Building a sextuple ratio balun

 

HOW DO YOU GET SIX SEPARATE IMPEDANCE SETTINGS OUT OF A TRIPLE RATIO BALUN™?

EASILY!


I have designed and constructed a sextuple-impedance balun based on the Triple Ratio Balun™(TRB™) design developed by Alan Biocca (WB6ZQZ) and described in detail on his web site (here). 

The original TRB™, and this variation, actually include two separate components. On the left, in the circuit diagrams, is a 50Ω bifilar BalUn (Balanced to Unbalanced) choke, used to eliminate current flow on the outside shield of the coaxial cable. This is a desirable feature even without the adjustable impedance ratios also provided. On the right side is a quadrifilar UnUn (Unbalanced to Unbalanced) coil, used to adjust the impedance match between the antenna and transmitter. This feature is highly desirable with antennas that use loading coils, such as the Buddipole system, because such coils reduce the inherent impedance of an antenna, – far below 50Ω on lower bands.

Biocca’s kit is no longer available; Buddipole now sells a switchable balun that is electrically equivalent to his kit, but using a rotary switch for selection of impedances, rather than Anderson PowerPole connectors. There are several differences between the balun I built and the one Buddipole makes. Most notably, mine mounts at the antenna, whereas the Buddipole version is at ground level. This is one of those tradeoff issues in any design project;  I have more “topside weight” because the balun is mounted on the Tee, but I avoid the losses than can occur in a 25 foot run of RG-58 cable when there is a high SWR. (Losses can be 50% or more under some circumstances.)

The cost for this home-brew balun is about $25.

After rearranging the components in his original circuit diagram (upper image) while preserving the electrical relationships, I realized that the UnUn portion of the TRB is essentially the equivalent of a resistor voltage divider network, except that the impedance match of any given combination of UnUn coil elements is a function of the square of the ratio of turns, rather than a linear relationship. Thus, what Buddipole describes for simplicity’s sake as a three-ratio balun with impedances of 50, 25, and 12.5 ohms, or 1:1, 2:1 and 4:1, is actually 50, 28.1, and 12.5 ohms (as Biocca has discussed).  See below for a detailed explanation of the impedance ratios.

I also realized that the impedance range of the original configuration could be extended to provide six impedance settings, rather than three, without reconfiguring the UnUn coil, simply by connecting one additional tap (drawn in red on his original schematic). In the revised diagram,  B-1 and B-2 are the bifilar choke balun coils. Q1 – Q4 are the coils of the quadrifilar UnUn. In this version, the “common” or ground side of the circuit is not directly soldered to the end of the UnUn, but rather to a connector that can be plugged into Connector E (the original configuration) or D. Connecting the common side of the circuit at D creates a three section UnUn rather than four sections, adding two new impedance ratios (22.2Ω and 5.5Ω). The new connector at D also provides an additional low impedance configuration for the four-section UnUn, 3.125Ω. The 5.5Ω and 3.1Ω configurations are useful for low-band vertical monopoles that require high-impedance loading coils. The following table documents the various tap configurations, their impedances (and how calculated), and the bands on which each balun setting is potentially useful. The additional taps combinations available provide a better match on the 30 meter band (22 Ω) and for highly loaded antenna configurations on 80 and even 160 meters. In the following table, “Net Impedance” is the product of Ratio Squared times 50Ω.


The 5.5Ω tap combination (C and D) is ideal for matching 80 meter antenna configurations,

which can’t be matched readily at better than 2:1 SWR with the 12.5 Ω balun configuration. 

The 3.125Ω configuration is theoretically useful for matching a 160 meter vertical monopole Buddipole configuration, although I haven’t built the 150 µH loading coil that would be required to implement it. (For the curious, such a coil could be made with 54 turns of #16
wire at 1/4” spacing on a 5.5 inch diameter plastic pipe form.) The resulting vertical antenna would rise 20 feet above the Tee, and would match well to the 3.125 Ω impedance setting on the Balun. It models about 2.1 dBi maximum gain at a 20° takeoff angle. Not surprisingly, given the monster loading coil, it has a bandwidth for any given configuration measured in Hertz, not even tens of Hertz, so it would not very functional in the real world, although it might earn an entry into the Guinness Book of World Records if built and used.

The completed balunxis built around two F140 (1.4” O.D) ferrite toroid cores, Mix 61, wound with 7 feet of #16 magnet wire. It is mounted on a sheet of acrylic plastic with wire tie straps. The plastic base is attached with screws to my version of the Buddipole Tee itself. It could be smaller, but I decided to make it the same width as my home-brew Tee. This also gave me enough room to eqiup the balun with SO-239, BNC female, and N-female panel jack, which are wired in parallel. The Anderson PowerPole connectors are held in position with two APP PowerPole Mounting Wings, SKU 1399G9. This assembly is NOT waterproofed; I live in Southern California, so I haven’t bothered with that, since it almost never rains here. In other climates some sort of water-tight covering, even a few layers of plastic bubble wrap and rubber bands, would be sufficient for portable and temporary installations. Those planning to construct this balun will serve themselves well by reading Biocca’s construction notes carefully and studying the photos on his web site. 

Thanks to Alan Biocca,  WB6ZQZ, for the concept, well-documented construction notes, and permission to use his original circuit diagram on this web site.