MFJ-269C Instruction Manual LF/HF/VHF/UHF SWR Analyzer
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To measure the resonant frequency of any Impedance Stub or Transmission Line, select the first (or
opening) measurement mode in the Main menu (or use the protocol outlined in 5.5 Advanced- 2).
Connect the stub under test to the Antenna port of the MFJ-269C.
Note
: The line must be open circuited at the far end for odd multiples of 1/4 wave stubs (i.e. 1/4, 3/4, 1-
1/4, etc.). It must be short circuited for all half-wave stub multiples (like 1/2, 1, 1-1/2, etc.).
If a balanced line is used, operate the MFJ-269C from internal batteries and keep its case isolated by a
few feet from other conductors or earth. Do not attach any wires (other than the feedline). Use the
Antenna connector shield for one lead and the center pin for the other. Two wire balanced lines must be
suspended in a straight line a few feet away from other metallic objects or ground.
If coax is used, the cable may be piled or coiled on the floor and external power can be used with the
analyzer (isolation is not necessary). Coaxial lines connect normally, with the shield grounded.
When tuning critical stubs, gradually trim the stub to frequency -- and adjust the feedline or stub using
the following method:
1. Determine the operating frequency and calculate the theoretical length for the feedline or stub.
2. Cut the stub or feedline 20% longer than the calculated length.
3a. Quarter-Wave: Measure frequency of the lowest resistance and reactance, or the lowest impedance
for odd quarter wave stubs. To fine tune, look only at the X= display and adjust frequency for X=0,
(or as close to X=0 as you can). That frequency should be about 20% below the desired operating
frequency if everything worked as planned during the length calculation.
3b. Half-Wave: Measure the frequency of highest Zo where the analyzer overflows and Z>1500 appears.
4. Divide the measured frequency by the desired frequency.
5. Multiply the result by the length of the feedline or stub to find the required length.
6. Cut the stub to the length calculated in step 5, and confirm lowest X is on he desired frequency.
The Distance to Fault mode can also be used. It will directly display the line length in degrees at any
frequency you choose. See section 5.5 Advanced 2.
7.2 Velocity Factor of Transmission Lines
The MFJ-269C accurately determines velocity factor of any transmission line. Select the Distance to
Fault mode in Advanced-2 (see section 5.5).
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