This is a departure from the normal 1:1 Isolation/choke balun and utilizes a bifilar pair designed to maximize interwinding breakdown voltage. Note that, by design, this balun is not wound with a 50 ohm pair and is primarily used with ladder line, twin lead or window line feeding antennas such as multiband dipoles (doublet) and/or multiband loops.
The balun functions to transform balanced to unbalanced feedline for the transition to coax while providing high levels of choking impedance to stop common mode currents from entering equipment. It is best installed outdoors to minimize the chance of RF entering the operating postion from the balanced feedline. This model is also an excellent choke balun for use between an antenna and a remote tuner installed at the antenna feedpoint.
With wire antennas such as this being used primarily on the lower HF bands, choking impedance has been shifted to provide maximum performance on these bands. However, this ferrite mix still provides excellent common mode control on the upper HF bands where very high choking impedance levels are not as critical.
The "ts" model is specially configured to provide isolation between a remote tuner and antenna at the feedpoint. The studs on input and output make installation/insertion very easy. The balun may also be ordered without an enclosure (1171x) for upgrading the balun in a tuner.
My thanks to Owen (VK1OD) for his recommendation and assistance with this design. Click on the graph below to view the full choking impedance curve.
Click to Enlarge
Testing done with an AIM-4170uhf Network Analyzer
This balun is best utilized with a quality tuner that can provide matching over a wide impedance range. If you have a less expensive tuner with limited range, consider using the Model 4114 balun to reduce the required impedance match.
- T match antenna tuners perform quite poorly if the resistive component of load impedance is very low, even worse if there is also a high capacitive reactive component. This is a common load found in shortened dipoles fed with open wire line.
- The traditional approach is to use a 4:1 balun on such antennas, but antenna tuner loss is worse with low resistive loads.
- The need to tolerate high voltages occurs when the balun happens to be located near a differential voltage maximum.
- High common mode impedance for effective reduction of common mode current.
- This balun is intended for use with an antenna tuner which will manage SWR and transformation mismatches.
- Differential insertion loss is not a significant parameter in this application, the dissipation in a current balun is primarily from the common mode current which is installation dependent and not easily predicted.
For a good graphical explanation of why this designs works well, read through Steve's (G3TXQ) website page
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