wattage on k3 vs kpa500
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Hi
Maybe some one can answer this. I set my k3 to 60 watts and measure 60 watts on my kpa500 meter with the amp in standby trnsmitting into a dummy load. When I switch to an antenna with a 3:1 swr the k3 still says 60 watts but the kpa500 now reads 80 watts with the amp still in standby. So which is correct or neither ? Thanks Richard ______________________________________________________________ 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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Here's my take:
Most if not all RF power meters will only be accurate at a 1:1 SWR (when there is resonance). The actual power can more accurately determined by reading the voltage and current and then only if the amp efficiency is known. It's likely that neither the K3 or KPA500 meter is entirely accurate; the K3 would sense the 'bad' SWR and start a power foldback and the amp is a random location on the feedline away with the mismatch causing a sensing error. (Adding another random feed length would likely give a different reading.) >From memory, at 3:1 you have approximately a 25% 'reflection'. I didn't do the math to verify, but in this case happens to be the same as shown as added power in the KPA500. Perhaps someone else can add more. The K3 is producing the power and should be the more accurate reading. However, an improvement of 3:1 should be obtained before either the K3 or the more importantly the KPA500 applies any significant power (in fact the amp will 'complain' and switch to standby at ~2.1:1). That higher SWR in the real world comes back as heat, which is bad and shortens the life expectancy of the final(s). Make sense? Rick WA6NHC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Richard Sent: Wednesday, November 23, 2011 6:40 PM To: [hidden email] Subject: [Elecraft] wattage on k3 vs kpa500 Hi Maybe some one can answer this. I set my k3 to 60 watts and measure 60 watts on my kpa500 meter with the amp in standby trnsmitting into a dummy load. When I switch to an antenna with a 3:1 swr the k3 still says 60 watts but the kpa500 now reads 80 watts with the amp still in standby. So which is correct or neither ? Thanks Richard ______________________________________________________________ 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 |
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In reply to this post by RICHH
Hi,
Does the K3 have an antenna tuner and is it enabled? If it does, your readings may be entirely accurate. The K3 delivers 60 watts into the matched load it sees. The KPA500, however is on the unmatched side and sees 80W forward power and 20W reflected which is the 25% reflected power equivalent to a 3:1 SWR. I think you should be able to measure reflected power on the KPA500. And what is the SWR reported by the K3? The other possibility is that with a real antenna you have RF in the shack sufficiently strong to cause power measurements to vary along the transmission line. AB2TC - Knut
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In reply to this post by Rick WA6NHC
Most likely the K3 is putting out 60 watts, real watts. The wattmeter is also probably correct reading 80 watts forward, with 20 reflected, equal to a net 60 watts. And 60 watts is radiated by the antenna. No power get reabsorbeds by the amp because it is presenting a congugate match. i.e. any power coming back from the antenna sees a perfect mismatch at the transmitter and gets re-reflected back to the antenna. Explained 100 times better than I can by Walt Maxwell in his books "Reflections" or maybe the current one is "Reflections III" something like that. Anyway the point is, none of this rf power heats up the finals. ______________________________________________________________ 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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If I may, let me reinforce and paraphrase Ron's comments.
Today's transceivers are designed to work into a 50 ohm resistive load. That means they will develop full power into a load which measures an SWR = 1.0. Now, most transmitters are designed to work properly into a SWR = 2.0 load - that means that they will develop full power output - however, the efficiency may suffer, so the transmitter current draw may increase when the load differs from a 1-1 SWR. The power output will remain the same if the SWR is not 1-1, but there will be more heat produced by the PA because the efficiency has been reduced. The reflected power is not dissipated in the finals (read the antenna article on my website www.w3fpr.com), but the fact that the finals are not operating at the full efficiency point, there will be increased heat - that is just the way the laws of physics work. 73, Don W3FPR On 11/24/2011 9:06 PM, Ron D'Eau Claire wrote: > I believe an ATU or other matching network provides a conjugate match. That > results in optimum power transfer because the RF current and voltage are in > phase and the SWR meter will read 1:1. > > When there is a mismatch, as indicated a high SWR, the finals in the rig do > not see the optimum load impedance for efficient power transfer so they do > not operate at optimum efficiency. Lower efficiency means that more of the > d-c power applied to the finals is consumed in heat instead of RF, and the > amount of heat the finals must dissipate goes up. I believe that's what led > many Hams to mistakenly believe that the "reflected" power was being > absorbed in the finals. > > The solution is, of course, to provide a proper termination for the finals. > When the antenna or other load doesn't present the right impedance, a > matching network (a.k.a. 'tuner' or ATU) can be inserted to provide one. > > Ron AC7AC > > > > -----Original Message----- > Most likely the K3 is putting out 60 watts, real watts. The wattmeter is > also probably correct reading 80 watts forward, with 20 reflected, equal to > a net 60 watts. And 60 watts is radiated by the antenna. No power get > reabsorbeds by the amp because it is presenting a congugate match. i.e. any > power coming back from the antenna sees a perfect mismatch at the > transmitter and gets re-reflected back to the antenna. Explained 100 times > better than I can by Walt Maxwell in his books "Reflections" or maybe the > current one is "Reflections III" something like that. Anyway the point is, > none of this rf power heats up the finals. > > ______________________________________________________________ > 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 |
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