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## Re: [Amps] Matching

 To: amps@contesting.com Re: [Amps] Matching Ian White GM3SEK Ian White GM3SEK Tue, 20 Dec 2005 08:56:47 +0000
 ```Steve Thompson wrote: > > >Peter Chadwick wrote: >> Steve said: >> >>As an example, take an impedance of 100 + j50 ohms, which >>you want to match to 50R resistive. >> >>I can see two L match solutions, which have a loaded Q of >>about 1.2. A T match can do it with a loaded Q of about 1.04. >>I can't see any L match that betters this. Can someone correct >>me if I'm wrong? >> >Can someone correct me if I'm wrong?< >> >> No - 'cos you're not wrong! >After more peering at Smith charts (considering matching some impedance >to a resistance), I think there's some impedances where you can't get >lower Q than an L match, and others where you can. There's nowhere that >an L match gives lower Q than 3 element matching circuits. > >It all depends if you are inside or outside the constant R and Y circles >for the resistance you're aiming for. And also whether the series output C of the T-network is helping or not. In the particular case of 100+j50, the series C tunes out the reactance and leaves 100+j0 to be matched. None of the L-networks can do this, so they are forced to match the equivalent *parallel* impedance instead, which is always higher (in this case 125 // j250). Hence the increase in loaded Q. If the impedance had been 100 -j50, then you'd need series L at the output to cancel the reactance and leave 100 ohms resistive. The series output C of a typical C-L-C T-network would then become a disadvantage, increasing the equivalent resistance that has to be matched, and thus increasing the working Q. Part of the confusion in this debate has been between an optimum configuration of L or T network, and the more limited capabilities of real-life ATUs which have a fixed configuration. I've certainly been guilty of mixing the two in the same posting. But beyond that, we're using working Q as a proxy for more practical power handling limitations: heat dissipation in variable inductors or arcing in variable capacitors. That adds a whole other level of complexity, leaving us with only one valid generalization: no ATU configuration can be "best" in all possible cases. -- 73 from Ian GM3SEK _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
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