MFJ-266 HF/VHF/UHF Antenna Analyzer
Instruction Manual
©
2011 MFJ Enterprises, Inc.
15
At the low and high ends of the transformer's frequency response range, SWR and reactance will
climb to unacceptable levels (< 1.2:1 is ideal). HF, VHF, and UHF tuned
transmission-line
transformers
may be tested in similar fashion by connecting one end directly to the analyzer and
terminating the far end. However, only precision RF terminations with known impedance
characteristics should be used above 50 MHz. Set up the analyzer for the desired range and
sweep the band of interest using the Tune control. Transmission-line transformers are “frequency
specific” and have much more limited frequency response.
7.9 Checking HF Baluns
A well-designed balun will have low SWR
and good balance
over its operating range. The MFJ-
266 can test both of these qualities using the setup shown below. Configure the unit to operate in
Analyzer
mode in the
HF
range. Connect the input (unbalanced) side of the balun to the
analyzer’s antenna connector. Connect a center-tapped resistive load to the balanced side (R1,R2
= 25
Ω
for 1:1 baluns, R1,R2 = 100
Ω
for 4:1 baluns). Using the
Tune
and
Band
controls:
B
C
Balun
MJ-266
A
Short Test Lead
R1
R2
(1.) Sweep the balun for SWR with the test lead disconnected from the load.
(2.) Connect the test lead to the mid-point (A) and re-sweep. There should be
minimal change.
(3.) Connect it to either side (B) (C). SWR will go up, but should go up equally on
both sides.
7.10 Checking Coax Cable
To check a length of coax cable for impedance error, connect one end to the analyzer and
terminate the far end with a precision (non-inductive) 50-Ohm resistive load. Set the MFJ-266 up
for
analyzer
mode and select the
VHF
tuning range. Rotate
Tune
across the VHF range while
watching the
Impedance Magnitude
reading. If the cable is 50 Ohms and in good condition, there
should be little change in the impedance magnitude readings. If there are significant fluctuations,
the cable is either not 50 Ohms or is badly contaminated. If readings cyclically swing between 25
Ohms and 100 Ohms, the cable is 75-Ohm coax.
