I believe the most significant contributor to key-down power sag is the increase in MOSFET channel resistance with temperature. As the MOSFETs heat, their ON resistance increases slightly, which drops output power slightly.
Fortunately this is a small effect, especially when measured in dB. 73 de Bob, K6XX > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Ron > D'Eau Claire > Sent: Friday, January 27, 2012 10:18 AM > To: [email protected] > Cc: [email protected] > Subject: Re: [Elecraft] KPA500 Power Output > > A quick way to check Jack's suggestion that the mains source > voltage may be > drooping is to monitor the HV on the KPA500 while > transmitting to see if it > decreases over time. > > Ron AC7AC > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Jack Brindle > Sent: Friday, January 27, 2012 9:55 AM > To: [email protected] > Cc: [email protected] > Subject: Re: [Elecraft] KPA500 Power Output > > Are you sure it is the KPA500 that is causing the drop off? > There are > a lot of variables here. The exciter could be scaling back, > the power > source (110/220) could be limiting you or something else may be > happening. As power cords heat, they tend to restrict the electron > flow. Especially if you are powered from 110V, that could be the > source of several problems. Be sure not to replace the > supplied power > cord with a smaller one - this is a recipe for reduced output > in itself. > > We have also seen transmitters decrease their output power > when keyed > down for a long time. This might be due to heating in the TX, ALC > action, or other things specific to the radio. > > I would suggest adding a wattmeter between the exciter and > the KPA500 > so you can monitor power between the devices. Also check the power > source wiring to make sure there are no limitations there. In > general, > look to see if there are other factors external to the KPA causing > power reduction. > > Jack B, W6FB > > > On Jan 27, 2012, at 8:57 AM, [email protected] wrote: > > > Sorry if this has been covered before, but I can't recall that it > > has. Is > > there any reason why power output > > falls off over time? Here's what I did: using MMTTY, I "diddled" > > into a > > 50-Ohm dummy load for one minute. > > I initially had a power output of 525 Watts which went to 500 W > > shortly and > > went down to about 465 W > > when MMTTY "diddling" timed out. I repeated that process later, > > monitoring > > current and noted that it also > > went down over time...makes sense! > > But my question is WHY? I was under the impression that I > needed to > > run the > > amp at 500 W on RTTY for > > optimum efficiency because in an earlier posting I noted > that at 150W > > output (in BPSK31), the amp was very inefficient and the fan was > > pretty loud. If > > I can't derate the output and I can't run full output, what > do I do > > with > > the KPA500 in the digital modes? > > > > Bert, N4CW > > > > > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:[email protected] > > > > 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:[email protected] > > 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:[email protected] > > 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:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
