MFJ-266 HF/VHF/UHF Antenna Analyzer
Instruction Manual
©
2011 MFJ Enterprises, Inc.
14
(7.) Re-cut the cable to that length.
*Note that the impedance value may not drop to zero, but it will begin to increase again
as you continue to tune past the null. If the null reading is broad, choose a frequency at
the center.
7.7 Determining Velocity Factor
If you have coax cable with an unknown velocity factor, you can determine it quickly using the
following procedure:
(1.) Set the MFJ-266 up in
Analyzer
mode (Section-3)
(2.) Set the
Range
to
HF
and the
Band
to
E
(Section-4)
(3.) Make a 1/4-
λ
stub from
9 feet
of the unknown cable and connect it to the
analyzer (open end)
(4.) Rotate
Tune
for minimum
impedance magnitude
reading. Write down the
frequency (MHz)
(5.) Divide 246 by this frequency to find the free-space 1/4-
λ
wavelength in feet
(L = 246
÷
f MHz)
(6.) Divide 9 (actual length) by free-space 1/4-
λ
wavelength to get the Velocity
Factor
(VF =9
÷
L)
Note that there is nothing magical about the 9-foot stub length, other than it falls conveniently
within the limits of Band E’s tuning range. Other lengths could be used. Shorter stubs will yield
poorer accuracy and long ones may needlessly waste useful cable.
7.8 Testing RF Transformers
Broadband HF-matching transformers wound for the 12.5 to 200 Ohm range may be tested using
the MFJ-266. Connect the 50-Ohm (primary) side to the analyzer connector using a short pigtail
and attach the appropriate resistive load across the secondary side (always use a non-inductive
resistor). Next:
MJ-266
Load
Transformer
(1.) Set the MFJ-266 up in
Analyzer
mode (Section-3)
(2.) Set the
Band Select
to
HF
and the
Band-Mode
to the desired frequency
range
(Section-4)
(3.)
Rotate
Tune
across the frequency range and note
SWR
. Change bands, as
needed.
