MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
17
To measure velocity factor:
1.) Connect the DUT and tune the analyzer VFO to a frequency where the analog Impedance meter nulls and
where reactance displayed on the screen approaches or crosses zero (X=0).
2.) Tap Gate once to enter your first X=0 frequency. The blinking 1st will change to a blinking 2nd.
3.) Tune the analyzer higher (or lower) in frequency until the Impedance meter reads the next null and the display
again crosses zero. If you can't obtain X=0, then a minimum of a few ohms is acceptable.
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.