MFJ-269C Instruction Manual LF/HF/VHF/UHF SWR Analyzer
33
If balanced line is used, operate the MFJ-269C
only
from internal batteries and keep it a few feet away
from other conductors or earth. Do not attach any wires (other than the stub) to the unit. Use the
Antenna
port shield for one lead and the center pin for the other. Two-wire balanced lines
must
be suspended in a
straight line a few feet away from metallic objects and ground.
Coaxial lines can lay on the floor and external power used (isolation net required). Coax is connect
normally, with the shield grounded.
The
Distance to Fault
mode measures the
Electrical Length
of a transmission line when a
Vf
of 1 is
entered. To obtain
Vf
factor for an unknown cable, you must know both the
Electrical
and
Physical
length
of the line. If the
Length in Feet
displayed (with a Vf entry of 1.0) is 75 feet, and the transmission line
actually measures 49.5 feet long, the velocity factor will be 49.5 divided by 75, for a result of 0.66
Vf
.
Note:
The far end of the line may be either
open
or
shorted
, but the line can not be terminated in any
impedance
other than an open or short
.
To confirm reliability, make two or more groups of measurements on different starting frequencies at
least one octave apart. If measured distances agree, they are almost certainly very reliable.
Use the following method:
1.
Using procedures in 5.5
Advanced-2
, measure distance to fault with
Vf
set at 1.00.
2.
Measure the
Physical Length
of the line in feet.
3.
Divide the measured
Physical Length
by the display reading.
Example:
27 feet (actual physical length) divided by 33.7 feet (measured electrical length) equals .80.
The velocity factor is .80 or 80%.
4.
If the analyzer is now set to
Vf= .80
and the line measured, the result should be the correct physical
length.
7.3
Impedance of Transmission Lines or Beverage antennas
The impedance of transmission lines between a few ohms and 1500 ohms can be directly measured with
the MFJ-269C. Lines of higher impedance can be measured if a broadband transformer or resistance is
used to extend the MFJ-269C’s range. Select any measurement mode that indicates resistance (R=) and
reactance (X=).
If a balanced line is used, operate the MFJ-269C
only
from internal batteries and keep it a few feet away
from other conductors or earth. Do not attach any wires (other than the feedline) to the unit. Use the
Antenna
connector’s shield for one lead and the center pin for the other. Two-wire balanced lines
must
be suspended in a fairly straight line a few feet away from metallic objects or ground.
Setup for coaxial line need not be isolated, and Beverage antennas may be directly connected to the MFJ-
269C.
Using fixed resistances: