MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
18
Line Impedance Using an Intentional Mismatch:
The test resistance should be somewhere near (but different from) the line's anticipated impedance. When choosing
a resistance value, consider the limits of the analyzer's measurement range (7 - 650 ohms) and stay well within it.
1.) Connect the DUT to the Antenna connector.
2.) Terminate the far end of the DUT with a non-inductive test resistance.
3.) Tune VFO to the lowest frequency where the Impedance and Resistance indicators both null.
4.) Fine tune to find where X is closest to 0 and R is at its minimum value (null).
5.) Press Gate to confirm
θ
= 0°. Write down the Resistance (R) value as R1.
6.) Tune up in frequency to find a distinct Impedance peak. Once again, X should approach or equal 0.
7.) Fine tune for the highest Resistance (R) value and confirm
θ
= 0°. Record Resistance (R) as R2.
8.) Multiply R1 x R2 and find the square root of the product. The result is the line's characteristic impedance.
Example: R1 = 37 ohms. R2 = 66 ohms. 37 x 66 = 2442. The square root = 49.42 ohms.
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).
7.4
Adjusting Tuners
Use a short patch cable to connect the analyzer's Antenna jack to the tuner's 50-ohm input. This connection may
also be made with a manual RF switch to facilitate rapid changeover, provided the switch has >50-dB port
isolation. When using any in-line switch, make sure there is no possible way for the transmitter and the analyzer to
become inadvertently connected!
Important Warning: Always connect the common (rotary contact) of the external RF switch to the tuner. The
switch must connect either the MFJ-259C or the station transceiver to the tuner. Transmitting equipment must
never be configured in a manner where it could accidentally become connected to the MFJ-259C.
1.) Patch the analyzer Antenna jack to the tuner input connector.
2.) Connect the desired antenna at the tuner output connector.
3.) Tune the analyzer VFO to the desired frequency and leave it there.
4.) Select the analyzer's basic SWR, Resistance (R), Reactance (X) Mode.
5.) Adjust the tuner's controls until the analyzer's SWR indicators show unity (1:1).
6.) Turn off and disconnect the MFJ-259C. Re-connect the tuner to the transmitter.
7.5
Adjusting Amplifier Matching Networks
To use the MFJ-259C for testing and adjusting RF amplifiers or other matching networks without applying
operating voltages, see the instructions below. Note that all tubes and other components should be left in position
and connected so that all sources of stray capacitance will remain unchanged.