MFJ MFJ-731 User Manual
MFJ-731 Analyzer Filter Instruction Manual
MFJ-731 Analyzer Filter
INTRODUCTION
The MFJ-731 Analyzer Filter is designed to null AM broadcast stations, and
attenuate all other types of out-of-band interference to Antenna Analyzers. This
unit works with all brands of analyzers.
The MFJ-731 Analyzer Filter is also ideal for any application requiring a low
power band-pass band-reject filter that provides a single adjustable deep notch
between 550 and 1800 kHz, while passing an adjustable range of frequencies
1.8-2.0, 3.5-4.0, and 7-30 MHz.
OPERATION
Some models of the MFJ-259, and all models of the MFJ-259B, disable the
internal oscillator when in the frequency counter mode. Use the following
procedure with these instruments. Be sure to follow the steps in exact order as
described below.
1. Connect the "SWR ANALYZER" port of the MFJ-731 Analyzer Filter to the
"ANTENNA" port of the analyzer.
2. Be sure the red "oprerate-tune" switch is placed in the "tune" (in) position.
3. Connect the antenna to the "ANTENNA" port, set the "BAND" switch to the
desired frequency range, set the "TRAP" control to the 0 (full counterclockwise) position.
4. Turn the analyzer power on, and adjust its frequency to the desired
frequency.
5. Adjust the "PASS" control for a 1:1 SWR reading, with an indicated "X" of
zero ohms. If necessary, move the "BAND" switch to another range that
allows nulling reactance and SWR readings.
6. Switch to the "operate" mode, and place the antenna analyzer in the
Frequency Counter mode (MFJ-259B and some models of MFJ-259 only).
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MFJ-731 Analyzer Filter Instruction Manual
7. If you see any reading on the "SWR" meter of the 259 or 259B analyzer,
adjust the "TRAP" control for a null.
8. If you adjusted the "TRAP" in step 7, place the MFJ-731 Analyzer Filter
back in the "tune" mode, measure SWR with the analyzer, and repeat steps 4
and 5 before continuing.
9. With the MFJ-731 Analyzer Filter's "operate-tune" switch in the "operate"
position, you can now measure SWR and impedance of the antenna system.
NOTES:
A.) On frequencies above 14 Mhz, accuracy is improved if you touch up the
"PASS" control with an accurate and reliable external dummy load as a final
adjustment step.
B.) Because of internal losses, this unit adds a slight amount of parallel
resistance to the load. Expect the resistance readings to be reduced a very
small amount from the actual values. Normal reduction is about two ohms
with a fifty ohm load.
C.) Once the correct setting is obtained, it is not necessary to readjust the
"TRAP" control if the measurement frequency is changed. It is necessary to
check, and perhaps readjust the "PASS" setting, if measurement frequency
is changed.
USE WITH OTHER ANALYZERS
Unfortunately, other analyzers might not allow you to measure return RF with
the internal oscillator disabled. The MFJ-731 Analyzer Filter can still be used
with these instruments, but you will have to adjust the analyzer filter's trap while
measuring the offending signal's level at the "SWR ANALYZER" port with a
receiver, oscilloscope, or a by a trial and error method.
The trial and error method requires adjusting the "TRAP" control for lowest
SWR reading, switching to "tune", and adjusting the "PASS" control for lowest
SWR after the trap is adjusted. This loop should be repeated a few times, until
the lowest SWR reading is obtained.
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