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Page 1 of 25BALUNSWØINKFebruary 2009
BALUNSBY
VIRGIL LEENERTS
WØINK
ARRL Technical SpecialistEmail – [email protected]
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TOPICS
• The BALUN & Antenna Problems
• What is a BALUN?
• The Current BALUN
• The Voltage BALUN
• Why & How to use a BALUN?
• Selecting a BALUN
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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.
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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.
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A Typical BALUN
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Common Mode Currents
• How are common mode currents generated?
• Why are common mode currents undesirable?
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Generation of CM CurrentFrom an Antenna
Reference Plane - Mother Earth - Ground
Transmitter
Tran
smission
Line
I1 I2id
id
id id
id
Icm
Dipole Antenna
Common Mode Current is generated on the transmissionline because of imbalance of displacement currents (id)around center of dipole antenna.
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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!
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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.
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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.
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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.
Reference Plane
Coaxial CableZcc
Eg Eg E1 E2
R1 R2
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Balanced Load – Balun
Reference Plane
Coaxial CableZcc
Eg Eg E1 E2
R1 R2
Ferrite Beads
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.
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Common Mode – No Balun
Reference Plane
Coaxial CableZcc
Eg Eg
Eg/x
Zcm
Icm
Let Zcc be 0 ohms which is in series with Zcm.Then Icm = Eg/x / Zcm
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Common Mode – Balun
Reference Plane
Coaxial CableZcc
Eg Eg
Eg/x
Zcm
Icm
Ferrite Beads
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
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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.
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1:1 Voltage Balun
E1
E2
BalancedOutput
UnbalancedInput
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.
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4:1 Voltage Baluns
Unbalanced
Input
Unbalanced
Input
BalancedO
utput
UnbalancedO
utput
Examples of 4:1 voltage baluns that can be connected for either balanced or unbalanced output.
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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.
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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!
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Balun Installation
Reference Plane - Mother Earth - Ground
Transmitter
Tran
smission
Line
I1 I2id
id
id id
id
Icm
Dipole Antenna
BA
LU
NB
ALU
NA balun connected at thispoint allows displacement current toflow on the transmission line.
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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.
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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.
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Make A BalunA 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.
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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.
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END OF PRESENTATION
QUESTIONS?
THANK YOU!
