1. Home
  2. Electronics Design
  3. Coupling of air core coils
  4. Page 2

Coupling of air core coils

I'd like you to work with me a little more on this, because I don't understand if we agree or not.

Remember we are inducing a B field in the coil(s) with an electromagnetic wave not with a current. The B field then induces current in the wire.

Is that said so it is understandable?

So if at some point the B field is going right to left through the center of a solenoid coil with two windings, one wound contra to the other, the current in one wind will flow left to right, and in the contra coil it will flow right to left.

Do you agree with this?

I'm lost here the direction of what? B field, current flow?

If you mean, no physical property of the coil-as-wound determines the direction of the current, I would disagree and say a contra wound coil has current flow in the opposite direction than a conventional wound coil.

Yes, I agree, but if one coil has current flowing in the opposite direction it needs to be connected with that in mind.

Do we have agreement?

Thanks, Mikek

Reply to
amdx
Loading thread data ...

This was important and sent me off on really understanding why Ben had the strange connections. The answer is that a contra wound coil's current flows in the opposite direction when compared to a conventional coil. But it took me a long time to get that to sink in. Thanks!

This is from the another page and explains why the contra coil was developed/used. This is from Ben Tongue of Blonder-Tongue fame. "Here is the reason for this winding scheme: (Mike here, the scheme is a contra wound coil) This crystal radio set design connects component inductors #L(1) and #L(2) in series for the lower half of the BC band and in parallel for the upper half. If the two coils were wound in the same direction from the hot to the cold end, as was done in the crystal radio set described in Article #22, distributed capacitance would be low in the series connection (about 7.7 pF) and higher in the parallel connection (about 21 pF), mainly because the finish (ground end) of component inductor #L(1) is located close to the start (hot end) of component inductor #L(2). This reduces the Q at the high end of the BC band. If the coils are contra wound, as I call it, the lower distributed capacitance condition occurs in the parallel, not the series connection, resulting a Q increase of approximately 17% at 943 kHz. It is increased even more at the high end of the BC band." From this long article about a constant bandwidth crystal radio.

Most crystal radio coils will have a high Q at the low end and then Q drops as frequency increases, the contra wound coil is an effort to overcome some of this degradation.

Mikek

Reply to
amdx

Repeat from another post. And that change in parasitics is the reason for the contra coil. From Ben's webpage.

"Here is the reason for this winding scheme: (Mike here, the scheme is a contra wound coil) This crystal radio set design connects component inductors #L(1) and #L(2) in series for the lower half of the BC band and in parallel for the upper half. If the two coils were wound in the same direction from the hot to the cold end, as was done in the crystal radio set described in Article #22, distributed capacitance would be low in the series connection (about 7.7 pF) and higher in the parallel connection (about 21 pF), mainly because the finish (ground end) of component inductor #L(1) is located close to the start (hot end) of component inductor #L(2). This reduces the Q at the high end of the BC band. If the coils are contra wound, as I call it, the lower distributed capacitance condition occurs in the parallel, not the series connection, resulting a Q increase of approximately 17% at 943 kHz. It is increased even more at the high end of the BC band." From this long article about a constant bandwidth crystal radio.>

formatting link
Most crystal radio coils will have a high Q at the low end and then Q drops as frequency increases, the contra wound coil is an effort to overcome some of this degradation. Mikek

Reply to
amdx

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.