MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
17
4.) Tap
Gate
again, and the display will show electrical distance in feet.
5.) Using a tape measure, find the physical length of the DUT in feet.
6.) Divide your
physical length
measurement by the
electrical length
to calculate
Vf
.
For example, if you measure 27 feet of cable and the
DTF
mode displays 33.7 feet, divide
physical length
(27) by
electrical lenght
33.7 feet:
Vf = 27/33.7 = 0.80
. Note that
Velocity Factor (Vf)
may also be expressed as
Velocity of
Propagation (Vp).
Velocity of Propagation is an equivalent term expressed as a percentage
(Vp = 80%)
.
7.3
Impedance of Transmission Lines or Beverage antennas
This procedure will show you how to measure the impedance of transmission line ranging from a few ohms to 650
ohms directly -- and also how to measure even higher-impedance lines using a broadband transformer or resistance
to extend the analyzer's range. To measure the necessary parameters, use the analyzer's default SWR --
Resistance
(R) and Reactance (X) Mode
augmented by the
Impedance Magnitude
function (access by pressing and holding the
Gate
switch).
Methodology:
When random-length transmission line is terminated by a load of the same impedance, no transformation occurs
between the cable's near end and far end. However, as soon as a mismatch is introduced at one end, the impedance
transforms to a value higher or lower than the line's
characteristic impedance
at the opposite end. If viewed on a
Smith Chart, the transformed impedance would literally trace a circle around the characteristic impedance of the
transmission line with every 360-degree phase shift. The greater the mismatch in ohms, the greater the amplitude (or
diameter) of the circle. We can use this behavior to determine the impedance of an unknown line. Taking one
approach, we intentionally introduce a resistive mismatch at the far end of the line, measure the impedance
transformation over a 360 phase rotation at the near end, and calculate the impedance at the center. Or, we introduce
different trial loads of known resistance at the far end until we find a value where the cable becomes "flat" across a
wide range of frequencies. In essence, we create a circle and then shrink it down until only the center remains.
As recommended for other transmission line tests,
coaxial cable
may be piled or coiled on the floor and the analyzer
operated on internal or external power. For
balanced line,
run the analyzer on internal batteries, keep it away from
other conductors or earth, and don't attach stray wires other than the DUT. Connect the balanced DUT with one lead
to the analyzer's ground stud and the other to the
Antenna
center pin. The DUT
must
be suspended and kept a few
feet away from metallic objects and ground. Beverage antennas must be connected directly to the analyzer.
Line Impedance Using an Intentional Mismatch:
The test resistance should be somewhere near (but different from) the line's anticipated impedance. When choosing a
resistance value, consider the limits of the analyzer's measurement range (7 - 650 ohms) and stay well within it.
1.) Connect the DUT to the
Antenna
connector.
2.) Terminate the far end of the DUT with a non-inductive test resistance.
3.) Tune
VFO
to the
lowest frequency
where the
Impedance
and
Resistance
indicators both null.
4.) Fine tune to find where
X
is closest to
0 and
R
is at its minimum value (null).
5.) Press
Gate
to confirm θ = 0°. Write down the
Resistance (R)
value as R1.
6.) Tune up in frequency to find a distinct
Impedance peak
. Once again,
X
should approach or equal 0.
7.) Fine tune for the
highest
Resistance (R)
value and confirm θ = 0°. Record
Resistance (R)
as R2.
8.) Multiply R1 x R2 and find the square root of the product. The result is the line's characteristic impedance.