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#1




LC calculation
Hi,
Can somebody walk me thru the calculation of an LC circuit where the capacitor is tapped down on the coil? I see this often done for bandspreading purposes. Tnx, Bill WX4A 
#2




LC calculation
Howdy,
I'm guessing that it can be solved like this... Consider the autotransformer action of the tapped inductor. Then divide the tap capacitor (C2) value by the square of the turns ratio (N) before adding it to the primary capacitance of the parallel tuned circuit (C1.) F=2Pi*sqrt(L(C1+C2/N^2)) I found a rigorous solution in chapter 8 of Alternating Current Circuits by K.Y. Tang but it's too messy to type. 73, Grumpy exray wrote in : Hi, Can somebody walk me thru the calculation of an LC circuit where the capacitor is tapped down on the coil? I see this often done for bandspreading purposes. Tnx, Bill WX4A 
#3




LC calculation
Thanks Grump...gonna save that for my files but I think I need to
clarify my question. One cap only...not one across the tank and another on the tap although thats an interesting idea. Maybe this schematic will help. http://www.antiqueradios.org/gazette/pix/sw31sch.jpg Bill Grumpy The Mule wrote: Howdy, I'm guessing that it can be solved like this... Consider the autotransformer action of the tapped inductor. Then divide the tap capacitor (C2) value by the square of the turns ratio (N) before adding it to the primary capacitance of the parallel tuned circuit (C1.) F=2Pi*sqrt(L(C1+C2/N^2)) I found a rigorous solution in chapter 8 of Alternating Current Circuits by K.Y. Tang but it's too messy to type. 73, Grumpy exray wrote in : Hi, Can somebody walk me thru the calculation of an LC circuit where the capacitor is tapped down on the coil? I see this often done for bandspreading purposes. Tnx, Bill WX4A 
#4




LC calculation
Howdy,
Oh, ok! I have seen a tapped down band spread capacitor circuit so I assumed... I wonder why they did that? Oh, looking at it the another way, the grid impedance loading the parallel tuned circuit is transformed by the turns ratio. Probably to provide some voltage gain from that stage. If the capacitance of the grid is negligible, then the resonance is determined by the inductance from the portion of the winding paralleled by the variable capacitor. If you want to include the grid capacitance in parallel with the variable capacitor, multiply it by the turns ratio. Otherwise the solution is essentially the same. 73, Grumpy exray wrote in : Thanks Grump...gonna save that for my files but I think I need to clarify my question. One cap only...not one across the tank and another on the tap although thats an interesting idea. Maybe this schematic will help. http://www.antiqueradios.org/gazette/pix/sw31sch.jpg 
#5




LC calculation
Grumpy The Mule wrote:
If the capacitance of the grid is negligible, then the resonance is determined by the inductance from the portion of the winding paralleled by the variable capacitor. So what about the 'top' portion of the coil being effectively in series with the tank? Wouldn't that affect the tank values? Tnx? 
#6




LC calculation
Howdy, Maybe not. Imagine the top portion disconnected, it would have very little effect. It's no more than a small capacitor swamped out by the tuning capacitance. I think if the grid capacitance times the square of the turns ratio is small compared to the tuning capacitance, it can be ignored. Because there are other strays that will require a bit of pruning in any case. The most significant effect would be when the tuning capacitor is at its minimum value. So it's here that it may effect the design of the inductor, requiring a bit less inductance for the desired upper band limit compared to the other tuned winding. There are no padding capacitors on that schematic. So I suspect there are seperate tuning capacitors for each tuned winding on the transformer. Because of this I believe it's not that critical. You could measure the grid capacitance with the tube mounted in its socket on the chassis. Then add that capacitance times the turns ratio to the value of the tuning capacitor when you calculate the necessary inductance value. Or build the coil so you can stretch it (slip a few windings further apart) a little once the thing is running. Then secure them with a bit of wax. 73, Grumpy exray wrote in : Grumpy The Mule wrote: If the capacitance of the grid is negligible, then the resonance is determined by the inductance from the portion of the winding paralleled by the variable capacitor. So what about the 'top' portion of the coil being effectively in series with the tank? Wouldn't that affect the tank values? Tnx? 
#7




LC calculation
Howdy,
Heh! I didn't look at the url for that schematic, it's an SW3! I love National Radio gear. I had two HRO500's, sold one, and there's an FRR59 in my basment. I regret having sold my pristine RBL receiver which beat the pants off either the HRO500 with LF10 or the Racal RAL7 with RA237 for LF/VLF work. My highschool's club had an NCX1000 which I lusted after... er, but only in my heart. But back to the SW3. I recall padder capacitors mounted on the top of the RF stage coil former. I don't know the value of the dual gang tuning capacitor but armed with that, Wheeler's formula and these sites, you could calculate the effect of the grid circuit to some extent. http://www.io.com/~nielw/sw3coils.htm http://www.antiqueradios.org/gazette/swevol.htm 73, Grumpy 
#8




