MFJ-249C Instruction Manual
HF/VHF SWR Analyzer
9
6.3
Impedance of Transmission Lines or Beverage antennas
The impedance of transmission lines between 15 and 150 ohms can be measured with the MFJ- 249C, a 250 ohm
potentiometer, and an ohm meter. Lines of higher impedance can be measured with a higher resistance
potentiometer if a broad band transformer is used (see the section on testing transformers) to transform the line
impedance to approximately 50 ohms.
1.
Measure the 1/4 wavelength frequency of the transmission line to be tested as in Testing and Tuning Stubs
section.
2
. Terminate the far end of the transmission line with a non-inductive 250 ohm potentiometer.
3
. Connect the transmission line to the MFJ-249C "ANTENNA" connector and set the analyzer
to the 1/4 wave frequency.
4
. Observe the SWR as you vary the "TUNE" from end to end of the "FREQUENCY" range
selected.
5
. Adjust the potentiometer until the SWR reading varies as little as possible, over the "TUNE"
range. Note that the
value
of the SWR is not important. Only the
change
in SWR as the
frequency is varied is important.
6
. The value of the potentiometer will correspond closely to the line impedance.
Line Impedance Using a Potentiometer or Resistor Decade Box:
Limit these measurements to the HF region because stray reactance from a potentiometer or decade box could
become a significant source of error at VHF frequencies. Also, use only non-inductive resistances (no wire-wound
resistors). If needed, you may install a broadband transformer of know performance accuracy to extend the
measurement range.
1.) Connect the DUT to the
Antenna
connector.
2.) Terminate the far end with a potentiometer or resistance decade box.
3.) Adjust the analyzer's
VFO
frequency and note
only
the SWR change (SWR need not be low).
4.) Adjust termination resistance until SWR remains constant over the widest possible range.
5.) The resistance of the termination resistor is the line impedance (or surge impedance of the system).
6.4 Adjusting
Tuners
The MFJ-249C can be used to adjust tuners. Connect the MFJ-249C "ANTENNA" connector to the tuner's 50 ohm
input and the desired antenna to the normal tuner output. This connection can be made with a manual RF switch to
facilitate rapid changeover.
WARNING
: Always connect the common (rotary contact) of the switch to the tuner. The switch must connect
either the MFJ-249C or the station equipment to the tuner.
The Station Equipment Must Never Be Connected To
The MFJ-249C.
1.
Connect the MFJ-249C to the tuner input.
2.
Turn on the MFJ-249C and adjust it to the desired frequency.
3.
Adjust the tuner until the SWR becomes unity (1:1).
4.
Turn off the MFJ-249C and re-connect the transmitter.
6.5
Adjusting Amplifier Matching Networks
The MFJ-249C can be used to test and adjust RF amplifiers or other matching networks without applying operating
voltages. The tubes and other components should be left in position and connected so that stray capacitance is
unchanged. A non-inductive resistor that equals the approximate driving impedance of the tube is installed between
the cathode of the tube and the chassis, or a resistor should be connected between the anode and the chassis that
equals the calculated plate impedance of the tube. The appropriate network can now be adjusted. The antenna relay
(if internal) can be engaged with a small power supply so that the coax input and output connectors are tied to the
networks.
