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## [AMPS] L-Net calculations: Am I doing something wrong?

 To: [AMPS] L-Net calculations: Am I doing something wrong? jono@enteract.com (Jon Ogden) Mon, 15 Feb 1999 20:23:14 -0600
 ```OK, In order to match the high impedance of the 4-1000A to a lower impedance so that I have better loaded Q in my tank circuit, I decided to take the advice many people had given me and that is to make an L-network to step the impedance down to a more workable level. This L-network would consist of the output capacitance of the tube as the shunt (parallel) element and a small inductor as the series element. OK, so at 5KV, the 4-1000A's plate impedance is 4600 Ohms = R1 (from Eimac Specs) The Cplate of the 4-1000A is 7.6 pF (from the ARRL handbook) Let's choose L (the series inductor) to be 1 uH (a suggested value by others). What secondary impedance do we obtain? OK: In an L-net Q=R1/Xp At 28 MHz: Xp=1/(2*pi*28E6*7.6E-12)= 747.91 Ohms Therefore Q=4600/747.91 = 6.15 R2 = Xs/Q (R2 = output impedance) At 28 MHz: Xs=2*pi*28E6*1E-6 = 175.9 Ohms R2 = 175.9/6.15 = 28.6 Ohms Now 28.6 Ohms is unrealistically small in order to properly get a good match with realistic components. OK, so what would a good R2 be? Let's try for 2500 Ohms as this will give decent Qs with the component values I have for the tank circuit. Therefore: Q is still 6.15 since neither R1 nor Xp have changed. Xs = R2*Q = 2500*6.15 = 15375 Ohms At 28 MHz: Ls = Xs/(2*pi*28E6) = 15375/(2*pi*28E6) = 87.4 uH This won't work either since it is a HUGE coil. OK, what gives? Am I missing something here or doing a calculation wrong? Everyone says they've seen this "trick" done in the ARRL handbook and other texts. Can someone be specific as to what handbook (what year)? I sure can't find it yet in the 1999 issue or in my 1989 issue. This idea won't work unless I am totally missing the concept of how to do this. So what am I missing or what have I misunderstood that folks have suggested? 73, Jon KE9NA ------------------------------------- Jon Ogden KE9NA http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!! "A life lived in fear is a life half lived." -- FAQ on WWW: http://www.contesting.com/ampfaq.html Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
 Current Thread [AMPS] L-Net calculations: Am I doing something wrong?, Jon Ogden <= [AMPS] L-Net calculations: Am I doing something wrong?, Terry Gaiser [AMPS] L-Net calculations: Am I doing something wrong?, Jon Ogden [AMPS] L-Net calculations: Am I doing something wrong?, Rich Measures [AMPS] L-Net calculations: Am I doing something wrong?, Terry Gaiser [AMPS] L-Net calculations: Am I doing something wrong?, Jon Ogden [AMPS] L-Net calculations: Am I doing something wrong?, Rich Measures [AMPS] L-Net calculations: Am I doing something wrong?, Jon Ogden [AMPS] L-Net calculations: Am I doing something wrong?, Rich Measures [AMPS] L-Net calculations: Am I doing something wrong?, Peter Chadwick [AMPS] L-Net calculations: Am I doing something wrong?, Rich Measures