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D:\A NJQRP Projects\Balun\Balun W1CG Low Power Balun Designed by: Charles Greene, W1CG Kitted by: the NJQRP Club Copyright 2002 by Charles Greene, W1CG and the NJQRP Club. All rights reserved. W1CG Low Power Balun Kit 1 A Low Power 4:1 Current Balun Kit by Charles Greene, W1CG [email protected] Over the last two years I have build, designed, tested and built many baluns -- and have had lots of fun doing it! The perfect, tiny, low loss balun still eludes me, however, but I keep on trying. One of earliest baluns I built just happened to have good performance. So I investigated its design features, improved its performance, rebuilt it using readily available materials, exten- sively tested it and I am presenting it here as a construction project for all homebrewers to enjoy. INTRODUCTION coiled on a magnetic toroidal core or rod it is The open wire balanced transmission line broader band than the uncoiled balun, and is widely used on wire antennas in an at- the length of the transmission line is less. As tempt to reduce losses and unwanted radia- there are balanced currents in the transmis- tion from the transmission line and as a small sion line, the net flux in the core is zero, so lightweight alternative to coax for portable high efficiencies can be achieved. (Ref 1, Chap use. It benefits from a balun (BALanced to 1). A schematic of a 4:1 current balun is UNbalanced device) to convert the unbal- shown in Figure 1A. anced output from an antenna tuner to the In 1957, Ruthroff (Ref 1, Chap 1) used balanced input of the transmission line. The the property that a potential gradient exists operator who wants a small, lightweight balun along the length of a single transmission line for back packing or fix station use for low to introduce the “voltage” balun. By con- power or QRP operation doesn’t have much necting the wires so that the direct voltage choice. Either he buys a large, heavy com- adds to the delayed voltage, a 1 to 4 voltage mercial balun or else searches the Internet ratio is achieved. A schematic of a 4:1 volt- for the design of a home brew balun, the age balun is shown in Figure 1B. The years performance of which is unknown. To help following the introduction of the voltage fill this void, this project concerns a small, balun saw its increasing use over the more low power 4:1 current balun that is easy to complex current balun. It is only recently build and has good performance from 160 that there is resurgence in use of the current (B) through 10 meters. A 4:1 balun is a good balun due to its inherent advantages. match for feeding an Off-Center-Fed antenna and performs well feeding an open wire trans- There are two problems with the voltage Figure 1. Two types of coiled transmission line 4:1 baluns using ferrite or pow- mission line, so that is the type presented. balun that limit its usefulness. Because its ered iron cores: (A) The Guanella or Current balun; a two core balun like the one principle of operation depends on a phase described in the text is shown. (B) The Ruthroff or Voltage balun. BACKGROUND shift in the voltage along a transmission line, There are essentially two types of coiled the phase shift becomes inadequate to sus- transmission line baluns using ferrite or pow- tain the voltage ratio as the frequency in- current to the load and increases losses. working into a high impedance load. For ered iron cores. The first one was introduced creases. The second disadvantage is a result example, the balun described in this article by Guanella in 1944 and is known as the The current balun operates on the prin- of the first. In an attempt to extend the high has a SWR of less than 1.08:1 into a non- “current” balun. This balun consists of two ciple of summing the voltages in each of four frequency range, fewer turns are used until inductive load of 200 ohms from 700 KHz coiled transmission lines with the inputs con- coils, and eddy currents in the windings limit there is barely adequate inductance for suffi- to 30 MHz. I tested a single core of this two nected in parallel and the outputs in series. its highest frequency. Therefore, more turns cient choking action to limit the primary cur- core current balun with the coils connected The impedance ratio is 1:N^2 where N is the can be added until eddy currents start to limit rent at the lowest frequency. Working into as a voltage balun (shown in Picture 1) and number of wires making up the transmission operation at the highest frequency. The ad- higher impedance makes matters worse as checked its frequency coverage. The low line. Two wires have an impedance ratio of ditional turns significantly increase choking the choking impedance does not change and frequency performance of the voltage and four times and three wires an impedance ra- action at the lower frequencies, and the ef- it becomes a smaller percentage of the total current baluns is similar, but the performance tio of nine times, for example. Guanella also fectiveness of the balun does not decrease as impedance. This causes too much of the of the voltage balun starts to fall off above showed that when the transmission line is rapidly as that of the voltage balun when wrong current to flow which decreases the 19 MHz. It has a SWR into a 200 ohm non- 2 W1CG Low Power Balun Kit W1CG Low Power Balun Kit 3 load), but the SWR was low down to 500 #22 wire harder to work with than the #24 KHz, which is well below the desired fre- wire, and any improvement in wire loss quency. This indicates that the design should would be slight. However, I resolved to check have fewer turns. So the next balun of the the loss of all three baluns during loss mea- design had 13 turns. The 13-turn balun has surements, and use that as one of the criteria an inductance of 101 uh and its SWR is low in selecting the wire size for the balun of and is flat from 700 KHz to 30 MHz. design. The wire size can be either #24 or #26 It is possible to construct a 4:1 current enamel insulated wire. I constructed a balun balun by winding both pairs of windings on of each, and their performances are essen- a single core. However, in my experience, tially the same. The #24 wire is easier to the performance of the single core 4:1 cur- work with and has slightly less wire loss, so rent balun is not as good as the two-core that is what we will use. To verify the wire current balun. The SWR is high and is not size to give 100 ohms for Zb, I wound a flat across the HF spectrum. Therefore, this balun using #22 wire and measured its per- balun is wound on two FT114-43 cores (Fig- Picture 1. A single core of the two core current balun formance which was a little less than that of ure 1A). Now that we have a preliminary with the coils connected as a voltage balun. the other two. It had a somewhat higher design, we need to describe how to wind it and then test its performance. inductive load of 1.2:1 and higher above 19 at the lowest frequency to choke off the pri- SWR with a 200-ohm load. I also found the MHz. In addition, Lewellen reported that mary current. For the 50 to 200-ohm balun, his tests showed that the current balun has Zo should be 500 ohms at the lowest fre- CONSTRUCTION improved performance over the voltage balun quency. The inductance to produce this im- under all conditions. (See Reference 4.) pedance at 1.8 MHz is 44 uh. Parts List prevent marring the surface of the core. Lead A PRACTICAL DESIGN Now we will pick a size. For ease in con- o 2 Ferrite toroidal cores, FT114-43 the long end of the wire pair through the hole Some design guidelines are set forth in struction and for a small balun for portable and without twisting the wires pull them the current literature (Ref 1, 2) for building o 6-ft #24 NYSOL magnet wire use, we will pick a size of about 1-inch. In away from the core and cut them to the same high performance baluns. I will avoid the math o 6 4” nylon cable ties order to get sufficient inductance (44 uh) we length. and the theory and just use some approxi- will need a material with a mid range perme- o 1-ft 3/64” heat shrink tubing mations as practical guidelines. As can be ability. We can not use low loss powered Tools 4) This is the first turn. Each time the wire seen from Figure 1A, when working into a iron material with permeability in the range n Vise (recommended) passes through the center is counted as one balanced load, each pair of windings sees half of 1 to 75 because we cannot get enough turn. Place a 1/8” length of heat shrink tub- n Large soldering iron or solder pot for in- the load Zl: 100 ohms for a 50 to 200 ohm turns of #24 or #26 enamel insulated wire on ing over the wire pair where it passes around sulation stripping 4:1 balun.
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