MFJ-266C HF/VHF/UHF Antenna Analyzer
Instruction Manual
Version 2A
15
2012 MFJ Enterprises, Inc
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-266C 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-266C. 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-266C 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.
At the low and high ends of the transformer's frequency response range,
SWR and reactance will climb to unacceptable levels (< 1.2:1 is ideal). HF,
VHF, and UHF tuned
transmission-line transformers
may be tested in
similar fashion by connecting one end directly to the analyzer and
