MIRA Amateur Radio Basic Course

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MIRA Amateur Radio Basic Course

• Chapter 7 Transmission Lines
• HC0700.ppt

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Review of Last Session

• Answers to Ch. 6 Propagation
• If no more we proceed to Ch 7 Transmission Lines

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7.1 Characteristics of Transmission Lines
We need to deliver the radio frequency (rf )
energy from our transmitter to an antenna that is
likely several metres away. We also need to carry
the rf signal from antenna back to our receiver
when listening. A transmission line to connects
the transceiver and antenna Transmission Lines
exist in several styles that try to
optimize Minimize loss from the line
itself Minimize radiation from line Display
stable electrical characteristics
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7.2 Characteristic Impedance
Impedance 'Apparent resistance' of a circuit in
response to resistance and reactance in an AC
circuit ( ch 4.13 )? Let us look back to
Series and Parallel Resonance to better
understand
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7.2 Characteristic Impedance
Impedance 'Apparent resistance' of a circuit in
response to resistance and reactance in an AC
circuit ( ch 4.13 )?
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7.2 Characteristic Impedance
Let us take the 'child on a swing' idea a bit
further Push too early? Push too late? Push by
a swinging a golf club at the seat? Push with a
bulldozer? Moral To be effective we must match
the 'push' to the
resonance of the swing
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Impedance 7.2 Characteristic
Conductors near one another exhibit capacitance
and inductance which may have unintentional
effects on the flow of RF. One example is the
Transmission Line.
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7.2 Characteristic Impedance
Characteristic impedance depends on the physical
shape of the conductors For best energy transfer
the input and output impedance should match the
characteristic impedance of the line. 'Copper
Losses' reduce efficiency slightly Higher
frequencies travel over the conductor ( skin
effect ) rather than through
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7.3 Balanced Transmission Lines
'Open Wire Feeder' - Two parallel conductors
separated by 1 to 10 cm. TV ' Insulated Twin
Lead' 'ladder line' look like rungs on a
ladder. - Very low loss with high SWR
tolerance - Affected by adverse weather -
Requires a 'balun' to connect to transmitter -
Must be kept clear of the ground, structures etc.
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7.4 Unbalanced Transmission Lines
Coaxial Cable or 'Coax' a centre conductor
inside a woven shield that confines the RF.
Characteristic Impedance depends on physical
construction of cable. Hams usually use RG 213
or RG 58 which exhibit about 50 ohms
impedance. Most common form of transmission
line Likely requires a 'Balun' at antenna
end Waterproof and may be buried Easily brought
through walls into buildings
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7.5 Coaxial Cable connector and other concerns
Special connectors are fitted to coax to provide
an RF tight and impedance matched
transition. PL 259 - male connector to SO 239
chassis connector BNC - 'Twist off' quick
disconnect connector for RG58 N type
Waterproof high quality critical
applications Avoid cable TV fittings and
cable Waterproof connectors that are use outside
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Installing a PL259 Connector onto RG 8 Coaxial
cable Courtesy Doug VE7DEE
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7.6 Line Losses
All lines exhibit loss of signal depending upon
the - physical construction - frequency of
operation As cable ages, losses increase All
cable is not created equal! Loss is is expressed
in Decibels/100 ft ( db/ft )? A loss of 3db is
equivalent to the loss of half the signal
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Decibels Courtesy Tony VE7AJN
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7.7 Baluns
BALUN - a specialized transformer used to match
RF sources to other components and control
unwanted radiation.. Want the antenna to
radiate not the transmission line. May need
to transform impedances
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11 Current Balun

• This is the simplest current balun, consisting of
  two coils of wire connected as shown.
• The coils may use an air core or a ferrite core.
• Often a current balun is made by winding coaxial
  cable into a coil, with or without a ferrite
  core.
• The load impedance is not changed by the balun.

• The inductive reactance of the windings
  prevents common mode currents from flowing and
  ensures a balanced output
• The inductive reactance should be 10 times
  the load impedance at the lowest frequency of
  operation

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41 Voltage Balun

• This is the simplest voltage balun, consisting of
  two coils of wire connected as shown.
• The coils may use an air core or a ferrite core.
• Current flowing through the lower coil induces an
  equal and opposite voltage in the upper coil.

• The primary circuit contains N turns and the
  secondary 2N, so the input
  impedance is ZL(N/2N)2 ¼ ZL

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7.8 Standing Waves
We assess the quality of the match to the antenna
by measuring the SWR ( Standing Wave Ratio
)? Measured with a SWR meter Look for all
power to antenna none reflected back ( 11
)? Ideally 11 but 21 ok, more
than 31 fix it Antenna Tuner or Transmatch will
compensate
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7.9 So what do I do with my old RG-8 Coax?
Disposition of old coax is not a matter for
testing so let's just accept the review questions
and move on. Next time we will look at Ch 8
Antennas. Radio Amateurs seem to enjoy
discussing and working with antennas more than
any other aspect of the hobby. If you come up
with a really good design it might event be named
after you!
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Chapter 7 TRANSMISSION LINES 7.1
Characteristics of Transmission Lines 7.2
Characteristic Impedance 7.3 Balanced
Transmission Lines 7.4 Unbalanced Transmission
Lines 7.5 Coaxial Cable connector and other
concerns 7.6 Line Losses 7.7 Baluns 7.8
Standing Waves 7.9 So what do I do with my old
RG-8 Coax?