To:  amps@contesting.com 

Subject:  [Amps] Ferriteloaded plate choke 
From:  Will Matney <craxd1@ezwv.com> 
Date:  Sun, 09 Jan 2005 13:39:48 0500 
Listpost:  <mailto:amps@contesting.com> 
Hi, From my experience, If I recall, ferrite is nonconductive (it's actually a ceramic). The only coated or plasticized cores that I know of are iron powder and strip wound. If enough insulation is installed, a ferrite rod will give a large amount of inductance with a relatively small number of turns. The lower number of turns for a high inductance gives a much lower "distributed capacitance", which is a major factor in choke resonance at high frequencies. My experiments have proved that for an amp operating 16010 meters, a plate choke with an inductance of 8001200µH is entirely stable on all HF ham bands. The problem with a choke with this level of inductance and able to carry the high level of DC current would be very large, and have a high level of distributed capacitance. A hollow core of an insulating coil form can hold a 1/2"dia. ferrite rod about 34" long. This rod will bring the typical inductance of a 90µH choke, up to 1200µH, with the distributed capacitance of the original 90µH choke. However, the higher level of inductance would cause any resonances to be far out of any HF ham bands. Ferrite and iron powder cores are frequency selective. Also, they saturate at low flux densities, say around 1800 to 2700 gauss. This then means the max DC current flowing through the choke has to create that amount of flux or less. Generally, it's better to figure for a lesser amount, say in this case 250 to 500 gauss less. This way you know the core will not saturate at it's maximum current rating. The material selection used in most RF applications is numbers 61, 63 and 43. Their permeabilities are 61 = 125, 63 = 40, and 43 = 950. Their maximum saturation flux densities are 61 = 2350, 63 = 1850, and 43 = 2750. These have all been used in broad band transformers and would be used the same in RF blocking chokes. They recommend that 43 material be used for frequencies between 2 and 35 MHz, and the other two between 6 and 30 MHz. The iron cores mostly used is number 2 mix (color red) and 6 mix (color yellow) with the following characteristics. 2 mix, u = 10, and freq. = 1 to 30 MHz. 6 mix, u = 8, freq. = 10 to 90 MHz. As you can see the iron cores have a low permeability. Circuit Q will be degraded if a core type is applied below its recommended frequency. However, cores can be used above their suggested operating frequency if the designer is willing to sacrifice permeability and use extra turns of wire. The main thing is to use the proper formulas to determine both the number of turns and the core size. Number of turns; N = 100 x sqrt of uH / Al N = number of turns, Al = inductance index by the manufacturer. Maximum flux density (B max); B max = E rms x 10^8 / 4.44 x f x N x A + N x I x Al / 10 x A E rms = applied voltage f = frequency in Hz A = core area in square centimeters N = number of turns I = DC current in the choke Al = inductance index The area (A) can be figured by simply rearranging the equation and placing B max in place of A. The area can then be built up by stacking toroids, and adding the areas of each cross section, or finding a core with the correct solid size. Keep in mind that this area is the area of the cross section of any core where its length has nothing to do with its operation. On a rod, it is the area of it's circular part or section, not the length. The use of a toroid is actually better than a rod because the magnetic path is made whole, or connected, and not open as a rod is so the rod has higher losses. You will see that the inductance index (Al) of a rod is much different than a toroid of the same cross section.
Will Matney _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps 
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