MFJ-259 Instruction Manual
HF/VHF SWR Analyzer
8
Measuring the Feedpoint Resistance of Antennas
The approximate feedpoint resistance of a low impedance (0-500 ohms) resonant HF or VHF
antenna or load can be measured with the MFJ-259.
1.
Connect the MFJ-259 directly across the terminals of the unknown resistance. If the load
is unbalanced be sure that the ground is connected to the SO-239 "ANTENNA"
connectors outer shell. If the load is balanced it may be necessary to operate the MFJ-259
using internal battery power to allow the case of the unit to float above ground.
2.
Set the band switch for the desired frequency measurement range.
3.
Adjust the TUNE control until the SWR reads the lowest value.
4.
Read the resistance directly from the resistance meter.
5.
Double check the meter readings against each other. The SWR ratio should be
approximately equal to the ratio between the measured resistance and 50 ohms.
FINDING A SHORT CIRCUIT IN COAX CABLES
The MFJ-259 is very effective in locating a short circuit in coax cables. The method is as
follows:
1.
Connect one end of the cable to the MFJ-259 input.
2.
Turn on the MFJ-259 and start sweeping the frequency, starting at the low frequency end
of its range. Watch the RESISTANCE METER for a dip reading and record the frequency
for zero ohms.
3.
Continue the sweep for the second dip, which should be at twice the first dip frequency.
4.
Calculate the position of the short: Divide 492 by the first dip frequency (in MHz) and
multiply the result by the velocity factor of the coax cable being measured. The result
should be the location of the short (in feet).
5.
You can check this number by shorting the other end and repeating the process.
TESTING AND TUNING STUBS AND TRANSMISSION LINES
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.