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BALUNS

BALUNS. BY VIRGIL LEENERTS WØINK ARRL Technical Specialist Email – w0ink@arrl.net. TOPICS. The BALUN & Antenna Problems What is a BALUN? The Current BALUN The Voltage BALUN Why & How to use a BALUN? Selecting a BALUN. TYPICAL ANTENNA PROBLEMS. Radio Interference to nearby devices.

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BALUNS

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  1. BALUNS BY VIRGIL LEENERTS WØINK ARRL Technical Specialist Email – w0ink@arrl.net BALUNS

  2. TOPICS • The BALUN & Antenna Problems • What is a BALUN? • The Current BALUN • The Voltage BALUN • Why & How to use a BALUN? • Selecting a BALUN BALUNS

  3. TYPICAL ANTENNA PROBLEMS • Radio Interference to nearby devices. • Transmission line radiation. • The above are due to “common mode currents” on the transmission line. • Loss of power to antenna due to mismatch between coax and antenna. • BALUNS can address these problems. BALUNS

  4. BALUN – A Coined Word • Balun formed from BALance – UNbalance. • Name suggest device converts between “Balance <> Unbalance”. • BALUN is name of device that can be many things like a common mode choke, unbalance to balance transformer, and a step up or down transformer. BALUNS

  5. A Typical BALUN BALUNS

  6. CommonModeCurrents • How are common mode currents generated? • Why are common mode currents undesirable? BALUNS

  7. GenerationofCMCurrentFromanAntenna BALUNS

  8. ProblemsCausedByCMCurrent • Pattern Distortion – caused by radiation from transmission line due to common mode current on transmission line. • RFI to nearby devices like TV sets and telephones. • RF in the shack or transmitter location that can cause RF current to flow through the operator! BALUNS

  9. Types of BALUNS • There are two general types of baluns. • Current or choke type Baluns. • => Eliminates “common mode currents”. • Voltage type Baluns. • => Provides for impedance matching of transmission line to antenna. An example is the TV antenna 300 ohm to 75 ohm matching transformer. BALUNS

  10. CURRENT BALUNS • Current baluns are typically 1:1 -- Zout = Zin but can have other ratios. • Primary use is for reduction of common mode currents. • Can be use for unbalanced to balanced loads like dipole antennas and beams. • Typical construction is ferrite beads over coaxial cable or wires on ferrite core. BALUNS

  11. Balanced Load – No Balun Let Zcc be 0 ohms which shunts R1. Then E1 = 0 volts and E2 = EG. No balanced voltage to load R1&R2. BALUNS

  12. Balanced Load – Balun Let Zcc be Hi-Z due to ferrite beads on cable and R1 = R2. Then R1 is not shunted by Zcc. Now E1 = E2 – balanced voltage across load due to balanced load – not balun. BALUNS

  13. Common Mode – No Balun Let Zcc be 0 ohms which is in series with Zcm. Then Icm = Eg/x / Zcm BALUNS

  14. Common Mode – Balun Let Zcc be Hi-Z due to ferrite beads on cable which is in series with Zcm. The Icm = Eg/x / Zcm + Zcc. Note if Zcm where infinite, then Icm = 0 Amps BALUNS

  15. VOLTAGE BALUNS • Voltage baluns have varied input to output ratios and have true balanced or unbalanced outputs as determined by the design. • Primary use is for impedance matching of typically a transmission line to an antenna. • Typical construction is the use of coaxial cable or wires on a ferrite core. BALUNS

  16. 1:1 Voltage Balun Example of a 1:1 unbalanced to balanced voltage balun. Balanced output is due to transformer and not the load. There is essentially no common mode current rejection. BALUNS

  17. 4:1 Voltage Baluns Examples of 4:1 voltage baluns that can be connected for either balanced or unbalanced output. BALUNS

  18. WHY USE A BALUN? • Baluns are used for two primary reasons. • One : Eliminate “common mode current” on the transmission line! • Two : Matching antenna impedance to the transmission line. BALUNS

  19. Matching Antenna • Impedance matching antenna to transmission line improves power transfer and lowers SWR. • Note! A high SWR on a transmission line does NOT cause the transmission line to radiate! BALUNS

  20. Balun Installation BALUNS

  21. Selecting A Balun • ALWAYS use a current balun to eliminate common mode current and if matching is needed, current baluns can also have a ratio (4:1 or 9:1) as required to match antenna. • Use a voltage balun only for impedance matching of antenna to transmission line. • Becoming knowledgeable on how and why baluns work, will insure success. BALUNS

  22. Purchasing A Balun • The description of a balun, may or may not, include that it is current or voltage balun. • A 1:1 current balun is sometimes described as a choke balun. • An example is the catalog description of the W2AU ferrite balun usually does not indicate that it is a voltage balun. BALUNS

  23. Make A Balun A current balun can be made by winding several turns of coax to form a inductor. Another choice to make a current balun is to place a number of ferrite beads over coax. Need More Ideas: “GOOGLE” for one that meets your needs. BALUNS

  24. REFERENCES • Baluns: What They Do And How They Do It By Roy W. Lewallen, W7EL. Article in “The ARRL Antenna Compendium” Vol 1. • Understanding, Building, and Using Baluns and Ununs By Jerry Sevick, W2FMI. A book published by CQ Communications. BALUNS

  25. END OF PRESENTATION QUESTIONS? THANKYOU! BALUNS

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