MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
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Example:
27 feet (actual physical length) divided by 33.7 feet (measured electrical length) equals .80.
The velocity factor is .80 or 80%.
4.
If the analyzer is now set to
Vf= .80
and the line measured, the result should be the correct physical
length.
7.3
Impedance of Transmission Lines or Beverage antennas
The impedance of transmission lines between a few ohms and 1500 ohms can be directly measured with
the MFJ-269D. Lines of higher impedance can be measured if a broadband transformer or resistance is
used to extend the MFJ-269D’s range. Select any measurement mode that indicates resistance (R=) and
reactance (X=).
If a balanced line is used, operate the MFJ-269D
only
from internal batteries and keep it a few feet away
from other conductors or earth. Do not attach any wires (other than the feedline) to the unit. Use the
Antenna
connector’s shield for one lead and the center pin for the other. Two-wire balanced lines
must
be suspended in a fairly straight line a few feet away from metallic objects or ground.
Setup for coaxial line need not be isolated, and Beverage antennas may be directly connected to the MFJ-
269D.
Using fixed resistances:
1.
Terminate the line or antenna in a non-inductive resistance somewhere around the expected value.
2.
Connect the transmission line or antenna directly to the MFJ-269D
Antenna
connector. Adjust the
frequency (near the expected operating frequency) until the lowest resistance and lowest reactance is
measured.
3.
Record the impedance value.
4.
Adjust the frequency until the highest resistance and
lowest
reactance is measured.
5.
Multiply the highest resistance by the lowest resistance, and find the square root of the result.
Example:
The highest resistance is 600 ohms, the lowest is 400 ohms. 400 x 600 = 240,000. The
square root of 240,000 is 490. The impedance is 490 ohms.
Using a potentiometer or resistor decade box:
1.
Connect the MFJ-269D to one end of the system (in this case you can use a broadband matching
transformer).
2.
Adjust the frequency and note
only
the SWR change.
3.
Adjust the termination resistance until the SWR remains as constant as possible with very large
frequency changes around the operating frequency range.
4.
The resistance of the termination resistor is the surge impedance of the system.
The electrical length of the Beverage can be determined by using procedures outlined in Advanced 2.
7.4
Adjusting Tuners
The MFJ-269D can be used to adjust tuners. Connect the MFJ-269D
Antenna
port connector to the
tuner's 50-ohm input. Connect (or select) the desired antenna to the tuner output. This connection may be
made with a manual RF switch to facilitate rapid changeover, provided that switch has better than 50-dB
port isolation.