MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
16
7.0
TESTING AND TUNING STUBS AND TRANSMISSION LINES
7.1
Testing Stubs
To measure Resonant Frequency for a matching stub or transmission line, select the SWR/Impedance mode in the
Basic Menu (opening mode). Note that DUTs measuring 1/4
λ
and odd multiples (1/4
λ
, 3/4
λ
, 5/4
λ
, etc) are
terminated with an open circuit at the far end. DUTs measuring 1/2
λ
and even multiples (1
λ
, 1-1/2
λ
, 2
λ
) are
terminated with a short circuit.
Coaxial lines may be piled or coiled on the floor and the analyzer operated on external power. Coax lines are
connected to the analyzer's SO-239 connector with the shield grounded. For balanced line, run the analyzer on its
internal batteries, keep it a few feet away from other conductors and earth, and do not attach any stray wires (other
than the feedline itself). Connect the DUT with one lead to the analyzer's ground stud and the other lead to the
Antenna connector center pin. The DUT must be suspended and kept away from metallic objects and ground. When
tuning frequency-critical stubs, trim them gradually using the method outlined below:
1.) Determine the DUT's target frequency and calculate the length to formula.
2.) Cut the DUT 20% longer than your calculated length.
3.) At the far end, install a short for 1/2
λ
and even multiples -- or leave it open for 1/4
λ
and odd multiples.
4.) Tune VFO to find the frequency of the lowest Impedance null. To fine-tune the null, watch the Reactance (X)
digital display and adjust as close to zero as possible. If your calculations were correct and the feedline Vf
accurate, the null frequency should be about 20% below your target frequency -- reflecting the 20% in added
length.
Continuing:
5.) Divide the present frequency by the desired frequency to calculate a Scaling Factor.
6.) Multiply the Scaling Factor by the feed line’s present physical length to find the desired physical length.
7.) Cut the stub and confirm that the reactance null (X=0) is on the desired frequency.
7.2
Velocity Factor of Transmission Lines
To determine the Velocity Factor (Vf) of a transmission line, select the Distance to Fault mode by entering the
Advanced menu and pressing Mode twice. The opening screen is shown below:
Coaxial lines may be piled or coiled on the floor and the analyzer can be operated on external power. Coax lines
are connect to the analyzer's SO-239 connector with the shield grounded. For balanced line, run the analyzer on its
internal batteries, keep it a few feet away from other conductors and earth, and do not attach any stray wires (other
than the feedline). Connect the 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 away from metallic objects and ground.
Note that the far end of the line may be terminated with an open circuit or a short circuit, but should not be
terminated by any other impedance or resistance value.
To compute Velocity Factor, you must measure both the electrical length (DTF) and the physical length of the
line. Velocity factor is calculated by dividing the physical length by the electrical length. For example, if the
analyzer displays an electrical length of 75 feet and the physical length measures 49.5 feet, the velocity factor will
be: Vf = 49.5 / 75 = 0.66.
To check reliability, make two or more groups of measurements using different starting frequencies spaced at least
one octave apart. If measured distances agree, your result is confirmed. The more frequencies you use for
confirmation, the greater your assurance that the results are correct.