Transcript BALUNS

BALUNS
BY
VIRGIL LEENERTS
WØINK
ARRL Technical Specialist
Email – [email protected]
WØINK
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TOPICS
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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 Current
From an Antenna
id
I1
I2
Dipole
id
Transmission Line
id
Antenna
Icm
id
id
Common Mode Current is generated on the transmission
line because of imbalance of displacement currents (id)
around center of dipole antenna.
Transmitter
Reference Plane - Mother Earth - Ground
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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
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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.
Eg
Eg E1
E2
Coaxial Cable
Zcc
R1
R2
Reference Plane
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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.
Eg
Ferrite Beads
Eg E1
E2
Coaxial Cable
Zcc
R1
R2
Reference Plane
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Common Mode – No Balun
Let Zcc be 0 ohms which is in series with Zcm.
Then Icm = Eg/x / Zcm
Eg
Eg
Coaxial Cable
Icm
Zcm
Eg/x
Zcc
Reference Plane
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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
Eg
Ferrite Beads
Eg
Coaxial Cable
Icm
Zcm
Eg/x
Zcc
Reference Plane
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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
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.
E1
Unbalanced
Input
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Balanced
Output
E2
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4:1 Voltage Baluns
Examples of 4:1 voltage baluns that can be connected
for either balanced or unbalanced output.
BALUNS
Unbalanced
Output
Unbalanced
Input
Balanced
Output
Unbalanced
Input
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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
id
Dipole
id
Transmission Line
id
BALUN
I1
I2
Antenna
Icm
id
id
BALUN
A balun connected at this
point allows displacement current to
flow on the transmission line.
Transmitter
Reference Plane - Mother Earth - Ground
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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 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?
THANK YOU!
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