilar, only wire length and capacity matters: the coil becomes a transmission line BUT a signal can pass through the coil diametrically, through the inter-turn capacity, and we have a possible overlap of the two phenomena.

The coil in the photo is monofilar, it is connected to nothing but weakly coupled by induction to two turns of the red wire that can be seen, which is connected to a signal generator. Its resonance frequency is about 1.5 MHz and rather sharp.
By injecting the signal directly into the external end of the coil, not by induction, the resonance is at approximately the same frequency.

According to a measurement of a second coil of 55nF, bifilar this one, with copper strips 5 mm width, I estimate the capacitance of the first one in the photo, 1.5 cm width and tighter turns, to be more than 200nF.
With such a capacity associated with such an inductance, it is impossible to get a resonance at 1.5 MHz but only at a few KHz or tens of KHz. The resonance is therefore not related to the LCω² product.

To set things straight, I wanted to measure the transmission time. I injected a 16 ns pulse into the outer end of the coil (not in the photo). I connected the inner end to the ground with a 47 ohm resistor at the terminals of which I observed the output signal. The pulse repetition frequency was only 1 KHz so that possible echoes from one pulse do not interfere with the next.
To my great surprise, I didn't notice any delay! On the scope screenshot (20 ns/div), you can see that the pulse passes directly through the coil: the output is activated almost simultaneously with the input, and we see that the effect is capacitive thanks to the negative value of the output during the falling edge of the pulse. More surprisingly we don't see any other signals, no delayed pulses.

The coil length is 30 m. The quarter wavelength at 1.5 MHz is 50m. With a coefficient of velocity of 0.6, the coil length fits a quarter length resonance. It remains a pure speculation because the coefficient of velocity is strangely low and also because I don't observe echoes when I use pulses!

I think we need to clarify these elementary effects of a capacitive coil before we go further. For me it's really unclear, and yet I'm familiar with RF devices.

[ayeaye]

So it acts as a capacitor? But the oscillation in the end is because of coil?

[F6FLT]

My interpretation: it's like a RC circuit, C being the capacity of the coil and R the terminal load where we take the output signal. There is a slight delay due to the circuit time constant t=RC.

Since the capacitor is in parallel with the coil, the resistance of the coil discharges the capacitor.
As a first result, the falling edge of the input pulse causes the output pulse to drop below zero because at the time of the drop, the capacitor was already no longer charged to the maximum value required by the upper level of the input pulse.
As a second result, after the lowest point, which is a negative potential while the input pulse is always positive,  the discharge of the capacitor causes the signal to return to zero after the end of the input pulse, and I suppose that it is the part of the signal that you see as an "oscillation".

I think that the weirdness come from the fact that we have a special transmission line: like a transmission line it has a distributed inductance and capacitance, but unlike a real transmission line, the capacitance is instantly linking all "section" of the line together.

I'm not sure at all of what I said. May be I'm wrong. Other interpretations are welcome.
The fact that I don't see a pulse delayed by the time needed to move along the line and yet by using a sinusoidal signal, I get a resonance at a frequency without any relation to the LC time constant is very annoying!

[gyulasun]

Hi F6FLT,

Would you clarify little more precisely how you connected the bifilar (copper strip) coil? Are the other two ends
of the coil left floating independently or they are connected together and left floating?
From your measurements, if I understood it correctly, could it be deduced that your bifilar coil as you connected it
behaves like a series LC circuit (at 1.5 MHz) resonance?
I mean as if a series (lumped) LC is driven at one end from the "hot" output of a generator and its other end is
connected to a 47 Ohm resistor across which your scope probe is connected and the ground of the generator is
connected to the cold end of the 47 Ohm (and to the ground of the scope of course).

Thanks,
Gyula

[ayeaye]

Gyulasun, as much as i understand, this was, a pulse from the signal generator, through the coil, there was a resistor in series with the coil, and the voltage was measured on that resistor. Like on the figure below i guess. So it seems to be, like resistance was charged, then when the pulse ended, it discharged. But i'm not sure whether i understood rightly. What i asked was, in the end of the negative part of the signal, there is some ringing, waving or such, i ask whether this was caused by the coil.

Traces of the F6FLT experiment drawn with Inkscape, and converted to samples. 50 units per square (scale), the drawing precision is great enough.

ch 1

https://trinket.io/python/69a421854d

ch 2

https://trinket.io/python/61f11be8fe

On the next figures are the gnuplot plots of ch1, input part and output part.

F6FLT, what were the scales of ch 1 and ch 2 on your traces? I assumed 5 V and 0.2 V but i couldn't really read it from your screen image.

With that not right the calculations below are not correct at all. So below are the obviously absurd results with the assumed scale values.

Calculation of the input part and its output.

https://trinket.io/python/4d26084a5b

Input power was 21.5164 uW

Calculation of the output part and its output.

https://trinket.io/python/ea65242ecf

Output power was 0.0097 uW

It may be simpler to do it in that way in some respect, but only the signal generator output can take it, a ttl output cannot take it, thus the need for transistor. If one has no signal generator. But calculations are more complicated, and there is a need to measure two channels, which greatly increases the error, so doing it with transistor is clearly better.

Make the resistor much greater maybe, perhaps you waste so much power.

Please if you want to change the Trinket Python scripts above, make first your own fork, that is, get a different link, go there, and then change it there, but make sure that the link is different, don't change the script at the original link please.

Just in case i just write here the actual calculation parts when using the circuit below. For calculating input, two input files were used, for ch 1 and ch 2 lists. Assume that v is the voltage on the output of the signal generator, vl is the voltage on the coil, vr voltage on the resistor, pl power in the coil, and pr power in the resistor, R is the resistance of the resistor (in ohms), and e is the sum of powers at the instants of time. Voltages are in mV, thus dividing by 1000 when calculating power, to get mW.

Input power calculation.

vl = v - vr
il = vl / R
pl = vl * il / 1000
e += pl

Output power calculation.

pr = vr * vr / R / 1000
e += pr