MFJ 209 - Velocity Factor of Transmission Lines

MFJ-209 SWR Analyzer Instruction Manual
8
Connect the PL-259 to the "ANTENNA" connector of the MFJ-209 and adjust the line or stub
by the following method. For critical stubs you may want to gradually trim the stub to
frequency.
1 Determine the desired frequency and theoretical length of the line or stub.
2 Cut the stub slightly longer than necessary.
3 Measure the frequency of the lowest SWR. It should be just below the desired frequency.
4 Divide the measured frequency by the desired frequency.
5 Multiply the result by the length of the stub. This is the necessary stub length.
6 Cut the stub to the calculated length and confirm that it has the lowest SWR near the
desired frequency.
Velocity Factor of Transmission Lines
The MFJ-209 can accurately determine the velocity factor of any impedance transmission line.
Measure the velocity factor with the following procedure:
1. Disconnect both ends of the transmission line and measure the physical length of the line in
feet.
2. Set up the line to measure 1/4 stubs as in the section on Testing and Tuning Stubs, page 7.
3. Find the lowest frequency across all the bands at which the lowest SWR occurs. The dip
should occur slightly below the 1/4 wavelength frequency.
4. Measure the frequency from the MFJ-209. This is the 1/4 resonant wavelength frequency
of your transmission line. Note that you will get low SWR reading at all odd multiples of
1/4 wavelength.
Example: On a 27 foot line the measured frequency was 7.3MHz.
5. Divide 246 by the measured frequency. This gives you the free space 1/4 wavelength in
feet.
Example: 246 divided by a dip frequency of 7.3 MHz is 33.7 feet, the free space 1/4
wavelength
6. Divide the physical measured length of the feedline in feet by the free space 1/4 wavelength
calculated in number 5.
Example: 27 feet (physical length) divided by 33.7 feet (calculated length) equals .80 .
The velocity factor is .80 or 80%.
Free space 1/ 4 wavelength =
Low SWR frequency
Velocity Factor =
Free space 1/ 4 wavelength
Actual feedline length
246