MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
3
Note:
For a more complete description of features and test methods, consult the table of contents to find
the manual sections describing the particular measurement you wish to make.
1.2
Frequency Range
The unit's dual-range
Frequency
switches select the following oscillator bands with a small overlap:
0.10-0.16 MHz
2.1-4.7 MHz
67-113 MHz
0.26-0.52 MHz
4.7-10.9 MHz
113-155 MHz/UHF LO*
0.52-1.02 MHz
10.9-28 MHz
155-230 MHz/UHF HI*
1.02-2.1 MHz
28-67 MHz
*A
UHF
pushbutton switch located above the LCD display activates 415-470 MHz SWR coverage. See
section 3.4 for VFO operating specifics.
1.3
Accuracy Notes
If measurement errors occur, they will likely be caused by one of the following conditions:
1.
Signal ingress from external sources, usually from a strong AM broadcast station.
2.
Diode detector and A/D converter error.
3.
Stray impedance errors contributed by connectors, cables, and adapters.
Broad-band Voltage Detectors and External Interference:
Laboratory grade network analyzers use
expensive high-selectivity gain-stabilized receivers to avoid off-frequency interference and ensure
measurement accuracy. Building these sophisticated detectors into the MFJ-269D (or any small handheld
unit) would drive the price far beyond the reach of most hobbyists. As an alternative, we use broadband
detectors that provide accurate measurements at a much lower cost. The only drawback is that broadband
detectors can be sensitive to powerful out-of-band signals. Most of the time, out-of-band interference isn't
an issue, but occasionally a particularly powerful signal may be picked up by the antenna under test and
routed into the analyzer bridge circuit where it conflicts with the internally generated VFO signal. When
strong "signal ingress" such as this occurs, it may result in inaccurate readings.
The solution for out-of-band interference isn't simple. Increasing the analyzer's generator power would
help, but doing so causes the unit to draw significantly more power at the expense of reduced battery
operating time. Higher power may also cause on-air interference when testing antenna systems that
radiate efficiently or exhibit directivity gain. Using common low-pass or band-pass filters similar to those
used in transceivers also wouldn't work because they behave like transmission lines of varying
impedance on different frequencies. Using them would only introduce gross measurement inaccuracies.
MFJ-731:
Fortunately, most analyzer interference problems occur on the lower frequencies, with near-by
high power AM broadcast signals being the worst offender. When testing physically large antenna arrays
such as 160-meter verticals, these powerful outside signals may couple very efficiently into the analyzer's
bridge circuit. Other strong local signals may "get in" as well. To correct the problem, we offer the MFJ-
731 tunable filter, an accessory especially designed to attenuate off-frequency signals. The MFJ-731
permits accurate impedance measurements between 1.8 and 30 MHz with virtually no impact on
measurement accuracy.
Detector Errors:
At low voltages, detector diodes become non-linear. To address this issue, the MFJ-
269D uses special microwave zero-bias Schottky detectors with matched compensating diodes. Each unit