Coupling of air core coils

[amdx]

On 8/7/2019 2:08 AM, whit3rd wrote:

On Tuesday, August 6, 2019 at 8:13:32 PM UTC-7, amdx wrote:

But after to damn much thinking I see the currents flows opposite
direction even though the coils are on the same form.

Exactly right: B field has a current-direction dependence, and swapping terminals
changes it.

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?

No physical property of the coil-as-wound determines
that direction,

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.

unless you include all the electric connections.
You can connect two windings so the turns add, or so they subtract.

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

 

[amdx]

On 8/6/2019 12:23 AM, nuny@bid.nes wrote:

On Saturday, August 3, 2019 at 3:25:14 PM UTC-7, amdx wrote:
Hi all,
In the crystal radio world there is a contra-wound coil.
Two windings, one wound clockwise and the other counter clockwise.
This so the can be put in series or parallel.
I'm trying to understand why one of the windings is would counter
clockwise or contra wound.

No real reason to do so, but no real reason not to either.

I made a drawing and ask the question, because of the slight skew of
the winding wouldn't they couple better having the same skew vs, each
skewed opposite the other.
I made a simple drawing here,
https://www.dropbox.com/s/7am0jj5u4hm4red/coupling..gif?dl=0

By the way: you seem to have missed the bit in the instructions to have the "starts" together and the "ends" at opposite ends, but that doesn't matter except when you go to switch between series and parallel. You want to make sure where the "s" and "f" ends of both coils are.

(He has them "f-s" "s-f" but you have them "s-f" "s-f".)

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!

More info, > http://makearadio.com/coils/contracoils.php

Sorry, the whole thing looks to me like borderline woo-woo pseudoscience thought up by a hobbyist with no formal knowledge of EM physics. He doesn't seem to have mentioned any of the relative advantages and disadvantages of or other differences between pie-wound and solenoid coils, for instance. In pie-wound coils on the same form it should be immediately obvious that there will be no difference at all (again, as long as you get the phasing right) unless you're working at wavelengths so short as to be fractions of the winding length, same as with solenoid coils. Then you'll have nodes and peaks that must coincide.

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.
> https://web.archive.org/web/20160627181315/http://bentongue.com/xtalset/26St4bXc/26St4bXS.html

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

Mark L. Fergerson

 

[amdx]

On 8/6/2019 7:42 PM, tabbypurr@gmail.com wrote:

On Tuesday, 6 August 2019 19:31:11 UTC+1, amdx wrote:
On 8/6/2019 10:26 AM, tabbypurr wrote:
On Tuesday, 6 August 2019 14:31:30 UTC+1, amdx wrote:
On 8/6/2019 12:01 AM, tabbypurr wrote:
On Tuesday, 6 August 2019 03:20:29 UTC+1, amdx wrote:
On 8/5/2019 9:05 PM, whit3rd wrote:
On Sunday, August 4, 2019 at 5:30:17 PM UTC-7, amdx wrote:
On 8/4/2019 1:11 AM, amal banerjee wrote:
On Sunday, August 4, 2019 at 3:55:14 AM UTC+5:30, amdx wrote:

Hi all,
In the crystal radio world there is a contra-wound coil.
Two windings, one wound clockwise and the other counter clockwise.
This so the can be put in series or parallel.

Actually, it's so the turns count can be N1+N2 or (flipping them) N1 - N2, or anywhere inbetween.
It's a continuously variable inductor.

Your question has been answered repeatedly by various people. If you don't believe us, ok, build & test one.

Ok I'm assuming the answer is "it makes no difference" then why do the
gurus say that proper way to build it is contra wound? Other than they
are wrong.

Mikek

They aren't perfectly coupled. Contrawinding changes the parasitics. So they won't be identical.


NT

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.>
https://web.archive.org/web/20160627181315/http://bentongue.com/xtalset/26St4bXc/26St4bXS.html
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