Balancing IB-PI coils

[Tinkerer]

Balancing IB-PI coils. How difficult is it?
I got used to do it, so it seem easy. But many do not dare to try to build an IB coil because they think it is very difficult and needs high precision coils.

Today I ripped my experiment coil apart to build the new one. So I tried a quick experiment.
Could I balance just ANY coil?

a long time ago I wound a 1mH low capacitance coil. It did not work for what I had in mind so it was just left on the shelf, still on the winding form.

Could I balance that with the 325uH polypropylene spaced TX coil?

Well, yes it did balance. Took about 30 seconds. Would it be sensitive too?
Would it discriminate FE?

Attached are pictures.
#1 the 2 coils in 00 position
#2 the targets
#3 No target
#4 the gold ring
#5 the steel ring.

Tinkerer

[Tinkerer]

Balancing IB-PI coils

Lets have a look at the pictures.
The scope pictures show a waveform that is reasonably balanced. The Sampling window shown is 100uS.
The trace is thick, this means noise. The reason is that the coils are not shielded.
The gold ring target gives a negative response, the steel ring target gives a positive response.
The response to the steel ring is relatively weaker, unlike with a traditional PI, where FE or steel gives the strongest response.
The response of the gold ring is short in time, corresponding to a short TC.

The coils: TX 325uH and RX 1000uH, very different but they balance. The balance is achieved by overlapping the 2 coils. The current in the coils runs in opposite direction or 180 degrees apart in phase.

The sensitive part of this coil arrangement is the part where the 2 coils overlap. As you can see, this is a small area, yet the coils are large and cumbersome.

This is the reason that we usually make the IB-PI coils concentric, that means the 2 coils use the same center point, or DD, that is, we take 2 round coils and shape them like D's and then overlap the straight part of the 2 D's.
This makes a narrow but long overlap and therefore a sensitive region of the same shape.

I stop here for today. There are many on this forum who know more about DD coils than I do, please chip in and give the "Unknowing" in search of information access to your knowledge.

Tinkerer