Transcript Baluns - ycars

Baluns, Chokes and UNUNs
Presented by
Andy AE8J
What we will cover
• Common mode chokes
– Why we need them
– Feed line currents
• Differential mode current
• Common mode current
– Measuring common mode current
– How common mode chokes work
– Types of common mode chokes
• Air core chokes
• Bead chokes
• Toroid chokes
• Baluns
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Current baluns
Voltage baluns
Impedance ratios
How to test a balun
• Ununs
Baluns, Chokes and UNUNs
The unfortunate truth
• “No other area of technology is so vastly misunderstood as
Baluns (and RF Chokes). The web is full of all kinds of misleading
and/or wrong information on this topic. At least 50% written in
the Internet about baluns is false.”
– Rick Westerman, DJ0IP / NJ0IP
• “Don’t believe anyone who claims to be an expert on baluns.”
– Bernie Huth, W4BGH
• “Baluns can be difficult components to analyze and understand
fully.”
– Steve Hunt, G3TXQ
• “Very few people fully understand this technology and as a result
it is still far from reaching its full potential.”
– Jerry Sevick, W2FMI
Why do we need them ?
• Antenna system balance
– Many ham radio antennas are balanced
• λ/2 Dipoles, Yagis, Loops, Quads, etc.
• Twin lead, Ladder line, Window line
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Coax is unbalanced transmission line
Antenna may not be symmetrical
Feed line is not perpendicular to the axis of the antenna
Proximity of surrounding objects, ground conditions
Feed point impedance of many types of antennas does not
match 50 ohm coax
• Folded Dipoles, OCF Dipoles, Windom, End feds, etc.
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COMMON MODE CURRENT in a feed line is caused by an unbalanced
antenna system
– Radiation pattern distortion
– Feed line radiation, RFI
– Noise pickup on receive
Transmission line currents
• There are two methods or “modes” by which RF current flows
in an antenna feed line
– Differential mode current (good)
• Occur when antenna is balanced and feed line is balanced
– Feed line current is equal in amplitude and opposite in polarity
– Maximum transfer of energy from feed line to antenna on transmit
– Maximum transfer of energy from antenna to feed line on receive
– Common mode current (bad)
• Occur when the antenna and/or feed line is not balanced
– Radiation pattern distortion
– Feed line radiation, RFI, RF in shack, RF burns & shocks
– Becomes more of a problem at higher power levels
• Noise pickup on receive
Differential Mode Currents
(good current flow)
Balanced antenna fed with balanced transmission line
Differential mode currents I1 and I2 are equal in amplitude and opposite in
polarity. The resulting electromagnetic fields in the feed line, because of their
close proximity, cancel each other out. No radiation from the feed line.
No low impedance path to ground
Common Mode Current
(bad current flow)
Balanced antenna fed with unbalanced transmission line
Differential currents I1 and I2 cancel each other inside the braid. Because of “skin effect”
outside of braid becomes a third conductor. There is no opposing current to cancel I3.
Provides a low impedance path to ground
Radiation Pattern Distortion
λ/2 Dipole with common mode distortion
λ/2 Dipole no common mode distortion
Measuring common mode currents
• RF current meter MFJ-854
– Differential mode currents in
coaxial feed line are equal and
opposite so they cancel each
other out
– RF current meter only measures
common mode currents flowing
on the outside of coax braid
Common Mode Chokes
• Air core chokes (Ugly Baluns)
Coaxial Choke
Can be effective for single band antennas
Air core choke impedance is all reactive and frequency dependent
Not the best choice for multi-band antennas
How common mode chokes work
• Current flowing through a wire develops a magnetic field
around it. Magnetic lines of
force are referred to as FLUX
• Coiling the wire concentrates the magnetic field increasing the
FLUX DENSITY
• Flux density can be further intensified by placing a magnetic
material inside the coil
• Flux density presents a high impedance (Z) to common mode
currents
Magnetic Core Material
Ferris materials respond strongly to a magnetic field.
Iron oxide, Nickel, Zink, Manganese and Cobalt are
combined with ceramic to form a variety of magnetic
materials or “ferrite mixes” with varying properties.
The ability of a material to intensify a magnetic field is
referred to as PERMEABILITY (µ).
The higher the permeability the greater the inductance.
