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Author Topic: Litz Mobile Loading Coils  (Read 428 times)
K6STI
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Posts: 18




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« on: July 30, 2019, 02:55:09 PM »

I've updated my coil inductance and Q calculator to handle Litz wire with 0.02 mm strands. This lets it analyze Litz loading coils for 40-meter mobile antennas.

For example, I designed a 24.4 uH coil with 1000 strands of 0.02 mm wire manufactured by Elektrisola. The coil uses about 15 feet of the wire on a polystyrene form 3.7" in diameter and 2.7" long. Q is > 2000. A Master Mobile Air Dux coil of the same inductance uses #12 solid wire and has a measured Q of 541 (without the cylindrical plastic enclosure). It is 2.5" in diameter and 5" long. Restricting the Litz coil to the same diameter drops the Q to about 1850.

Litz wire with #48 strands is more common. This wire is suitable for 75-meter mobile loading coils. A 50 uH coil for 3.8 MHz using 1100/48 Litz yields Q > 2400.

RF resistance for the Litz coils is about 0.5 ohm. This means that antenna efficiency is almost completely determined by ground losses.

The program includes an automatic optimizer that varies selected coil parameters to maximize Q while keeping inductance constant. The documentation includes measurements for a couple dozen solid-wire coils made with an HP 4342A Q meter. Average program accuracy was 2.4% for inductance and 7% for Q.

The program runs in Windows and can be downloaded here:

http://ham-radio.com/k6sti/coil.zip

Brian
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N5YM
Member

Posts: 18




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« Reply #1 on: July 31, 2019, 04:21:10 PM »

Thanks Brian. 

Your efforts in producing this application are appreciated by those of us who like to mess around building some of our own equipment.

Mike
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K6STI
Member

Posts: 18




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« Reply #2 on: July 31, 2019, 05:36:53 PM »

Thanks, Mike.

I forgot that I had measured the Master Mobile coil at 2.52 MHz, not 7.15 MHz where I modeled the Litz coil. At 2.52 MHz, Q measured 541 (2.5" leads). The program calculates Q as 508 for the same lead length. The two values are reasonably close so I'll use the program to predict the Q at 7.15 MHz since the HP 4342A can't measure it there. Q calculates as 705 and inductance as 30.1 uH (no leads). A 30.1 uH coil made of 1000/.02mm Litz wire has a calculated Q of 2015. So the proper Q comparison between the Master Mobile coil and a Litz coil with the same inductance is 705 and 2015.

Brian
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