Ferrite Core Material & Baluns - HomeBrew Forum at eHam.net

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eHam.net Forum : HomeBrew : Ferrite Core Material & Baluns

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Ferrite Core Material & Baluns	Reply
by WD4HXG on October 5, 2009	Mail this to a friend!
For years I have wound small baluns using beads made from Fair-Rite 73 material for use at lower HF frequencies. While looking into making a balun to operate with a KW it became painfully obvious that 2.5 inch cores were not available in 73 material. Fair-Rite in their app notes provided graphs showing losses as a function of frequency for differing core materials. 73 material was the gem of the batch in the HF frequency range but due to a phenomena called 'Dimensional Resonance' 73 material cores are only available in small diameter sizes. Anyone have experience with high power losses in the HF frequency range (1 MHz to 30 MHz) using Fair-Rite material mix 31?

RE: Ferrite Core Material & Baluns	Reply
by W8JI on October 6, 2009	Mail this to a friend!
What type of balun are you making? Different types have different core requirements. Where are you using it? Different applications have different stresses on the balun.

RE: Ferrite Core Material & Baluns	Reply
by HFRF on October 7, 2009	Mail this to a friend!
I have used 61, 43 ferrite material for 40 years with good results used for making baluns and inductors. Iron power -2 material works ok but you need more turns to get usable inductance.

RE: Ferrite Core Material & Baluns	Reply
by WD4HXG on October 8, 2009	Mail this to a friend!
The intended application is for use in a Guanella design covering from 1 MHz to 30 Mhz hopefully with less than 0.25 dB loss at 50 ohms in and out. I had specifically looked at 2.5 inch diameter cores in an effort to increase the mass for thermal considerations but 73 material is not used to manufacture cores of that size. 31 material was suggested as a substitute but the specs for 31 material has a Curie temp limit 30 degrees lower than 73 material. Past core heating problems when operating with VSWR's of 2:1 is troubling in light of the reduced Curie temperature. 260 degrees F seems high but smaller cores have heated rapidly in previous use to the extent that they burned the ends of the fingers used for testing. Additionally the desire to use 2.5 inch cores was to facilitate winding when using flat windings to break the 110 ohm boundary imposed by bifilar wire with round conductors. The balun is intended for use in an antenna tuner at the input with mechanically linked dual L networks. However after reading the notes from W7EL and W8JI will require a revisit of that idea. During the meantime I have acquired 2.5 inch 31 material cores and will go ahead and build a couple of baluns for characterization on a network analyzer. Thanks for the feedback.

RE: Ferrite Core Material & Baluns	Reply
by KU3X on October 13, 2009	Mail this to a friend!
If you are going to make a Guanella balun using a ferrite core, this is what you need. Amidon FT 240-61 material is permeabilty 125 and is good for 6 to 80 meters. You need to wrape 12 turns through the core. If you need to cover 160 meters, Amidon came out with a replacement core for the 250 permeabilty core they had before. The replacement core now is 300 permeabilty. That core number is FT 240-K. You will need to wrap that core with 11 turns to cover 160 meters. A lot of hams use a #43 material and it seems to work good. But the proper core to use is the K core. If you do not want to use coax to make your balun, use two #16 or #15 enamel covered wire, 12 bifilar turns taped close together. #16 enamel wire closely taped together come out to 50 ohms of transmission line. I use #15 thermelized wire and it is close enough to 50 ohms and has a very high voltage breakdown. The enamal wire I use has a working voltage of 12,800 volts at room temp. Amidon has a $50 minimum order and I think the K core is about $18.50 each? Good luck with your project. Barry, KU3X

RE: Ferrite Core Material & Baluns	Reply
by W8JI on October 13, 2009	Mail this to a friend!
You understand of course the ferrite core adds a combination of reactance and resistance to the system. Even a very small amount of series equiv loss resistance, or a fairly high parallel equiv resistance, can get pretty hot. The Q of a 73 materials reaches unity around 2 MHz, where it has about equal resistance and reactance. What you want, to minimize heating, is to increase the Q. As core stresses get higher you want a better Q core, and that generally involves using a larger core length (window depth) in parallel with the winding to keep impedance high. This is because lower loss cores also offer less impedance per linear length in parallel with the winding. You might get about 100 ohms per inch of window depth on 2 MHz with a 73 mix, and only 10 ohms with a 61 mix, so you need either more turns or a deeper core or a combination of the two to get the same impedance. Once you have that impedance the 61 material would heat significantly less because it has a much higher reactance compared to loss resistance, plus you use more material to spread the heat out. 31 materials might work, or if you have really high common mode excitation of the core you might have to drop much lower in ui until you get the loss tangent low enough. The exact material would depend on the frequency, the impedance needed, and the voltage from end-to-end on the winding. Tom

RE: Ferrite Core Material & Baluns	Reply
by WD4HXG on October 18, 2009	Mail this to a friend!
I need to look into relationships between various Losses, Impedance Measurements and Initial Permeability. Obviously I am overlooking something here and until that is sorted out I will just be blundering along. Thanks for the tips. Will run some mesurements this week and be back to ask more questions.

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