MFJ MFJ-213 - Antenna Matching; Matching Antennas through a Tuner; Antenna Impedance Readings; Unpredictable SWR

MFJ-213 HF/VHF/UHF Antenna Analyzer Instruction Manual
6.6 Antenna matching
If your antenna doesn’t exhibit 1:1 SWR at the minimum SWR frequency,
then some mismatch is present relative to 50-Ohms. For simple dipoles and
ground-independent verticals, mounting height above ground may be the
primary cause. Generally, it’s best to ignore SWR readings under 2:1 and
mount these antennas as high as possible where they’ll perform best. For
antennas that feature adjustable matching networks (Yagis etc), SWR can
usually be improved by following the manufacturer’s antenna setup
instructions. Note that matching and tuning settings may interact, so
readjustment of both the antenna’s element length and matching network
may be needed to obtain best results.
6.7 Matching antennas through a tuner (ATU)
If your antenna can’t be tuned or matched to an acceptable SWR level by
making physical adjustments, then an external antenna tuner (ATU) should
be installed. The MFJ-213 may be used in conjunction with the tuner to
make adjustments without the need to transmit test signals over air. Simply
connect the analyzer to the tuner input (radio side) through a short patch
cable. Select the Analyzer mode, set up the Band, and Tune for the desired
frequency of operation. Then, adjust the antenna-tuner's controls following
the manufacturer's recommendations until SWR approaches 1:1. Remove the
analyzer, reconnect the radio, and the load will be pre-matched to the radio's
50-Ohm operating impedance.
6.8 Antenna Impedance Readings
The MFJ-213 displays complex impedance and impedance magnitude
readings on the same screen with the SWR reading. However, when
measuring through coax, remember that the impedance readings are phase-
shifted values appearing at your end of the cable and not the actual feedpoint
impedance of the antenna itself (Section-5.4). As a “work-around” strategy,
it’s possible to measure the antenna’s actual impedance remotely if the
feedline is cut to an exact electrical half wavelength. In a half-wavelength
line, the phase shift is a full 360 degrees, which electrically rotates the
analyzer’s calibration plane back into alignment with the load. However, this
strategy only works at one frequency and errors compound quickly if your
cable is multiple half-wavelengths long. As a practical matter, unless you
have an advanced working knowledge of transmission-lines, Smith charts,
and impedance matching theory, it’s best to ignore impedances and rely on
SWR for routine antenna-system adjustments.
6.9 Unpredictable SWR
A change in feedline length shouldn't shift your antenna's minimum-SWR
frequency or have much impact on the SWR readings. If it does, suspect a
significant mismatch between the antenna and coax, or more likely, poor
Version 1 10 © 2012 MFI Enterprises, Inc