I have searched the archives and have not been able to find a definitive
answer. I've now completed the construction of the basic KX1 and would like to have a fairly reasonable measurement of the output power on 20M and 40M I have a very decent dummy load but do not have a very accurate RF Power meter. I do have a decent DMM and could measure the voltage drop across the 50 ohm dummy load - I'm not sure how accurate the DMM would be at 7 or 14 MHZ. Is there a recommended method to measure the power output without an RF power meter ? Thanks in advance, Ron, AC2C ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
You might use a diode, such as a 1N914, 1N4148 or (preferably) a 1N5711
to rectify the voltage developed across the load and then measure the DC out of the diode. With a high Z load such as a DMM, the DC voltage will be close to the RF peak voltage. A standard DMM, even a good one, will not provide usable results above a few hundred KHz, unless it's a specialized instrument intended for RF measurements. Jack K8ZOA Ron Bower wrote: > I have searched the archives and have not been able to find a definitive > answer. > > > > I've now completed the construction of the basic KX1 and would like to have > a fairly reasonable measurement of the output power on 20M and 40M > > > > I have a very decent dummy load but do not have a very accurate RF Power > meter. I do have a decent DMM and could measure the voltage drop across the > 50 ohm dummy load - I'm not sure how accurate the DMM would be at 7 or 14 > MHZ. > > > > Is there a recommended method to measure the power output without an RF > power meter ? > > > > Thanks in advance, > > Ron, AC2C > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
In reply to this post by Ron Bower
Ron,
Use a diode detector. A 1N4148 diode will do fine up to about 12 watts. Connect the anode of the diode to the center conductor of your coax *at* the dummy load (use a coax TEE adapter if you have no other access). From the cathode end of your diode, connect a 0.01 uF capacitor to the shield side of the coax. Your DMM leads connect across the capacitor. Measure the forward voltage of the diode with your DMM if it has that function, otherwise, assume that the diode forward voltage is 0.6 volts. With this probe, the DVM will read the peak RF voltage. You can calculate the power from the DMM reading as follows: The peak voltage across the resistor is equal to the DVM reading plus the diode forward voltage, so add them. You may then calculate the RMS value of the result above. Then square that result and divide by the resistance. Or you may accept (or derive) the formula will be as follows with no intermittent RMS calculation. P = (Vdvm+Vdiode)^2/2R = (Vdvm+Vdiode^2/100 for a 50 ohm dummy load OR, if you assume the diode drop is 0.6 volts P=(Vdvm+0.6)^2/100 73, Don W3FPR Ron Bower wrote: > I have searched the archives and have not been able to find a definitive > answer. > > > > I've now completed the construction of the basic KX1 and would like to have > a fairly reasonable measurement of the output power on 20M and 40M > > > > I have a very decent dummy load but do not have a very accurate RF Power > meter. I do have a decent DMM and could measure the voltage drop across the > 50 ohm dummy load - I'm not sure how accurate the DMM would be at 7 or 14 > MHZ. > > > > Is there a recommended method to measure the power output without an RF > power meter ? > > > > Thanks in advance, > > Ron, AC2C > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
In reply to this post by Ron Bower
Ron,
I just went through this scenario. With the help of Don Wilhelm I was able accurately measure my power out on my KX1 and K3. I first used a homebrew load with "free" resistors given away at a club meeting. They only worked at low frequencies. At 20 meters the SWR to far off to be of use! I bought an Elecraft DL1 because it uses one of the correct types of resistors for this application and it has a built in VDC test point! The only thing to be wary of is that the DL1 test point is at the 25 ohm point in the series of resistors!!! Don's formula (and mine) differs from that found in the DL1 manual. Elecraft: P(in watts) = ((Volts * 1.414) +(.15))^2 / 50 Don: P(in watts) = ((Volts) + (diode drop))^2 / 25 Jack: P(in watts) = (((Volts) + (diode drop) * 2)) / (square root of 2)))^2 / 50 There are 3 points to be aware of. 1. Use the square root of 2 and not the 1.414 figure. 2. Measure your diode drop. 3. Combine the drop with the VDC before doing any other calculations. 4. If you do not use Don's formula, DIVIDE by the square root of 2. The Schottky diode in my DL1 measures .330 mv. A 1N34a (germanium diode) has a drop of about .3 volts. A Schottky's drop is a little more but offers more linearity! In any case, the VDC voltage must be divided by the square root of 2 to get the RMS. In other words the VDC(0-Peak) is 1.414(approximately) times the RMS voltage If you measure the VDC at the 50 ohm point then my formula becomes: P(in watts) = ((VDC + (diode drop)) / (square root of 2))^2 /50 After getting an accurate measurement of my KX1's power I found that the KX1 ATU was showing values that were too high. By adjusting C9 and R1 I was able to get the KX1 and the DL1 to agree! BTW, I also used the DL1 to calibrate my K3 at low power. Now the K3, my DL1 and my Versa Tuner V all agree! -----Original Message----- .....snip..... I've now completed the construction of the basic KX1 and would like to have a fairly reasonable measurement of the output power on 20M and 40M I have a very decent dummy load but do not have a very accurate RF Power meter. I do have a decent DMM and could measure the voltage drop across the 50 ohm dummy load - I'm not sure how accurate the DMM would be at 7 or 14 MHZ. Is there a recommended method to measure the power output without an RF power meter ? Thanks in advance, Ron, AC2C .....snip.... ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
In reply to this post by Ron Bower
______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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