HM-9 Torroid Coil - QRP Forum at eHam.net

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eHam.net Forum : QRP : HM-9 Torroid Coil

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HM-9 Torroid Coil	Reply
by K8AXW on October 10, 2009	Mail this to a friend!
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

RE: HM-9 Torroid Coil	Reply
by NJ2E on November 8, 2009	Mail this to a friend!
Al, Try this web site http://www.kitsandparts.com/xtoroids.html The frequency is needed only for a resonance. Regards, Don NJ2E

RE: HM-9 Torroid Coil	Reply
by K8AXW on November 10, 2009	Mail this to a friend!
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

RE: HM-9 Torroid Coil	Reply
by WB6BYU on November 10, 2009	Mail this to a friend!
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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