MFJ MFJ-249 - Page 8

MFJ-249 SWR Analyzer Instruction Manual
To measure the SWR on a predetermined frequency adjust the "i UNE" and "FREQUENCY"
knobs until the counter displays the desired frequency. Read the SWR from the "SWR" meter.
To find the lowest SWR adjust the frequency until the SWR meter reaches the lowest reading.
Read the frequency of the lowest SWR directly from the counter display.
Note:. The internal oscillator of the MFJ-249 will not produce ar: entirely stable reading on the
frequency display due to short term drift in the oscillator. This drift is normal and does
not affect the accuracy, results or usefulness of this instrument since a null will even be
maintained on the highest Q (narrow bandwidth) antennas or.loads.
It is perfectly acceptable to ignore decimal place readings more than two places to the right of
the decimal point in the counter (ten's of kilohertz) above 15 MHz and thrée places to the right
of the decimal point (kilohertz) below 15 MHz.
Frequency Counter
The frequency counter used in the MFJ-249 will typically measure frequencies between a few
hertz and 200 MHz. The frequency counter is designed to accept TTL input voltage levels but
will respond to voltages as low as a few hundred millivolts. The counter function is accurate
to 1 part per million at room temperature.
To use the frequency counter turn the power "ON" and press the "INPUT" button on the top
panel until a "B" appears in the upper right corner of the counter display. The BNC jack is
now selected for input to the counter.
The sample time period is selected by momentarily pressing the 'GATE" button near the upper
left corner of the top panel. The counter displays the average frequency over the sample time
period. The red LED in the upper right corner of the front panel flashes when the count cycle
is complete and the display is updated. The sample time is displayed in the lower left corner of
the counter display. At power up, the sample time period normalizes at .0! seconds.
Additional count periods of 0.1, 1.0, and 10 seconds can also be selected.
Connect the signal to be counted to the BNC type Jack labeled "FREQUENCY COUNTER
LNPw
Warning: The frequency counter has a CMOS input device that can be easily damaged. To
avoid damage to the counter while using the "FREQUENCY COUNTER INPUT"
jack the user must observe the following precautions: NEVER exceed 5 volts
peak input. NEVER apply an input signal with the power switch off.