‰  NOW 40 WPM  ‰  TEXT IS FROM JANUARY 2015 QST  PAGE 49‰
THEM PERFECT FOR HEADSETS, FOR EXAMPLE, BUT EQUALLY APPROPRIATE TO BE USED
IN LARGER DESKTOP STRUCTURES IN PLACE OF A DYNAMIC ELEMENT.  ELECTRETS NEED
POWER THE ONE DRAWBACK OF AN ELECTRET MIC IS THAT IT NEEDS AN OPERATING
VOLTAGE, TYPICALLY IN THE RANGE OF 5 8 V DC, AT LOW CURRENT.  DYNAMICS, ON
THE OTHER HAND ARE SELF POWERED, JUST MAKE SURE YOUR RADIO DOESNT APPLY DC
TO THE MIC LEADS IF YOU USE ONE.  MANY RADIOS INCLUDE AN OPERATING VOLTAGE
FOR ELECTRETS RIGHT AT THE MIC CONNECTOR

SETUP YOU WILL NEED A PC OR LAPTOP RUNNING WINDOWS, AND AN INTERNAL SOUND
CARD OR EXTERNAL USB SOUND CARD.  I PREFER EXTERNAL USB SOUND CARDS THAT
USE THE TEXAS INSTRUMENTS SOUND CHIP BECAUSE THEY REQUIRE NO DRIVER
INSTALLATION.  EXAMPLES INCLUDE THE BEHRINGER UCA 202 AND ADS RDX 150.  SET
YOUR COMPUTER CLOCK TO UTC WITHIN 1 SECOND.  THIS IS NECESSARY BECAUSE WSPR
TRANSMITS ON EVEN NUMBERED MINUTES 0, 2, 4, ETC.  CORRECT UTC CAN BE FOUND
ON THE INTERNET AT WWW. USNO. NAVY.  MIL/.  OTHERWISE, A TIME CLOCK
SYNCHRONIZED TO THE NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY NIST
RADIO STATION, WWV, WORKS WELL.  YOU DONT HAVE TO BE CONNECTED TO THE
INTERNET TO TRANSMIT WSPR.  BUT TO SEE WHERE YOU ARE BEING RECEIVED YOU
NEED TO CHECK WSPRNET. ORG, ENTER YOUR CALL SIGN, AND VIEW THE MAP THAT
LOOKS LIKE THE ONE SHOWN IN FIGURE 1.  WSPR SOFTWARE YOU CAN DOWNLOAD AND
INSTALL THE FREE WSPR SOFTWARE FROM THE WSPR SITE.  THE SOFTWARE DISPLAYS A
BLACK WINDOW FOR ERROR MESSAGES AND A MAIN SCREEN SHOWN IN FIGURE 3.  CLICK
SETUP AND ENTER YOUR CALL SIGN, POWER LEVEL, SOUND CARD SETTING, AND
MAIDENHEAD GRID LOCATION.  CONNECT YOUR AUDIO CABLE, DC POWER SOURCE, AND

LOSCOPE WILL SHOW 0R56 V PEAK TO PEAK.  NOTE THE POSITION OF THE PC VOLUME
SLIDER CONTROL SO YOU CAN RETURN TO THIS SETTING IF IT IS CHANGED.  THIS
LEVEL, WHILE NOT ABSOLUTELY CRITICAL, SHOULD BE IN THE RANGE OF 0R14 TO
0R24 V RMS.  THIS ADJUSTMENT CAN BE USED TO INCREASE OR DECREASE THE POWER
OUTPUT OF THE TRANSMITTER OVER A SMALL RANGE.  NEXT, WITH AUDIO SIGNAL
REMOVED AND 12 V DC POWER OFF, ADJUST BOTH BIAS POTENTIOMETERS R18 AND R19
FOR 0 TO GROUND.  SWITCHING TYPE MOSFETS DO NOT HAVE PRECISELY SPECIFIED
GATE VOLTAGE THRESHOLDS.  TYPICALLY THEY VARY FROM 2 TO 4 V.  HENCE, THE
BIAS FOR EACH MOSFET NEEDS TO BE SET SEPARATELY.  STARTING AT ZERO BIAS
WILL ENSURE THAT THE MOSFETS ARE OFF INITIALLY.  THEN INCREASE THE BIAS
VOLTAGE TO THE POINT WHERE THEY ARE JUST BARELY CONDUCTING.  HERE IS THE
SIMPLE PROCEDURE.  FIRST CONNECT A 50 RF LOAD RESISTOR ACROSS THE ANTENNA
TERMINAL.  CONNECT YOUR VOM DC MV SETTING BETWEEN GROUND AND TEST POINTS
TP1 AND TP2 IN TURN TO MONITOR FOR THE ONSET OF CONDUCTION OF EACH OF THE
MOSFETS.  WITH AUDIO SIGNAL REMOVED, TURN ON THE 12 V DC TO THE
TRANSMITTER.  ADJUST R18 FOR A 1 TO 2 MV READING AT TP1 ON THE VOM.  THIS
CORRESPONDS TO ABOUT 1 TO 2 MA OF CURRENT IN Q2.  NOW MOVE YOUR VOM TO TP2
AND ADJUST R19 FOR 2 TO 3 MV.  A HIGHER SETTING WILL REDUCE IMD, BUT
TRANSISTORS WILL GET HOTTER.  YOUR WSPR TRANSMITTER IS READY TO GO.  WSPR
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