Design & Performance of a Choke Balun and Balanced Transmission LineAuthor: R.J.Edwards G4FGQ © 15th June 2004
A choke balun is a pair of wires wound alongside each other around a ferrite core. The two wires together form a single-conductor choke, simultaneously behaving as a two-wire balanced transmission line. The choking action allows two different circuits to be connected together via the line without regard to the grounding arrangements of either. At one end of the balun can be an unbalanced circuit with one terminal grounded. At the other end the circuit can be floating relative to ground or it can be held firmly balanced against ground.
A choke balun works between indefinite impedances. Its short transmission line is not used for impedance-matching input and output circuits although a transformation does occur. It can be wound on a ferrite rod. The type modeled in this program is wound on a ferrite ring. Flexible, stranded, twin speaker cable or similar can be used, or two single plastic-insulated wires running together.
In this program the balun is located between an unbalanced matching network (a tuner with one side grounded), and a twin feedline to a balanced antenna. The balanced side of the balun is floating with no definite ground connection.
A Note about Impedance Z
When the feedline is considered as a single wire, a terminating impedance Z at the antenna end will affect longitudinal current along the wire. The impedance presented to the balun will differ considerably from Z. The 'single wire' line 's Zo will be involved. In general, there will be standing waves on the wire.
Termination Z is formed by the whole antenna structure which may be grounded at some point. The magnitude and angle of Z can be described as indeterminate. At HF, Z probably lies in the range 100 to 1000 ohms decreasing with frequency. For a simple structure such as a 1/2-wave dipole at resonance, a guess of 200 ohms is good enough. The impedance seen by the balun is transformed to an entirely different value by Zo and the uncertain length of the 'single' wire.
In conjunction with balun imperfections, Z causes unequal currents to flow in the pair of line wires. It's of varying importance. The effect is associated with radiation from the feedline but with little knowledge of its magnitude.
So as a side issue to balun design, this program attempts to quantify the percentage of unbalance current on the transmission line. But at best, due to the many uncertainties, the results can only be said to lie in the 'right statistical ball park'. Percent current unbalance is defined by 100*(I1-I2)/(I1+I2). It will be found to vary randomly and widely with frequency and feedline length.
Miscellaneous Operating Notes & Assumptions
- Enter sensible data. E.g., wire diameter must be less than the diameter over the insulation. A low Mu, HF grade of ferrite core with high volume resistivity is assumed, but a high Mu, LF grade enables a high balun inductance for use on 160m band.
- Balun wire-to-wire insulation must withstand the same voltage as on the feedline. Ordinary plastic insulants are assumed, but Teflon is better for high power.
- Longitudinal or unbalance currents are synonymous with common-mode currents. Random resonances on the line and/or antenna increase or affect the unbalance. In general, behaviour is satisfactory when an antenna is being properly fed. Percent current unbalance is calculated at the balun-feedline junction. Percent current unbalance can vary along the feedline. This is not computed.
- Space turns evenly around the core with a little extra space at the ends. Don't forget to check that number of turns does not exceed maximum possible. Avoid a completely full, tightly packed core. Remove one or two turns.
- The 'single-wire' line will have a Zo of the order of 450 to 550 ohms. When, by chance, Z = 500 ohms the crude termination will damp down resonances.
- Percent of input power lost in the balun line will help with assessing power rating.
- During operation the program calculates Zin of the feedline when terminated by the antenna, Zin of the single-wire line terminated by Z, and Zin of the balun as presented to the tuner.
The ferrite core cross-section is rectangular in shape with rounded corners. Rounded corners allow wire plus thick insulation to bend around a small radius. Three easy-to-measure dimensions are needed to fully specify a ferrite core - the ring's outer diameter, the core's radial thickness, and its axial length. A typical core cross-section has an axial length 1.75 times radial thickness, but the program can deal with rectangles of any practical aspect ratio. Rounded corners are assumed to reduce the rectangular core area by 8 percent. For a circular cross section enter a square of the same cross-sectional area. A square is equivalent to a circle when its side is 0.886 times the diameter. Exchanging a circle for a square will slightly modify line length, etc.
A very uncertain parameter in manufacturers' data is the resistance of the loss component in shunt with the ferrite ring inductance. This not only affects estimation of percent current unbalance, but also the core power loss and rating.
Core dimensions and number of turns are non-critical. Variation of 10% in any parameter will not substantially affect performance. Some external factors affecting behaviour have been omitted from input data because collecting data and incorporating it in the program is not worth the effort involved.
Hit 'U' for an example of a balun plus a 300-ohm feeder to a folded dipole.
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it from Your Computer
This program is self-contained and ready to use. It does not require installation. Click this link BalChoke then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.
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