MFJ-249C Instruction Manual
HF/VHF SWR Analyzer
8
3.) Select any analyzer operating mode that will display
SWR
.
4.) Read the
SWR
and adjust the
VFO
tuning for minimum
SWR.
Write down the frequency.
5.) To re-tune your antenna accurately without a lot of cut-and-try, calculate a
Scaling Factor.
5.) First, determine if the antenna needs to be shorter (higher in frequency) or longer (lower in frequency).
6.) To make it
shorter ( higher)
, divide the
present frequency
by the
desired frequency
(scaling factor <1).
8.) To make it
longer (lower)
, divide the
desired frequency
by the
present frequency
(scaling factor >1).
9.) Multiply the
scaling factor
by the
present length
to calculate the new length.
For example, suppose your 132-foot dipole has low SWR on 3.750 MHz and you want to move it to 3.900 MHz. It
needs to be shorter to tune higher, so you calculate the scaling factor:
3.750/3.900 = 0.95.
Next calculate the new
length:
132 feet x .96 = 125.7 feet
. Note that scaling only applies to full-size verticals and dipoles that don't use
loading coils, traps, stubs, resistors, capacitors, or capacitance hats. Antennas with these features should be adjusted
according to the manufacturer’s instructions.
6.0 TESTING AND TUNING STUBS AND TRANSMISSION LINES
6.1 Testing
Stubs
The proper length of quarter and half wave stubs or transmission lines can be found with this unit and a 50
Ω
carbon
resistor. Accurate measurements can be made with any type of coaxial or two wire line. The line does
not
have to be
50 ohms. The stub to be tested should be attached with a 50
Ω
noninductive resistor in series to the center conductor
of the "ANTENNA" connector with a coaxial line. The shield should be grounded to the connector shell. For two
wire lines the 50
Ω
resistor connects in series between the ground shell of the PL-259 and one conductor. The other
conductor of the balanced line connects directly to the center pin of the connector. Coaxial lines can lay in a pile or
coil on the floor, two wire lines
must
be suspended in a straight line a few feet away from metallic objects or
ground. The lines 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.)
and
short circuited for half wave stub multiples
( like 1, 1-1/2, etc.). Connect the PL-259 to the "ANTENNA"
connector of the MFJ-249C 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.
6.2
Velocity Factor of Transmission Lines
Velocity Factor of Transmission Lines
The MFJ-249C 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.
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.
Read the frequency from the frequency counter display. 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..
5.
Divide 246 by the measured frequency. This gives you the free space 1/4 wavelength in feet.
