MFJ-266B HF/VHF/UHF Antenna Analyzer
Instruction Manual
2012 MFJ Enterprises, Inc.
Version 1D
12
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
isolation between the feedline and the antenna. Isolation problems
typically occur when unbalanced coax line is connected directly to a
balanced element such as a dipole or a loop, and the outer surface of the
coax shield literally becomes a part of the antenna. If the length of the
shield happens to presents a low impedance path, it can load the element
significantly and shift the minimum-SWR frequency unpredictably. It will
also introduce needless mismatch, divert transmitted RF back toward the
operating position, cause RFI problems in the residence, and increase
unwanted noise pickup in receive mode. The best way to decouple the
outer surface of the shield from the antenna element is with a balun.
Current-type baluns work best because they have higher power-handling
capability and less loss than other types. An effective current balun could
be as simple as a few loops of coax taped together at the feedpoint, but
for best common-mode rejection, a Guanella-style balun wound on a
ferrite core is recommended.
7.0 ADVANCED FUNCTIONS
Here are some of the MFJ-266B advance functions. Note that some of
these procedures involve connecting component leads to the unit's N
connector. For these connections, we suggest making up a very short N-
male coaxial pigtail or obtaining a type-N dual binding post adapter to
prevent damage the center-contact of the analyzer connector.
7.1 Frequency Measurement
The MFJ-266B features a precision counter that accurately measures the
frequency of RF signals between 1 and 500 MHz with up to 100-Hz
resolution (see Section-3.3 for setup). To conduct measurements, connect
your signal source to the unit’s antenna jack. The counter will typically
lock onto any signal 30 mv (-20 dBm) or stronger, with an upper signal
measurement limitation of 1 volt (or +10 dBm). Any input signal
exc10 dBm automatically triggers a Danger – High FS warning
on the analyzer display (high field strength). This is a notification to
reduce the signal level as quickly as possible.
Warning: Avoid connecting any external RF source more powerful
than 10 dBm (or 10 mW) to the analyzer's antenna connector.