MFJ-266B HF/VHF/UHF Antenna Analyzer
Instruction Manual
2012 MFJ Enterprises, Inc.
Version 1D
8
very low SWR (or low RF-return voltage). For example, one analyzer
may read 1.2:1 while another reads 1.1:1 when checking the same
antenna. The MFJ-266B is electronically compensated to minimize
detector error, but be aware of the potential for minor differences.
5.4 Calibration-Plane Error
The analyzer’s calibration plane is the point of reference where all
measurements have the greatest accuracy (gain reference=0 dB, phase
shift = 0-degrees). For basic hand-held units like the MFJ-266B, the
calibration plane is fixed at the antenna connector. As such, any
measurement made through a cable will displace the load from the
calibration plane and introduce some amount of error. For SWR readings,
error is mainly caused by losses in the cable. Specifically, SWR will read
somewhat lower through a length of cable than with the analyzer
connected directly to the direct load because the forward and reflected
stimulus signals are attenuated in the feedline. The more loss there is in
the cable, the greater the error. Most of the time, this inaccuracy isn’t a
problem because the SWR you measure with the analyzer is the same
SWR the radio will encounter when connected. However, if you wish to
know the antenna’s actual feedpoint SWR for documentation purposes,
the analyzer should be connected directly to the feed point through a
short pigtail.
Calibration-plane error has a much more significant impact when
attempting to measure impedance values because of phase rotation in the
cable. In fact, impedance readings can swing dramatically, depending on
the cable’s electrical length and the severity of the load’s mismatch with
reference to 50 Ohms. For accurate impedance data, always connect the
analyzer directly to the antenna or device you’re testing using the
shortest lead possible.
5.5 Sign Ambiguity (± j)
Most hand-held analyzers (including the MFJ-266B) lack the processing
capability to calculate the reactance sign for complex impedance (Z = R
± j). By default, the MFJ-266B displays a plus sign (+ j) between the
resistive and reactive values, but this sign is merely a placeholder and
not a calculated data point. Although the analyzer’s processor can’t
calculate sign, it can often be determined with a small adjustment of the
TUNE control. To determine sign, TUNE the analyzer up-frequency
slightly --
(1.) If reactance decreases, the sign is likely to be ( - ) and the reactance
capacitive (X
C)
.
(2.) If reactance increases, the sign is likely to be ( + ) and the reactance
inductive (X
L
).