MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
19
To measure input circuits:
Install a non-inductive resistor equaling the approximate driving impedance of each
individual tube between the cathode of each tube and the chassis.
To measure tank circuits:
Install a resistor equal to the tube's calculated operating impedance from the anode to
the chassis using short leads.
The antenna relay (if internal) may be engaged using a small external power supply. Closing the relay will connect
the amplifier’s external RF input and output connectors to the amplifier’s internal RF-matching networks. The
appropriate network may now be adjusted. When the analyzer reads 50 ohms and 1:1 SWR at the operating
frequency with the proper amounts of capacitance to set the system Q, the networks are functioning properly.
Caution: The driving impedance of most amplifiers will change as the input drive level is varied. Do not attempt
to adjust the input network with the tube in an operating condition using the low level of RF generated by the
MFJ-259C stimulus generator!
7.6
Testing RF Transformers
The MFJ-259C can test any RF transformer presenting a 25-100 ohm termination on one of its windings. Connect
the 25-100 ohm winding to the analyzer's Antenna jack using a very short 50-ohm pigtail (<1° phase shift). The
other winding (or windings) should be terminated with a low-inductance resistor equal to the desired load
impedance. Sweep the analyzer's VFO across the DUT's intended operating range. Use the basic SWR, Resistance
(R) Reactance (X) Mode (plus the Impedance Magnitude [Z] option) to evaluate the DUT's impedance and useable
bandwidth. You may also measure the transformer's efficiency by comparing the source voltage generated by the
MFJ-259C to the load voltage using standard power-level conversions.
7.7
Testing Baluns
To test balun performance, connect the analyzer Antenna jack to the balun's 50-ohm unbalanced input. Terminate
the balanced side with two equal-value load resistors connected in series to make up the required load impedance.
For example, to test a 200-ohm (4:1) secondary, use a pair of 100-ohm carbon (non-inductive) resistors in series, as
shown below in Fig A:
Balun
>
A
C
B
Clip Lead
50 Ohms
Unbal
R1
R2
Balun
Clip Lead
50 Ohms
Unbal
R1
R2
<
A
C
Fig A
Fig B
A properly designed current balun works best for maintaining current balance. It also has the highest power
capability and lowest loss for given materials. To evaluate the balun (DUT), measure SWR while connecting the
grounded clip lead to point A, B, and C. When functioning properly, a current balun will exhibit low SWR over its
entire operating range with the clip lead installed at any of those three positions.
A well designed voltage balun should show low SWR over its operating range with the clip lead installed at
position B, but show poor SWR with the clip lead is installed at A or C (note, however, that the SWR readings
should measure about the same whether connected to A or C). A voltage balun should also be tested using the
configuration shown in Fig B, with the resistors in parallel. If it is operating properly, SWR will be remain low
with the resistors connected from either output terminal to ground.