MFJ MFJ-213 - Calibration-Plane Error; Sign Ambiguity (+ J)

MFJ-213 HF/VHF/UHF Antenna Analyzer Instruction Manual
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-213, 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-213) lack the processing
capability to calculate the reactance sign for complex impedance (Z = R + j).
By default, the MFJ-213 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 (Xc).
(2.) If reactance increases, the sign is likely to be ( + ) and the reactance
inductive (X:).
Version | 7 © 2012 MFI Enterprises, Inc