WШINK

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BALUNS

• BY
• VIRGIL LEENERTS
• WØINK
• ARRL Technical Specialist
• Email w0ink_at_arrl.net

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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 ltgt
  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
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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.
• gt Eliminates common mode currents.
• Voltage type Baluns.
• gt 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 11 -- 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
R1R2.
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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.
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Common Mode No Balun
Let Zcc be 0 ohms which is in series with
Zcm. Then Icm Eg/x / Zcm
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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
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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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11 Voltage Balun
Example of a 11 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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41 Voltage Baluns
Examples of 41 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
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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 (41 or 91) 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 11 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 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.
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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?
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