Ferrite beads - Elmers Forum at eHam.net

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Ferrite beads	Reply
by VE1CZ on March 31, 2009	Mail this to a friend!
Seeing that choke baluns are somewhat effective I have decided to try building a a line isolator.I see these on the commerical market and was wondering what type of ferrite beads and how many are needed to make an effective isolator for HF?

RE: Ferrite beads	Reply
by WA3SKN on March 31, 2009	Mail this to a friend!
I am not sure what you mean by a "line isolator". Are you planning on an RF transformer for HF or a power transformer? -Mike.

RE: Ferrite beads	Reply
by W5DXP on March 31, 2009	Mail this to a friend!
"Line isolator" is what the folks at Radioworks call a choke. http://www.radioworks.com/ccwcover.html Ferrite beads are inefficient compared to ferrite toroids. I would build the isolator using ten turns of small coax on a toroid like the FT-240-77. One needs a lot of beads to achieve that same 10 turns-squared toroidal choking action. -- 73, Cecil, w5dxp.com

RE: Ferrite beads	Reply
by N1LO on March 31, 2009	Mail this to a friend!
What Cecil said. He nailed it! --...MARK_N1LO...--

RE: Ferrite beads	Reply
by N1LO on March 31, 2009	Mail this to a friend!
http://www.eham.net/forums/elmers/215920

RE: Ferrite beads	Reply
by VE1CZ on April 1, 2009	Mail this to a friend!
Now that sounds like what I need , so tnx for the replys. Bob

RE: Ferrite beads	Reply
by N1LO on April 1, 2009	Mail this to a friend!
10 turns through 1 toroid = 100 single-pass beads (!), and type 77 is much more effective (lossier) at HF than type 31.

RE: Ferrite beads	Reply
by G3TXQ on April 1, 2009	Mail this to a friend!
And because Type 77 material is lossy, make sure you follow Cecil's advice and use small coax for the winding. K9YC warns against using bifilar windings on lossy material because of the significant differential-mode flux leakage into the core and the possibility of overheating. Steve

RE: Ferrite beads	Reply
by VK1OD on April 1, 2009	Mail this to a friend!
Elmer said "10 turns through 1 toroid = 100 single-pass beads (!)". That quite ignores the dimensional difference between a typical toroid that would accomodate 10 turns of coax, and a typical bead used in a W2DU style balun. Further differences may include core material and the effects of stray capacitance. Neither toroids nor beads are standardised. The concept that impedance is proportional to turns squared ignores the effect of stray capacitance which is the factor that limits the number of turns that can be used effectively at higher frequencies. Whilst Walt, W2DU, described his balun design with 50x#73 (A$50) or 25x#43 beads, they were only 5mm long. My favorite beads for such baluns are 20mm and 25mm long, so just 12 beads (total A$9) is sufficient for most applications. (http://www.vk1od.net/balun/W2DU-LF1260/index.htm) There is no standard equivalence that 1 toroid = 100 beads. The W2DU style has some advantages in some applications, including that the effects of self capacitance will be less than cramming many turns of coax on a small toroid. The term "isolater" seems to refer to a 1:1 current balun or common mode choke, and it seems mainly used by some manufacturers of antennas that include magic proprietary components (eg the isolator). Owen

RE: Ferrite beads	Reply
by G3TXQ on April 1, 2009	Mail this to a friend!
>>VK1OD wrote: The concept that impedance is proportional to turns squared ignores the effect of stray capacitance which is the factor that limits the number of turns that can be used effectively at higher frequencies. << Owen, of course that's correct, but despite the stray capcitance - or perhaps because of it - multiple turns on a single ferrite toroid can deliver impressive performance across the HF spectrum in a conveniently small form. K9YC measured the impedance of a choke comprising 14 turns of coax on a single T240-31 core as follows: 1MHz 3500 ohms 5MHz 5500 ohms (peak Z) 10MHz 5000 ohms 20MHz 2500 ohms 30MHz 1500 ohms K9YC also measured the impedance of W2DU-style chokes using type 31 material. He needed 40 beads (total length 4ft) to get close to Z>1000 ohms from 1-30MHz. From the data I've seen published, if you needed Z>3000 ohms across 160m to 20m, multiple turns on a toroid would be by far the more practical way to achieve it. Steve

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