MFJ MFJ-213 - Tuning a %4-Wave or 4-Wave Coaxial Stub

MFJ-213 HF/VHF/UHF Antenna Analyzer Instruction Manual
used. Shorter stubs will yield poorer accuracy and long ones may needlessly
waste useful cable.
7.5 Tuning a '4-Wave or '2-Wave Coaxial Stub
To accurately tune a coaxial stub, begin by calculating the free-space length
at the stub's intended operating frequency:
For 1/4-A in inches = 295] + MHz
For 1/4-A in feet = 246 + MHz
For 1/2-A in inches = 5902 + MHz
For 1/2-A In feet = 492 + MHz
Next, multiply the free-space length times your cable's velocity factor.
Finally, add at least 10% to this length for a margin of error (better too long
than too short). Cut the cable to this initial length. Connect one end of the
cable to the analyzer's Antenna connector. For a 1/4-A stub, leave the far
end open. For a 1/2-A stub, short the far end. Next:
(1.) Set the MFJ-213 to Antenna mode (Section-3)
(2.) Initially, set the Band and Tune for the desired stub frequency (Section-
4)
(3.) *Tune down in frequency to find lowest impedance-magnitude reading
(a 1/4-A open or a 1/2-A short look like a short or low impedance load).
(4.) Write your measured frequency down.
(5.) Divide the measured frequency by the desired stub frequency to obtain a
correction factor
(6.) Multiply the present stub length by the correction factor to get the
desired stub length.
(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.
(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-) wavelength in
feet (L = 246 +f MHz)
(6.) Divide 9 (actual length) by free-space 1/4-2 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
Version 1 13 © 2012 MFI Enterprises, Inc