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Re: [Amps] Measuring RF Power

To: amps@contesting.com
Subject: Re: [Amps] Measuring RF Power
From: Bill Fuqua <wlfuqu00@uky.edu>
Date: Tue, 22 Mar 2005 16:22:38 -0500
List-post: <mailto:amps@contesting.com>
Now, how about the next example. RMS of a sine wave using only 4 samples 
but this time use 170 volts peak value and 144 ohms (100 watt light bulb). 
I have rounded the following to the nearest tenth to make it easier to read 
but if you wish to carry it out to all digits it works perfectly.
  phase    V        V Squared           Power into 50 ohms
0            0              0                           0
90          170           28900                    200.7
180         0              0                           0
270        -170           28900                   200.7
Total       0               57800                   401.4
Average   0              14450                   100.3
Sq.root    0              120.2

Power = Vrms Squared/R= 14448/144 = 100.3
How about that. only 4 samples.
By the way that is how most power inverters produce a "modified Sine Wave".
The important point is that the RMS and PEAK values are correct. Electronic 
devices with capacitive input filters are happy and so are other things 
like light bulbs, motors ,etc.
          They use a high frequency inverter (switcher) to get the 170 
volts DC and then use a pair of H-Bridge power MOSFETs to put the proper 
voltages on each of the 120 volt output connectors. One thing, the cheaper 
ones do not have a "hot and return" they have two hot terminals that are 
180 degrees out of phase. This is bad news if you are using a hot chassis 
AC/DC set. You are going to have 60 Volts RMS on the chassis no matter 
which way you insert the plug.

73
Bill wa4lav
   

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