MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
13
5.5
Distance to Fault mode
From
Advance Mode
, enter
Distance to Fault
by tapping
Mode
twice. The identification screen is shown below:
This mode is useful for finding the physical or electrical length of a random piece of cable, or for finding the distance
to a transmission line fault.
Unbalanced lines
need not be isolated during testing and may be coiled on the floor. For
balanced lines
(ladder line, open-wire feeders, etc), run the MFJ-259D on internal batteries and keep it at least a few
feet away from earth and other conductors. Attach only the DUT (no other wires) -- one lead to the center pin of the
Antenna
connector and the other to the case via the grounding stud. Suspend two-wire balanced feeder in a straight
line so it remains clear of metallic objects and ground by at least a few feet.
The
Distance to Fault
test measures the
electrical distance
to the transmission line's abnormality. To obtain the
physical distance
, multiply
electrical distance
times the feed line’s specified
Velocity Factor
(Vf). For example, if
the analyzer display reads DTF = 75 feet and the transmission line is typical RG-8 with a specified velocity factor of
0.66, the physical distance to the fault will be:
75 x .66 = 49.5 feet
.
An open or a short-circuit at the fault yields best accuracy -- some resistive and reactive loads or L/C terminations
may skew the results or simply not work. To conduct the test, follow the specific sequence of steps outlined below.
To measure fault distance:
1.) Connect the DUT to the analyzer's
Antenna
connector. From
Advanced
mode, tap the
Mode
switch twice to bring
up the
Distance to Fault
ID screen. The top line of the working screen will display the
VFO
frequency in MHz
followed by
1st
blinking at a rapid rate. The
1st
indicates the analyzer is waiting for entry of the first
X=0
frequency. The lower line displays
DTF
(no data showing) and
X
with the
Reactance
reading in ohms.
2.) Begin by searching for the lowest
VFO
frequency where the
Impedance
meter shows a sharp null and where the
reactance value on the screen approaches zero (X=0). If possible, find an
exact frequency
where X=0:
3.) When you've found the first X=0 null, tap the
Gate
button once to enter it. The blinking
1st
will change to a
blinking
2nd
. Now, tune to the next higher X=0 null. A minimum reading of a few ohms is acceptable if the X
display won't zero.
4.) Press
Gate
again, and the display will indicate the
DTF
. Multiply the
DTF
times the velocity factor of the cable
(
Vf)
to get the
physical DTF
(distance in feet).
5.6
Resonance Mode
From the
Advanced
menu, tap the
Mode
switch three times to bring up the
Resonance Mode
ID screen. The top line
of the working screen displays the
VFO
frequency in MHz followed by
SWR
. The lower line displays
Resistance (R)
42.648 MHz 2nd
DTF X=1
21.324 MHz 1st
DTF X=0
Dist. to fault
11.3 ft x Vf