++++++++++++++++++ To: DolfinDon at aol.com Cc: dbarker at visioncomm.net, elecraft at mailman.qth.net Date: Sat, 16 Feb 2002 12:22:08 -0700 Subject: Re: [Elecraft] Power measurement with a 'scope From: Gary L Surrency Direct conversion from pk-pk to rms voltage: Vrms = Vpp x 0.35355 (ARRL 1998 Handbook conversion factor, page 6.6) Gary Surrency AB7MY On Fri, 15 Feb 2002 20:19:01 EST DolfinDon at aol.com writes: > In a message dated 2/15/2002 6:54:40 PM Central Standard Time, > dbarker at visioncomm.net writes: > > > I have a 100 MHz 'scope (Tektronix 465, Ebay, about $60). How is > QRPp > measured using a scope and a dummy load ++++++++++++++++++ From: DolfinDon at aol.com Date: Fri, 15 Feb 2002 20:19:01 EST Subject: Re: [Elecraft] Power measurement with a 'scope To: dbarker at visioncomm.net, elecraft at mailman.qth.net In a message dated 2/15/2002 6:54:40 PM Central Standard Time, dbarker at visioncomm.net writes: I have a 100 MHz 'scope (Tektronix 465, Ebay, about $60). How is QRPp measured using a scope and a dummy load +++++++++++++++++++++++++++++++++++++++++++++++++++++ Hi A scope measures peak to peak voltage to convert to watts. First measure the peak to peak voltage at the dummy load. Convert to peak by dividing by 2 then convert to rms volts by dividing by 1.414. Then convert to watts by squaring the rms voltage and dividing by 50 ohms Don Brown KD5NDB +++++++++++++++++++++ Reply-To: From: "Ron D' Eau Claire" To: "Elecraft" Subject: RE: [Elecraft] Power measurement with a 'scope Date: Fri, 15 Feb 2002 17:25:46 -0800 > I have a 100 MHz 'scope (Tektronix 465, Ebay, about $60). How is > QRPp measured using a scope and a dummy load... > Dan / WG4S / K2 #2456 The oscilloscope accuracy is very important. Power = E^2/R. That is Power is equal to the voltage squared divided by the resistance. Any error in the voltage reading is squared! So if your scope is 5% off, the resulting calculated power will be 20% off! It's real easy to have a 5% or 10% error in the voltage reading if that scope has not been properly calibrated recently, or if you aren't careful about how you line up the trace on the graticule. Frankly I'd suggest that you can do better with a simple peak-reading voltmeter. Also, you are dependent upon the resistance of the dummy load. Don't assume that it is 50 ohms, as John observed. However, it won't hurt it it's not too far off - the K2 output filters tolerate a fair variation in the output load. Of course, you want to read RMS volts to calculate RMS power, which is what you are looking for if you want what most Hams refer to as the "output power". Ron AC7AC K2 # 1289 ++++++++++++++++++++++ Date: Sat, 16 Feb 2002 02:45:37 +0100 From: "Ingo, DK3RED" To: Elecraft Subject: [Elecraft] Re: Power measurement with a 'scope Hello Dan. > ... How is QRPp measured using a scope and a dummy load ... It's easy! Step zero: Preparation Connect the dummy and switch the K2 to transmit (press the key). Step 1: Vpp (peak-to-peak voltage) Messure the peak-to-peak voltage on the dummy with the scope. i.e. Vpp = 14.14 volts Step 2: Veff (effective voltage) Veff = 0.707 ( Vpp / 2 ) Veff = 0.707 ( 14.14 / 2 ) Veff = 5 volts Step 3: R (resistor/dummy) R = 50 ohms Step 4: Power (output) P = (Veff * Veff ) / R P = (5 * 5 ) / 50 P = 0.5 watts P = 500 milli watts -- 72/73 de Ingo, DK3RED Don't forget: the fun is the power! dk3red at t-online.de http://www.qsl.net/dk3red ++++++++++++++++++ Date: Fri, 15 Feb 2002 23:38:07 -0500 From: "David A. Belsley" To: rondec at easystreet.com, Elecraft Subject: RE: [Elecraft] Power measurement with a 'scope > The oscilloscope accuracy is very important. > > Power = E^2/R. That is Power is equal to the voltage squared divided by > the resistance. > > Any error in the voltage reading is squared! So if your scope is 5% off, > the resulting calculated power will be 20% off! Let E be true voltage, M be measured voltage, and let e be the relative error (such as +-.05) defined by M = E(1+e). Let MP denote measured power and AP actual power. Then MP = M^2/R = (E(1+e))^2/R = (E^2/R)(1+e)^2 = AP(1+e)^2. (1+e)^2 = 1 + 2e + e^2, and for small absolute e (such as .05), this is approximately equal to 1+2e. Thus, the proportionate power error is approximately twice that of the voltage reading. A voltage error of absolute 5% would produce roughly an absolute 10% power error. Now, it's true, if you don't know whether your 5% voltage error is high or low, then the resulting power measurement could be 10% low or 10% high, giving, in some sense, a possible 20% error spread. But, of course, then the voltage measurement would, in the same sense, have been a 10% error, not a 5% error, and, again, the power error would be twice that of the voltage. best wishes, dave belsley, w1euy ++++++++++++++++++ Reply-To: From: "Ron D' Eau Claire" To: "Elecraft" Subject: RE: [Elecraft] Power measurement with a 'scope Date: Fri, 15 Feb 2002 21:40:08 -0800 > Now, it's true, if you don't know whether your 5% voltage error > is high or > low, then the resulting power measurement could be 10% low or 10% high, > giving, in some sense, a possible 20% error spread. But, of course, then > the voltage measurement would, in the same sense, have been a 10% error, > not a 5% error, and, again, the power error would be twice that of the > voltage. > > dave belsley, w1euy Thanks, Dave. I wasn't being rigorous in my example and I apologize. The point I was making is that because the voltage is squared in the process of calculating the power, the accuracy of the voltage measurement is much more important to the final result than one might realize at first. For example, if you are making exactly 5 watts out and measure 14.24 volts instead of 15 volts rms (reading 5% low) or 15.75 volts instead of 15 volts rms(reading 5% high) then the power calculation at 50 ohms load would be 4.06 watts at the lower side and 4.96 watts on the high side. That's a total range of error of just about 20%. That's pretty bad when one starts out thinking that 5% accuracy is surely adequate for a good, precise power measurement. Ron AC7AC K2 # 1289 ++++++++++++++++++ Reply-To: From: "Ron D' Eau Claire" To: "Elecraft" Subject: RE: [Elecraft] Power measurement with a 'scope Date: Sat, 16 Feb 2002 09:10:26 -0800 > Your example states 5W and 50ohm. So, a measured 14.25 is 10% low > [(15.8-14.25)/15.8]. Going to 10% high, we have 17.40v. These > yield power readings of 4.06 (19% low) and 6.06 (21% high). > > So, your idea was correct but the example had some problems. > > > As to the Cantenna resistance at HF, how do I measure that > without an accurate power meter? I figured it wouldn't have any > reactive components at all and the DC ohmmeter would do. > > Dan / WG4S +- 3 letters / K2 #2456 +- 0 Egad! Dan. I caught that error before sending it and then forgot to correct it! Okay. NO more arithmetic! The resistor in the Cantenna is supposed to be "non inductive" (i.e. non-reactive for this purpose). I have not run careful measurements on the Cantenna, but I would suggest that you are in very good shape if you use the d-c resistance value. That issue came up on here a year or so ago and, as I recall, several ops did use the d-c value and it agreed very well with other measurement methods. Many of the Cantenna's are a good 10 ohms low. After all, they were designed as a dummy load with a low SWR, not as a "precision" load. Ron AC7AC K2 # 1289 ++++++++++++++++ To: DolfinDon at aol.com Cc: dbarker at visioncomm.net, elecraft at mailman.qth.net Date: Sat, 16 Feb 2002 12:22:08 -0700 Subject: Re: [Elecraft] Power measurement with a 'scope From: Gary L Surrency Direct conversion from pk-pk to rms voltage: Vrms = Vpp x 0.35355 (ARRL 1998 Handbook conversion factor, page 6.6) Gary Surrency AB7MY On Fri, 15 Feb 2002 20:19:01 EST DolfinDon at aol.com writes: > In a message dated 2/15/2002 6:54:40 PM Central Standard Time, > dbarker at visioncomm.net writes: > > > I have a 100 MHz 'scope (Tektronix 465, Ebay, about $60). How is > QRPp > measured using a scope and a dummy load ++++++++++++++++++ From: "Dan Barker" To: "Elecraft" Subject: [Elecraft] RF Probe measurments - K1 Date: Mon, 18 Feb 2002 15:13:56 -0500 My KAT2 book said 15.8 volts. What revision you reading? >From the website, The Rev D manual (3/22/01) also says 15.8. My = calculator agrees! Dan / WG4S / K2 #2456 -----Original Message----- From: elecraft-admin at mailman.qth.net [mailto:elecraft-admin at mailman.qth.net]On Behalf Of James C. Hall, MD Sent: Sunday, February 17, 2002 10:23 PM To: elecraft at mailman.qth.net Subject: [Elecraft] RF Probe measurments - K1 Hello: Today I built an RF probe to calibrate power output on my K1 after = adding the antenna tuner unit. In the instructions for calibration, the 5.0 = watt setting on the meter is to corrolate to a DC volt reading of 14 volts = using the probe on the center pin of the dummy load. My question is how does = one come up with that corrolation, since the equation is P=3D E^2 / R . = Plugging the numbers in : (14 v)^2 / 50 ohms =3D 3.92 watts. Where is the = missing volts ? My probe saw 19 volts at max output which corrolates to 7.2 watts out - = and that's what the meter shows as well. Neat beginner project - the RF probe ! Jamie WB4YDL ++++++++++++++++++ Reply-To: From: "Ron D' Eau Claire" To: "Elecraft List" Subject: RE: [Elecraft] RF Probe measurments - K1 Date: Mon, 18 Feb 2002 16:03:19 -0800 > I'm not at home right now, but the KAT manual said 14 volts. > Hmmm - maybe a > fudge factor needs to be added, but 1.8 volts ?! Interesting when I made > the adjustment, the max voltage of 19 volts did agree with the formula for > 7.2 watts. Go figure ! > > Jamie > WB4YDL Whatever the manual says, YOUR calculation is right! 15.8 vrms across a 50 ohm load is, indeed, 5 watts (well, 4.99). The Elecraft probe includes a scaling resistor that should give you a direct readout in RMS provided that you use it in a voltmeter circuit with an 1l megohm input resistance. The accuracy is dependent upon those resistances both being close so the metering circuit reports 7.07% of the d-c voltage across the diode in the probe. Ron AC7AC K2 # 1289 +++++++++++++++++ Date: Mon, 18 Feb 2002 18:21:36 -0600 Subject: Re: [Elecraft] RF Probe measurments - K1 From: James Hall To: , Elecraft List Hi Ron: Well I thought maybe we were looking at a non-linear scale at low wattage - or something like that ;) I built the N5FC RF probe - http://www.io.com/~n5fc/rfprobe2.htm - and I was very pleased with the result. My KAT is Rev. C . Maybe have something to do with it. Thanks, Jamie WB4YDL ++++++++++++++++++