BALUNS BY VIRGIL LEENERTS WINK ARRL Technical Specialist

BALUNS BY VIRGIL LEENERTS WØINK ARRL Technical Specialist Email – w 0 ink@arrl. net WØINK February 2009 BALUNS 1

TOPICS • • • The BALUN & Antenna Problems What is a BALUN? The Current BALUN The Voltage BALUN Why & How to use a BALUN? Selecting a BALUN WØINK February 2009 BALUNS 2

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. WØINK February 2009 BALUNS 3

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. WØINK February 2009 BALUNS 4

A Typical BALUN WØINK February 2009 BALUNS 5

Common Mode Currents • How are common mode currents generated? • Why are common mode currents undesirable? WØINK February 2009 BALUNS 6

Generation of CM Current From an Antenna WØINK February 2009 BALUNS 7

Problems Caused By CM Current • 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! WØINK February 2009 BALUNS 8

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. WØINK February 2009 BALUNS 9

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. WØINK February 2009 BALUNS 10

Balanced Load – No Balun Let Zcc be 0 ohms which shunts R 1. Then E 1 = 0 volts and E 2 = EG. No balanced voltage to load R 1&R 2. WØINK February 2009 BALUNS 11

Balanced Load – Balun Let Zcc be Hi-Z due to ferrite beads on cable and R 1 = R 2. Then R 1 is not shunted by Zcc. Now E 1 = E 2 – balanced voltage across load due to balanced load – not balun. WØINK February 2009 BALUNS 12

Common Mode – No Balun Let Zcc be 0 ohms which is in series with Zcm. Then Icm = Eg/x / Zcm WØINK February 2009 BALUNS 13

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 WØINK February 2009 BALUNS 14

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. WØINK February 2009 BALUNS 15

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. WØINK February 2009 BALUNS 16

4: 1 Voltage Baluns Examples of 4: 1 voltage baluns that can be connected for either balanced or unbalanced output. WØINK February 2009 BALUNS 17

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. WØINK February 2009 BALUNS 18

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! WØINK February 2009 BALUNS 19

Balun Installation WØINK February 2009 BALUNS 20

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. WØINK February 2009 BALUNS 21

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 W 2 AU ferrite balun usually does not indicate that it is a voltage balun. WØINK February 2009 BALUNS 22

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. WØINK February 2009 BALUNS 23

REFERENCES • Baluns: What They Do And How They Do It By Roy W. Lewallen, W 7 EL. Article in “The ARRL Antenna Compendium” Vol 1. • Understanding, Building, and Using Baluns and Ununs By Jerry Sevick, W 2 FMI. A book published by CQ Communications. WØINK February 2009 BALUNS 24

END OF PRESENTATION QUESTIONS? THANK YOU! WØINK February 2009 BALUNS 25