W2FMI 6:1 LB300 Low Power Balun
6:1 Balun - 300 ohms balanced to 50 ohms unbalanced (coaxial)
A 6:1 balun matching 50-ohm cable to a balanced load of 300 ohms can be used when a folded dipole, made from 300-ohm ribbon, at a height of 0.22 wavelengths above ground (at o.33 wavelengths. the input impedance is close to 450 ohms) The balun can be placed at the feedpoint of the folded dipole or in the operating room if the feedline is also 300-ohm ribbon. For off-center fed antennas with 300-ohm impedance and for any 300-ohm unbalanced antenna, use the 6:1HB/U300. The HB/U300 is both a current and a voltage balun and can also be used to match 50- ohm coaxal to any 300-ohm balanced load.
This Balun is a low power, compound-balun transmission line transformer designed to match 50 ohm coaxial cable to a balanced load of 300 ohms. It consists of a 1:1.5 Unun (50:75 ohm) in series with a 4:1 (75 to 300 ohm) Guanella balun. The impedance ratio is consitant from 1.7 MHz to 45 MHz. The power rating is 150 watts continuous and 300 watts peak. The efficiency is 97 percent.
The schematic digram below illustrates the construction of this Balun. The 1:1.5 Unun on the left has six quantifilar turns on an F125-K The top winding is #16 Formvar SF wire. The other four are #18 Formvar SF wire. The 1:4 balun, on the right, has eight bifilar turns on an F150A-K. The wires (which are actually hook up wires) have a 12-mil coating on #19 wire. They are further separated by a 50-mil OD teflon tubing. The characteristic impedance of the windings is 150 ohms.
(NOTE - you can use about any size core that has a permeability in the 200 - 300 range. The 'K' mix is perfect for this, but is one of the more expensive cores and comes in limited sizing. The #61 cores should work well for this project. The important thing on sizing is that the number of turns with the correct wire guage can be placed on the core in a single layer without bunching up the conductors. Reducing the number of turns slightly will increase the frequency range of the transformer while reducing efficiency at lower frequencies.)
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