Ferrite Compositions
Ferrite Chokes
(Not to be confused with a ferret choking)
Ferret Choke
Ferrite Choke
W2DU Bead Choke
Toroid Choke (Balun)
Balanced output
Unbalanced input
Definition
Many companies often call the same item different
names to confuse you or make it difficult to compare the
same item between companies. So let’s set the record
straight and avoid the confusion with product names. A
feed line choke is also called a common mode choke, a
1:1 balun, a 1:1 unun, a current balun/unun, a line
isolator, a feed line current choke, and a 1:1 Guanilla
balun/unun/choke. All these names are for the same
item: a common mode current suppression device that
work to suppress common mode current on the outside
braid of your antenna feed line.
Baluns
•
A Balun is a type of broad band RF transmission
line transformer that can perform three basic
functions:
1.
Unbalanced to balanced conversion of current or voltage
2.
Rejection of common mode currents with some configurations
3.
Impedance transformation with some configurations
(impedance ratios other than 1:1)
• The word balun is a contraction of the term
“balanced to unbalanced”
Balun Impedance Ratios
1 to 1 BALUNs are used mostly with center-fed dipoles. (aka Chokes)
1.56 to 1 BALUNs are used for vertical antennas or antennas using a
counterpoise (aka UNUN)
2 to 1 BALUNs are employed with high-frequency Quads and loop antennas
(above 40 meters).
4 to 1 BALUNs are commonly used with Windom's, Off-Center fed dipoles,
and Sky-wire loops cut for 75/80 and 160 meters.
6 to 1 BALUNs are sometimes used with antennas that are fed with 300 ohm
feed line.
9 to 1 BALUNs are the BALUN of choice when feeding end-fed, long-wire
antennas fed with 450 ohm feed line.
Types of Baluns
• Two basic types of baluns
– Current baluns (Guanella)
• Places an inductance in series with the balanced terminals
• Forces equal currents at balanced terminals
• Impedance matching 1:1, 4:1, 6:1, 9:1, etc.
• Prevents common mode currents
• Lower insertion losses
• Preferred for most amateur applications (current fed)
– Voltage baluns (Ruthroff)
• Places an inductance across the balanced terminals
• Forces equal voltages at balanced terminals
• Impedance matching 1:1, 4:1, 6:1, 9:1, etc.
• Does not prevent common mode currents
• Higher insertion losses
• Mainly used in long wire and end fed type antennas (voltage fed)
Current Baluns
(Guanella)
• 1:1 Current Balun
• 4:1 Current Balun
4 : 1 Current Balun
(Dual Core)
Unbalanced input
Balanced output
Voltage Baluns
(Ruthroff)
• 1:1 Voltage Balun
• 4:1 Voltage Balun
4 : 1 Voltage Balun
Transmission Line Baluns
Testing a Balun
• Testing a balun with an MFJ-259B antenna analyzer
– LDG RBA-4:1 Voltage Balun
– Terminate balanced input with 200 Ω resistor
• Sweep analyzer thru frequency range
– Analyzer should read 50 ohms and low
SWR throughout frequency range
• What type of balun ?
– Testing a current balun
– Testing a voltage balun
Quotes from the experts
• “Current baluns, rather than voltage baluns, should be used whenever
possible. Current baluns provide better balance and often have lower loss.
Current baluns, especially 1:1 ratio baluns, tolerate load impedance and
balance variations much better than voltage baluns.”
• “Most commercially produced baluns are Voltage- type baluns. The
"Current-type balun™" is superior to simple 'Voltage-type' baluns in
several ways. The most important are better output balance and higher
feedline-to-antenna isolation. This means you enjoy improved antenna
radiation patterns and less problems with TVI, RFI and RF feedback.”
When is a Balun not a Balun?
When it’s a UNUN
4:1 Balun
4:1 Unun
Ununs
• Unbalanced to Unbalanced transformers
– Both input and output are unbalanced
– Does not provide common mode current suppression
– Used to feed unbalanced antennas with coax
• verticals, inverted L’s, long wires, etc.
– Available in 2:1, 4:1, 6:1 and 9:1 impedance ratios
UNUN Specification
Unbalanced input
Unbalanced output
UNUN for vertical antennas
• 32 ohm to 50 ohm impedance matching
Unbalanced input
Unbalanced output
Thanks for your attention