From: Kenton E Graham (kgraham@CSWNET.COM)
Date: Sat Aug 31 1996 - 09:50:34 EDT

Since I've had several requests regarding the mention I made on the wattmeter
that appears in "Joy of QRP", Here's some more info.... 73 de Ken K5ID
PS: a good 100 or 200 microamp meter works best....

>To: kf2ph@bnl.gov
>From: kgraham@CSWNET.COM (Kenton E Graham)
>Subject: Wattmeter
>Hi Nick,
>Here's a "word picture" of the wattmeter...
>tie the anode of a 1n62 or ecg109 diode to the 50 ohms in your dummy load.
>The cathode end is grounded thru a .01 disk cap (100v). The cathode also
>goes to another enclosure with the meter in it. I use a bnc connection to the
>other box. In the other box the bnc goes to an rfc-- 1 mh, 300 microh, or
>8 turns on a ft3743, not critical-- then to resistor of about 200-300k to the
>plus side of the meter. (the resistor can be changed to give full scale at
whatever power level you want). Minus side of meter to ground.
>That is the complete circuit. If you remove the 50ohm resistor, the
>wattmeter can be calibrated as follows:Use an adjustable power supply and a
>apply the voltage and meter where the 50 ohms was. correlate lots of
voltage points with meter readings. 10 volts is about 5 watts, 22 volts is
about 5 watts.
>Using the formula P=Vpk*Vpk/2R you will get power. So 10 volts will represent
>1 watt if you have exactly 50 ohms.... etc.
>The diode renders your dummy load useless for high power unless you
disconnect the diode with a switch or something, because the back voltage is
about 60v for the ecg109 and about 100v for the 1n62. Don't forget, or you
will blow the diode!!
>Of course, this is not an inline wattmeter; but it is very handy in setting
up or determining power levels offline..
> good luck and 73, Ken, K5ID

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