All about amateur radio RF transmission lines. This relates to Section 26 of the NZART Radio Syllabus and may be used to teach this section of the exam.

1. • Types of Lines
• Terminations
• Impedance
• S.W.R.
• Velocity Factor
• Baluns
• Antenna Switching
26 – Transmission Lines26 – Transmission Lines

2. Getting Power from One Place to Another
The Transmission Line does the Business!!

3. Transmission Lines – Coaxial Conductors
•Coaxial cable (coax) consists of two
concentric conductors. It is a single wire
surrounded by insulation and enclosed in an
outer conductor, usually a braid.
•This is an “unbalanced” line, the outer sheath
can be at earth potential, only the inner wire is
“hot”.
•The transmitter power radiating from the
antenna is less than that generated at the
transmitter due to losses in the transmission
line. with higher frequencies and with
increasing the length of the line.
•Most line loss occurs in the supporting
insulation so open-wire lines have lower losses
than heavily insulated line.

4. Voltage and Current Relationships
What is the voltage ( E ) between the two terminals – Maximum or
Minimum?
What is the current ( I ) between the two terminals – Maximum or Minimum?

5. Voltage and Current Relationships
Place a direct short circuit across the terminals – What happens to the
Voltage and what happens to the current?
What is the voltage ( E ) between the two terminals – Maximum or
Minimum?
What is the current ( I ) between the two terminals – Maximum or Minimum?

6. Transmission Lines – Terminations
The Voltage existing at the load for Open and Short Circuits

7. Transmission Lines – Impedance
• A transmission line has a characteristic “impedance”. This can range from 30 ohms for
coax to 600 to 1000 ohm for open-wire wide- spaced line.
•The characteristic impedance of a line depends upon the size and spacing of the
conductors. Note that the line above is “FLAT” – no variation in the ratio of voltage to
current (i.e. no variation in impedance at any point along the line
• Impedance is the ratio of the voltage to the current. A high voltage and low current
means a high impedance. A low voltage and high current means low impedance). Loads
attached to the distant end of a line have an effect on the impedance “seen” at the input to
the line.
• When a line is terminated at the distant end with a termination impedance that is the
same as the characteristic impedance of the line, the input to the line will be “seen” to be
the characteristic impedance of that line. Looking in to the input of this line, you “see” an
infinitely-long line. This is ideal for the optimum transfer of power from the transmitter
down the line to the antenna.

8. Transmission Lines – Short Circuit Termination
A signal starts off and travels down the line. It reaches the distant
end and finds the line to be Short-circuited!
What happens to the Voltage? Current? Impedance? at the load
end?
Travelling back down the line, the impedance (ratio of Voltage ( E )
to Current ( I ) ) will decrease until at the quarter-wavelength
point, the impedance will be seen to be ZERO.
The source will represent infinite voltage and zero current.

9. A signal starts off and travels down the line. It reaches the distant
end and finds the line to be Open-circuited!
What happens to the Voltage? Current? Impedance? at the load
end?
Travelling back down the line, the impedance (ratio of Voltage ( E )
to Current ( I ) ) will decrease until at the quarter-wavelength
point, the impedance will be seen to be ZERO
Transmission Lines – Open Circuit Termination

10. Transmission Lines – Velocity Factor
• A radio wave in free space travels with the speed of light. When a wave travels
on a transmission line, it travels slower through the dielectric/insulation.
• The slower speed at which it travels on a line is known as the “velocity factor”.
Typical figures are:
Twin line 0.82, Coaxial cable 0.66, (free space 1.0)
•So a wave in a coaxial cable travels at about 66% of the speed of light (as an
•example).
•In practice this means that if you have to cut a length of coaxial transmission
line to be a half-wavelength long (for, say, some antenna application), the length
of line you cut off will have to be 0.66 of the free-space length that you
calculated.

11. The Quarter-wave lengths of line, in effect INVERTS the
impedance at it’s termination
Transmission Lines – Quarter-Wave Lines

12. The input impedance of a Half-wave length of
line is a repeat of the distant end of the line.
Transmission Lines – Half-Wave Lines

13. • VSWR (Shortened to SWR) may be visualised by looking at the
forward and reflected waves in a line. If the termination of the
line does not match the line then not all the energy will be
transferred to the load and some will be reflected back down the
line. A pattern of peaks and troughs in the voltage will exist.
Transmission Lines – SWR

14. • The SWR Meter (Power Meter) is usually connected between the
Radio and the antenna with the output power of the radio going
through it. Tune for MINIMUM SWR
• The meters sample the waves going to the antenna and sample
the waves reflected back and the indication tells you that your
antenna is resonant and efficient (Or inefficient!)
SWR (Power Meter Connection)

15. Baluns ( BALanced Unbalanced )
A BALUN is a device to convert a balanced
line to an unbalanced line - and vice-
versa. It comes in a variety of types.
When a balanced antenna, such as a dipole,
is directly fed with coax ( an unbalanced line),
the antenna currents (which are inherently
balanced) may travel back down the outside
braid of the cable to the radio and RF
voltages may appear on the metal surfaces of
the radio.
RF burns are quite unpleasant!

16. Balanced & Unbalanced Feed LinesBalanced & Unbalanced Feed Lines
What is an unbalanced line?What is an unbalanced line?
What is a coaxial cable?What is a coaxial cable?
A flexible coaxial line contains:A flexible coaxial line contains:
What device can be installed to feed a balanced antenna with an unbalanced feedWhat device can be installed to feed a balanced antenna with an unbalanced feed
line?line?
What does the term "balun" mean?What does the term "balun" mean?