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PRODUCT REVIEW MFJ 9219 QRP TUNER, WATTMETER, AND DUMMY LOAD REVIEWED BY
PHIL SALAS, AD5X AD5X ARRL. NET  THE MFJ 9219 COMBINES AN ANTENNA TUNER, AN
SWR/ WATTMETER, AND A DUMMY LOAD INTO A COMPACT PACKAGE.  IT IS DESIGNED
FOR PORTABLE OPERATION AT POWER LEVELS UP TO 20 W.  MANY QRP LOW POWER
TRANSCEIVERS AND KITS DO NOT INCLUDE AN ANTENNA TUNER OR SWR METER, AND
ANTENNA INSTALLATIONS IN THE FIELD OFTEN REQUIRE COMPROMISES, SO THE MFJ
9219 IS A USEFUL ACCESSORY.  OVERVIEW THE MFJ 9219S ANTENNA TUNER IS A
STANDARD MANUAL T TUNER SERIES CAPACITANCE/ SHUNT INDUCTANCE/ SERIES
CAPACITANCE.  THE TWO VARIABLE CAPACITORS HAVE VALUES OF 20 TO 650 PF, AND
THERE ARE 12 SELECTABLE VALUES OF FIXED INDUCTANCE FROM 0R25 5H TO 6R5 5H.
THE WATTMETER IS ONLY FUNCTIONAL WHEN THE INTERNAL DUMMY LOAD IS USED.  THE
DUMMY LOAD CONSISTS OF EIGHT 2 W, SURFACE MOUNT RESISTORS.  IT WILL HANDLE
5 W FOR SEVERAL MINUTES AT A TIME, BUT IT IS LIMITED TO NO MORE THAN 10
SECONDS AT 20 W, AFTER WHICH IT MUST BE PERMITTED TO COOL FOR AT LEAST 2
MINUTES.  THE TRANSCEIVER INPUT RADIO IS A BNC CONNECTOR, AND THE OUTPUT
ANTENNA CAN EITHER BE COAXIAL ANOTHER BNC OR BANANA PLUGS FOR A RANDOM WIRE
AND GROUND.  FIGURE 1 SHOWS THE INTERIOR OF THE UNIT.  THE MFJ 9219 SWR
CIRCUIT IS BASED ON THE WHEATSTONE BRIDGE, A SIMPLE CIRCUIT THAT CAN
DETERMINE UNKNOWN IMPEDANCES IN RF CIRCUITS.  THEORY OF OPERATION FOR THE
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