=  NOW 40 WPM  =

DEPENDING ON THE MAXIMUM READING DESIRED.  I CALIBRATED MY METER FOR
A FULL SCALE MAXIMUM READING OF 15 V, SO I ADJUSTED THE EXTERNAL
POWER SUPPLY FOR 21R21 V.  THE PEAK OF THE RMS VOLTAGE IS 15 V
1R414.  CONNECT THE SERIES DETECTOR PROBE, THEN CONNECT TO THE
VARIABLE POWER SUPPLY.  ADJUST THE CALIBRATE TRIMMER FOR A METER
READING OF 15 V.  AFTER THIS, THE ANALOG METER WILL BE CALIBRATED
AND READY TO USE.  THE DIGITAL METER CALIBRATION IS ALMOST AS SIMPLE
AS THAT OF THE ANALO RF MEASUREMENTS USING HOMEMADE EQUIPMENT KEN
POLLOCK, WB3JOB THOUGH I HAVE ACCUMULATED MANY DIFFERENT METERS FOR
MEASURING RF VOLTAGE, THERE WAS STILL A GAP IN WHAT I WAS ABLE TO
MEASURE.  THANKS TO YEARS OF EXPERIENCE IN BUILDING SUCH DEVICES, I
EASILY FILLED THE GAP BY PUTTING TOGETHER WHAT I NEEDED.  MY
MEASUREMENT NEEDS COULD BE SATISFIED BY PROBES FOR VARIOUS
APPLICATIONS, SUCH AS RF VOLTAGES, FIELD STRENGTH, AND A SIMPLE
WAVEMETER.  ADDITIONALLY, A MEASUREMENT HEAD COULD BE CONSTRUCTED TO
DISPLAY THE READINGS.  PROBES I DESIGNED AND CONSTRUCTED FOUR BASIC
PROBES A SERIES DETECTOR PROBE, A SHUNT DETECTOR PROBE, A FIELD
STRENGTH PROBE, AND A WAVEMETER PROBE.  THE ACCOMPANYING SCHEMATICS
AND PARTS LIST CAN BE FOUND AT WWW. ARRL. ORG/ QST IN DEPTH.  THE
SHUNT PROBE USES A CAPACITOR IN SERIES WITH THE PROBE TO BLOCK DC
AND RESPOND ONLY TO AC.  THE SERIES DETECTOR PROBE PASSES DC AND IS
USED FOR CALIBRATION.  THE WAVE METER PROBE COVERS THE LOWER HF
BANDS, AND THE FIELD STRENGTH PROBE IS USEFUL FOR HF AND VHF.  EACH
PROBE USES A 1N34A GERMANIUM DIODE.  ONCE THE RF IS DETECTED, THE
SIGNAL IS PASSED THROUGH SERIES RESISTORS TO THE MEASUREMENT DEVICE.
WHEN A SINE WAVE AC IS DETECTED, A DC VOLTAGE IS PRODUCED THAT IS
1R414 TIMES THE RMS VOLTAGE.  THE SERIES RESISTANCES WILL HAVE A
VOLTAGE DROP EQUAL TO THE PEAK VOLTAGE MINUS THE RMS VOLTAGE.  FOR
EXAMPLE, IF 10 V RMS IS MEASURED, THE PEAK VOLTAGE WILL BE 14R14 V.
SELECTING THE PROPER RESISTANCE WILL RESULT IN A 4R14 V DROP, AND 10
V WOULD BE MEASURED BY THE INSTRUMENT.  THIS ACTS AS A VOLTAGE
DIVIDER WITH THE PROBE AS ONE RESISTOR, AND THE INPUT RESISTANCE OF
THE MEASUREMENT DEVICE AS THE OTHER RESISTOR.  I NEEDED TO MEASURE
HIGHER VOLTAGES, SO I ADDED ANOTHER PROBE SEE FIGURE 1 THAT WAS
SWITCHABLE FOR 1X OR 10X.  THE 10X ALLOWS ME TO MEASURE VOLTAGES UP
TO 200 V.  USE CARBON COMPOSITION OR NON INDUCTIVE RESISTORS FOR THE
DIVIDER.  MEASUREMENT DEVICES I OPTED TO BUILD MY OWN MEASUREMENT
HEADS WITH INPUT RESISTANCES OF 10 M.  I BUILT AN ANALOG METER AND A
MORE ACCURATE DIGITAL READOUT METER.  BOTH ARE PORTABLE, AND EACH
CAN USE A STANDARD 9 V BATTERY OR A WALL WART.  THE ANALOG DC METER
CIRCUIT SEE WWW.  ARRL. ORG/ QST IN DEPTH IS BASED ON AN ANALOG 0 TO
1 MA METER SEE FIGURE 2.  MY METER HAD A SCALE THAT WAS ALREADY
LABELED WITH 0 TO 15 V.  I USED AN LM324 QUAD OP AMP BECAUSE IT IS
DESIGNED FOR OPERATION AT LOWER SUPPLY VOLTAGES, AND IT HAS FOUR
INDEPENDENT OP AMPS.  ONE SECTION OF THE OP AMP IS CONFIGURED AS AN
INVERTING AMPLIFIER.  THE GAIN IS SET AT 0R1 BY THE 10
=  END OF 40 WPM TEXT  =  QST DE W1AW  <