Date: Mon, 19 Feb 2001 21:56:03 -0500 From: Charles Greene Subject: Re: [Elecraft] random wire ant At 02:08 PM 2/19/2001 -0500, Don Wilhelm wrote: >Charles and all, >Your core recommendation sounds great to me, but why #26 wire?? >I prefer to use much larger wire to reduce the resistive losses - like # 20 >or #22 for starters. Don and All, First of all, for less than 100 watts, the resistance of even #30 wire is insignificant in a 4:1 current balun. Most of the losses are dielectric but also are core material related. However, you may not want to use #30 wire for other reasons. Sevick writes and my tests bear it out, that the impedance of the transmission line making up the balun should be twice the impedance of the input or 100 ohms for a 50 ohm input. I did calculations, consulted tables in Sevick's book and came up with the result that the impedance of a transmission line on a balun is approx. 85% of that in open air. Close space enameled wire gives the following transmission line impedance when wound on a toroid: #22 = 75 ohms #24 = 88 ohms #26 = 98 ohms #28 = 110 ohms To get close spacing, twist once on the outside of the toroid for each turn or use heat shrink tubing. However, if you use insulated wire the spacing is greater and impedance greater. Using stranded, silver plated teflon insulated wire, I came up with the following: #18 = 108 ohms #22 = 122 ohms #24 = 135 ohms Sevick used #14 enameled with a teflon tubing for around 100 ohms, but he's talking KW baluns and I'm talking >100 watt ones. The next thing you need is enough inductance to provide 10 times input impedance at the lowest freq you are designing for, ie, XL = 500 ohms. This works out to be 44 uhy for 160 meters and 22 uhy for 80 meters. Actually, 5 times is an acceptable number but I use 10 times to be conservative. Then you look at the specs of the core material and read AL, the number of uhy or mhy per turn. Amidon catalog has the specs and the formulas for the number of turns you need for a given inductance which depends on the material and the size of the core. I have had excellent results with the -61 (mu = 125) and -43 (mu = 750) ferrite cores, and I have done some work with larger powered iron ones, - -2 material (mu = 10). The smaller powdered iron cores don't have enough mu for the lower part of the HF band but are fine for VHF. Type K ferrite material has a mu of 250, but it's smallest size is 1.25", and I figure I don't need that big a core. You can figure out how many turns of what size wire you can get on a given size toroid by calculating the inner circumference and the turns per inch of the wire you are using and see if that will give you the required inductance. The size wire you use is generally the size to give you around 100 ohms impedance. From above, that is #26 enameled and #18 insulated. However I have made some excellent baluns using #24 insulated and even #30 insulated, although the latter doesn't have a low SWR working into a 200 ohm resistive load (1.6:1). That doesn't appear to be important when working into a reactive load as presented by an all band dipole center fed with open line. You may ask why not a voltage balun. The current balun has a wider frequency range than the voltage balun. I never could get 160 to 10 with a voltage balun. 80-10 or 160-12 were the best. On the other hand, a typical current balun will cover 1 mhz through 30 mhz. For a couple examples, see N0SS's web page: http://home.earthlink.net/~n0ss/ 73, Chas, W1CG ++++++++++++++++++ Date: Sat, 01 Sep 2001 15:34:28 -0600 From: "James R. Duffey" To: Subject: [106238] Powdered Iron Cores!!!! Message-ID: Mime-version: 1.0 Content-type: text/plain; charset="US-ASCII" Content-transfer-encoding: 7bit Karl (Larsen, not Kanalz) - IN reply to your comments: "Hi Alan, the pictures on the web page sure show the toroid as being red which is an indication of a powdered iron device. I know for a fact that powdered iron cores make very poor baluns, that is, the balun doesn't display a balanced side. It should also be high loss." Well, let me make some comments about this and vent my frustation. 1. The toroid is not used as a balun core in the Z-Match Tuner. It is used to couple the two windings. You can find more details in an excellent article by Charlie "tuner" Lofgren in Antemma Compendium 5. I can't find my copy or I would quote the exact reference. 2. The efficiency of the Z-Match tuner is high, especially compared to other tuners, some of which use air core inductors. Bob kellog, AE4IC, did a nice set of antenna tuner measurements several years back. A summary is posted at: http://www.qsl.net/wd8rif/archives.htm#antennas The 6th article by Bob is particularly useful. 3. Iron cores can be used successfully as baluns if the balun is properly designed and used within the design parameters. Unfortunately, the iron core balun is usually used in voltage (transformer) 4:1 balun which is not optimum for amateur use for a variety of reasons, none of which are germain to this conversation. A 1:1 current (choke balun) is preferred See W7EL's article "Baluns: What they do and How They Do it" in Antenna Compendium 1. 4.The success of a current balun depends on supplying large inductance over a wide range of frequencies. It does not matter too much how one obtains this inductance. Air (very low permeability, certainly lower than iron cores) core baluns, such as discussed in the Handbook can be used effectively. These are not as broad band as some other balun types, but they are quite effective for tribanders, or 80/40 dipoles. 5. For a current balun, the loss of the core material does not matter. This is due to the fact that the core only sees the magnetic field induced by the common mode current that we are trying to eliminate. It doesn't matter if this is dissipated as heat, we are rid of it and that is that. Now for QRO use, the heat may cause failure in the balun, but at QRP levels, it will not matter. The core does not see the feedline current that we are trying to get to the antenna. The current is in oposite directions in the feedline and the induced magnetic field in the core is zero as a result. If we wind the balun with coax, the core is shielded from the transmission line currents by the coax. Again, the problem is with the balun design, not the core material. The difficulties you have seen with iron core baluns is due to the use of a 4:1 voltage balun design, not the iron core. In any case, the ZM-2 does not use the iron core as a balun. The ZM-2 is one of the most efficient tuners made for QRP use. You won't go wrong by obtaining one. - Dr. Megacycle KK6MC/5 -- James R. Duffey KK6MC/5 30 Casa Loma Road Cedar Crest, NM 87008 +++++++++++++++++++