LC calculation
Grumpy The Mule wrote:
But back to the SW3. I recall padder capacitors mounted on the top of the RF stage coil former. I don't know the value of the dual gang tuning capacitor but armed with that, Wheeler's formula and these sites, you could calculate the effect of the grid circuit to some extent. http://www.io.com/~nielw/sw3coils.htm http://www.antiqueradios.org/gazette/swevol.htm Only the bandspread coils have the trimmer on top and thats basically to set the rough frequency. The main tuning then becomes a bandspread tuning. I've forgotten all this stuff  I'll have to look back at my stuff...maybe the trimmer falls in series with the main tuning caps. Anyway, thats just an example that came to mind. I've seen others doing similar tricks for main tune/bandspread tune using the same cap but with a different coil configuration. Bill 
#9




LC calculation
On Nov 9, 8:36*am, Grumpy The Mule wrote:
Howdy, I'm guessing that it can be solved like this... Consider the autotransformer action of the tapped inductor. Then divide the tap capacitor (C2) value by the square of the turns ratio (N) before adding it to the primary capacitance of the parallel tuned circuit (C1.) F=2Pi*sqrt(L(C1+C2/N^2)) I found a rigorous solution in chapter 8 of Alternating Current Circuits by K.Y. Tang but it's too messy to type. 73, Grumpy exray wrote : Hi, Can somebody walk me thru the calculation of an LC circuit where the capacitor is tapped down on the coil? *I see this often done for bandspreading purposes. Tnx, Bill WX4A One thing to be a bit careful about is including the coupling between the coil sections. Since I don't know how those particular coils were designed, I can't say for sure, but in general the coupling between pieces of the coil isn't as high as you might think. You can make a good estimate for typical HF singlelayer aircore solenoid coils just using your favorite coil calculation. For example, consider a coil that's one inch diameter, two inches long, 20 turns per inch, and tapped at the 30th turn (1.5 inches) up from the bottom. Then the whole coil is about 16.32uH, the 1.5" part is about 11.54uH, and the top 0.5" is about 2.63uH. If the coupling were perfect between the sections, I believe the inductance of the whole would be about 11.54uH + 2.63uH + 2*sqrt(11.54*2.63)uH = 25.19uH. At 16.32uH for the whole coil, the implied coupling coefficient between those two sections is only about 0.20, and you need to be careful to not think of the tapped coil as simply a transformer with a 3:1 turns ratio, with implied close coupling between the sections. (This also illustrates why you can short out turns of a tank coil without totally killing the net inductance...) I trust if I've hosed the calculation too badly, someone will point out the error of my ways. ;) Cheers, Tom 
#10




LC calculation
Howdy,
That's an excellent point. I expect that the sections of the coil in question will not have very good coupling. I hadn't considered it! I calculate the same inductance values. Bah! In my previous posts the "(sqrtL*C)" should be "(1/sqrtL*C)" Heh... with a pencil and paper the formula turned out right. I noticed this when I calculated the resonance of the two values 16.32uH and 11.54uH with an arbitrary value of 50pF just to verify how I thought things should be. So from this I gather that the extension of the coil connected to the grid circuit will have even less effect on the tuning than I expected. The Easy Teenage NewYork method of solving this would be to put the problem into SPICE with the estimated coupling coefficent of 0.2. I've never liked shorting turns to reduce an inductance. Seems like an avoidable source of some losses no matter how you slice it. But for some long coils, like antennas, it works ok. I guess. K7ITM wrote in : One thing to be a bit careful about is including the coupling between the coil sections. Since I don't know how those particular coils were designed, I can't say for sure, but in general the coupling between pieces of the coil isn't as high as you might think. You can make a good estimate for typical HF singlelayer aircore solenoid coils just using your favorite coil calculation. For example, consider a coil that's one inch diameter, two inches long, 20 turns per inch, and tapped at the 30th turn (1.5 inches) up from the bottom. Then the whole coil is about 16.32uH, the 1.5" part is about 11.54uH, and the top 0.5" is about 2.63uH. If the coupling were perfect between the sections, I believe the inductance of the whole would be about 11.54uH + 2.63uH + 2*sqrt(11.54*2.63)uH = 25.19uH. At 16.32uH for the whole coil, the implied coupling coefficient between those two sections is only about 0.20, and you need to be careful to not think of the tapped coil as simply a transformer with a 3:1 turns ratio, with implied close coupling between the sections. (This also illustrates why you can short out turns of a tank coil without totally killing the net inductance...) I trust if I've hosed the calculation too badly, someone will point out the error of my ways. ;) Cheers, Tom 
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