On 11/3/2016 22:24, Ron D'Eau Claire wrote:
Since, at HF, there is very little dielectric losses most of the loss is resistive. That means the greatest loss is at whatever condition of frequency and impedance transformation produces the greatest circulating currents. You are quite right; RF resistance is different from dc resistance. It's generally much higher due to the "skin effect".
The core losses could also be significant.
One of the widest range ATU circuits is the T-match, but it is capable of producing a low SWR to the transmitter at settings the result I huge circulating currents in the coil. More than a few Hams have discovered the coil in their MFJ T-match tuner collapsed in a heap as the poly insulating bars on the coil melted at a fraction of the rated power. It's easy to have one consume more than 50% of the RF power applied.
The L network is the lowest Q matching network (without a transformer), so it has the lowest circulating currents. T and Pi networks have higher Q, but don't require a continuously variable inductor.
At extreme impedances, T and Pi network tuners can indeed be so lossy that most of the power is dissipated in the tuner itself. One of mine once achieved a match to a shorted feedline, with predictable results.
73, Scott K9MA
73, Ron AC7AC -----Original Message----- From: Elecraft [mailto:[email protected]] On Behalf Of K9MA Sent: Thursday, November 3, 2016 4:28 PM To: [email protected] Subject: Re: [Elecraft] Measuring Losses in an ATU I haven't tried it, but one way to measure the loss of the ATU would be to connect a second, identical ATU "backwards" to a 50 Ohm load. This second ATU would be tuned to present the highest or lowest resistive load possible. One way this could be done would be by connecting a resistor across the transceiver output, matching it, and reading out the ATU component values. The second ATU would be set to the same values, which should result in a match from the first ATU without retuning. The measured loss would be twice that of a single ATU. That said, it's very likely the maximum ATU loss would be at the highest frequency of interest and highest or lowest impedance the ATU can match. Highly reactive loads might make it worse. I suppose an automated test could explore all the possible combinations. This could all be simulated, of course, if good models of the components are available. I suspect most of the loss is in the inductors, but one needs to know more than their DC resistance. 73, Scott K9MA -- Scott K9MA [email protected] ______________________________________________________________ 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 Message delivered to [email protected]
-- Scott K9MA [email protected] ______________________________________________________________ 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 Message delivered to [email protected]
