Try running some AC current through the monofilar and series bifilar pancake and see if you can notice any difference.

Conrad:

I an offer you a suggestion about the winding of your bifilar coil.  I am going to assume that you are going to wind it on a spool.  Make a tiny hole about one-half way up on the side of the spool.  When you wind the coil and you come to the hole, just push a small loop of the pair of wires through the hole.  The wires exit the hole and then loop back inside.  Then just finish off winding the coil.

Then you can carefully strip off some of insulation and then tin the exposed wires with some solder.  You can used that as a tap point to see the voltage action going on inside the coil.  You can observe the increased voltage potential between adjacent wires like that.

MileHigh

@MileHigh: good idea.

To see the increased voltage potential between adjacent wires I have to lead out two adjacent wires?

In the monofilar coil, a loop of wire from two consecutive turns?

In the bifilar coil, a loop of wire from the pair of wires?

Greetings, Conrad

Coil formers are ready, see attached photo.

The aim is to wind ~ 200 mH coils.

In order to be able to count the windings (to have the same number on both coils) I need paper between each layer (which gives me a smooth surface to lay the next layer upon, also makes it easier to see if the wires are close together). The whole point is to have identical coils (besides monofilar and bifilar).

I guess this will increase self capacitance (increased space between wires)?

Greetings, Conrad

@Gyula:

This table says that paper has dielctric constant of 3 (air 1) http://members.gcronline.com/cbrauda/0007.htm.

The plastic sheets I have are similar.

It is probably hard to say whether the paper between layers will distroy some effect one would want to detect? It sure destroys the purpose of the test if the two coils are largely dissimilar. Therefore I like the paper because it insures "same number of turns".

I wound quite a lot of coils and paper (or plastic sheet) between layers are a big help. Without a separator between layers one needs thick wire not to mess up. But with thick wire the inductance goes down at the same size.

I got the idea of a seperator sheet between layers of windings when taking wire off old transformers to get the core. Although the wire was quite thick, there were seperator sheets. May be to absorb heat but I thought it was to keep the windings neatly spaced.

Greetings, Conrad

Conrad:

Let me make an ascii schematic of the coil.  I am going to assume that you are going to use a wire pair to wrap the bifilar coil - i.e. the source wire has two conductors.  So the way you will make the series bifilar coil is to connect the end of the 'left' wire to the beginning of the 'right' wire in the pair.

So we have the start, middle and end contacts.  Let's call them TP1, TP2, and TP3.

Then if you make a loop that is half way, you have two more contacts, let's call them LP1 and LP2.

Let's pretend the wire (pair) is 100 turns.

        <Turn1>                                   <Turn 100>
        <inner spool>      <middle>    <outer spool>
TP1--WWWWWWWWWW(LP1)WWWWWWWWWW--TP2
TP2--WWWWWWWWWW(LP2)WWWWWWWWWW--TP3
        <Turn 101>                              <Turn 200>


So, you notice that the TP1-TP2 voltage and the TP2-TP3 voltages already have 100 turns of the bifilar coil "distance" between each other.  So in theory you don't have to have LP1 and LP2 to see the "full bifilar separation."  The addition of the loop has limited value because there are other test points that do the same thing.  However, this is an exploratory mission, and they can't hurt, so why not add them?  (Note you only need one hole per spool.)

LP1-LP2 (turn 50 and turn 150) also have 100 turns "distance" but I am doubtful that you will see much difference between the loop sensing points and TP1-TP2 or TP2-TP3.

I suppose the question is will there be anything interesting when you look at the potential difference between the various test points.

Now let's assume that you are going to make a monofilar coil of 200 turns with the equivalent one-conductor wire.

So you have this:

        <Turn 1>                                  <Turn 200>
TP1--WWWWWWWWWW(TP2)WWWWWWWWWW--TP3

So you can see that you have a monofilar equivalent here.  There are 100 turns of "distance" between TP1-TP2 and also TP2-TP3.

In most cases you probably will see almost the same waveforms for both coils when you make voltage differential measurements that are 100 turns apart from each other.  It doesn't mean that there isn't any extra energy stored in the "capacitance" of the bifilar.  I used quotations just as a reminder that this capacitance is normally not active or visible at lower frequency ranges.  A capacitor with what can become a dead short across it's "plates" after a short amount of time is a strange breed of capacitor.

It's very important to mention a familiar theme:  If you are playing with the coils in their self-resonant mode, there may be observable differences but the whole operation at that frequency is out of the practical usable frequency range for the coil.  Is there any practical application for a bifilar (or monofilar) coil in self resonance?  Can you do anything with it?  I know it's the question that some people hate, but it's a perfectly valid question.

MileHigh

Conrad:

>>>To see the increased voltage potential between adjacent wires I have to lead out two adjacent wires?

For the bifilar you lead out two adjacent wires and for the monofilar you lead out one wire only.

>>>In the monofilar coil, a loop of wire from two consecutive turns?

As shown in the diagram, you just lead out one wire.

>>> In the bifilar coil, a loop of wire from the pair of wires?

Yes.

Going back to the frequency domain, lets say for a suggested test you connect your signal generator to a 50-ohm series resistor and then connect that across the coil.  As you sweep up in frequency the AC impedance of the coil will increase.  So more and more of the voltage drop will appear across the coil as the frequency increases.  You can try different resistors.  In all likelihood you will observe the same behaviour for both coils.  When you get to very high frequencies, you may start to see slight differences between the coils.

This is a basic bare-bones test that looks at the frequency response of the two coils.  That's dependent on the inductance of the two coils and since we know that the inductance is the same therefore the frequency response characteristics should be the same.

Permit me to take the stage for a second:  Now is a golden opportunity for someone to suggest some test and/or application that exploits the slightly larger tiny capacitance that has a higher potential difference between the two half-coils of the bifilar coil.  I am talking a real test that can be explained with sufficient clarity that Conrad can consider doing.

MileHigh

As far as filters go, a series LC circuit is a what's called a band-pass filter.   It lets a certain range of frequencies pass power from the input to the output:

(signal source) -> (series LC) -> load resistor -> Gnd.

A parallel LC circuit in the same setup is called a notch filter because it prevents a certain range of frequencies from passing power from the input to the output, i.e."the notch."

(signal source) -> (parallel LC) -> load resistor -> Gnd.

If you know the impedances of series LC and parallel LC circuits at resonance it should all make sense.