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Author Topic: Question on ferrites  (Read 6397 times)
W6EM
Member

Posts: 882




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« Reply #15 on: March 03, 2013, 09:53:59 AM »

Quote from: W6EM

Interesting postulate.  Please give me a technical reference, and not from an amateur radio publication.




Hard to find it in a non-ham publication, because any Engineering text would
leave it as an exercise for the reader.

But try this:  Terman* gives the inductance of two coupled coils in parallel as

Ltot = (L1 * L2 - M * M ) / ( L1 + L2 -/+ 2M )

      (we use the minus sign in the last term because the fields are aiding)

where L1 and L2 are the coils and M is the mutual inductance.  In the case
in question, L1 = L2, and M = k * the inductance of either winding, where k
is the coupling coefficient.

Unfortunately the formula reduces to 0 / 0 when k = 1 and M = L, but
if you solve for the limit as k -> 1 (or M -> L) you'll find that this reduces
to L * ( 1 + k ) / 2, so when k = 1, Ltot = L.

Or you can simply solve the original equation for M = 0.90 L and for M = 0.99 L
and draw your own conclusions about how close Ltot will be to L.

* Terman, Fredrick Emmons, Radio Engineers' Handbook, First Edition,
McGraw-Hill, 1943, pp 64-65


While it approaches the value of L, it does indeed "blow up" if one arrives at a k of 1.  Terman doesn't do a good job of explaining himself.  Perhaps a coupling coefficient of 1 cannot be realistically achieved.  So, in reality, L1 and L2 in parallel will always be something less that 1.  Which will yield a total inductance of something less than L, for n identical windings with the same sense in parallel.  BTW, the induced voltage in L2  from a test current in L1 opposes forced current flow, hence the subtraction of M.  Just the opposite if the winding direction were opposite for L2.
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W5DXP
Member

Posts: 3639


WWW

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« Reply #16 on: March 03, 2013, 01:31:40 PM »

...you can increase magnetic field intensity to a point beyond which flux density ceases to increase.

Sorry, I was talking about a linear system and didn't realize that non-linearity was being considered. When anything goes non-linear, all (linear) bets are off.Smiley
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WA3SKN
Member

Posts: 5526




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« Reply #17 on: March 04, 2013, 05:24:51 AM »

OK, so much for theory!
Sounds like you have interference to your smoke alarms in a high RF environment.  The simplest answer is to just try it!  If it works...Great!... if not... try something else.  The answer to interference is shielding, filtering and distance (moving the antenna).  And any combination that works is OK.
Good luck with the project!
73s.

-Mike.
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WX2S
Member

Posts: 746




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« Reply #18 on: March 08, 2013, 02:04:50 AM »

OK, so much for theory!
Sounds like you have interference to your smoke alarms in a high RF environment.  The simplest answer is to just try it!  If it works...Great!... if not... try something else.  The answer to interference is shielding, filtering and distance (moving the antenna).  And any combination that works is OK.
Good luck with the project!
73s.

-Mike.
Coda: I think I may have solved the problem after three attempts. Took a little research into how the detectors function. Posted here: http://www.eham.net/ehamforum/smf/index.php/topic,88568.msg658627.html#msg658627

73, -WX2S
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
G3TXQ
Member

Posts: 1530




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« Reply #19 on: March 08, 2013, 08:06:46 AM »

Try this Experiment:-

Wind 14 turns of Red/Black twin lead onto a T200-2 toroid and measure the inductances.

Inductance of Red winding = 2.7uH
Inductance of Black winding = 2.7uH
Inductance of Red + Black windings connected in parallel = 2.7uH

Steve G3TXQ
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