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Author Topic: HM-9 Torroid Coil  (Read 766 times)
K8AXW
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Posts: 3672




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« on: October 10, 2009, 08:07:01 PM »

Greetings all:

I bought a Heath HM-9 QRP watt/SWR meter on eBay only to find it was set up for VHF operation.

I have replaced all the parts for HF operation but have run into a wall on replacing the torroid coil.  It is rated at 29.5uH.  I can't determine how to wind one because I have no idea what frequency to plug into the torroid winding formula.

Any information or source of a 29.5uH coil would be appreciated.

73

Al - K8AXW
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NJ2E
Member

Posts: 35




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« Reply #1 on: November 08, 2009, 05:47:39 AM »

Al, Try this web site http://www.kitsandparts.com/xtoroids.html The frequency is needed only for a
resonance. Regards, Don NJ2E
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K8AXW
Member

Posts: 3672




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« Reply #2 on: November 10, 2009, 07:57:12 AM »

Hi Don,

Thank you for your reply.  I've seen this chart before but it (and still is) kinda confusing to me.  I'll get into it again (methodically) and see if I can come up with something.  In the meantime, I'm hoping Santa will bring me an inductance meter that I've been lusting after for over a year.  

I think with this chart and that meter will make a facinating project(s).

Will be back.

73

Al - K8AXW
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WB6BYU
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Posts: 13028




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« Reply #3 on: November 10, 2009, 08:34:11 AM »

Frequency in the toroid winding formula???

For each core type, the inductance is simply based on the
number of turns, not the frequency (at least for the first
order effects.)  If you are using a T-50-2 core the
referenced table shows the AL value is 49 uH / 100 turns.
(That's the red "49" under the "T50" column header.)

In your case you want 26.5 uH, so start with 26.5 / 49 = 0.54
as the proportion of the inductance at 100 turns. Because
inductance is proportional to the square of the number of
turns, take the square root of 0.54 = 0.74 and multiply
this times the 100 turns = 74 turns.

 I suspect, however, that in your application you'd want
to use a ferrite core for broad-band performance, in which
case you might choose an FT-50-43 core instead (the
purple numbers in the table.)  In this case you can
simply look down the column and see that your desired
inductance falls between 6 turns = 18.8 uH and 8 turns =
35.5uH.  So 7 or 8 turns should be close enough.  With
the smaller FT-37-43 core 8 turns would give you 26.9uH
according to the table.

So the first step is to choose the core type, then use
the table (or just the AL value for that core type) to
calculate the required number of turns to get the
inductance desired.
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