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#11
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Resontate frequency of parallel L/C
On 18 Nov, 11:12, Dave wrote:
Brian Howie wrote: | | ! -----!----- | | | | L C | | | | R R | | | | ------------ | | | I ran it through Spice ( laziness) - It doesn't resonate. Intuitively you think it should have a low Q resonance at 1.6MHz , but it doesn't Nice one. 73 Brian GM4DIJ finally come up The trick is to make R = sqrt(L/C) then the impedance is real everywhere. You can use any old values for L: and C, as long as you make R=sqrt(L/C); That equation is obviously know from transmission lines too..- Hide quoted text - - Show quoted text - David, After all those discussions where you have been baiting me on my antenna you finally come up with what I have been hitting you with. Rember my comments where a radiator can be any shape, any elefation as long as the element is in equilibrium. Finally the penny has dropped with respect to the LC ratio! Art |
#12
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Resontate frequency of parallel L/C
Cecil, W5DXP wrote:
"Since the two resistances are equal, seems to me the resonant frequency would be where the two reactances are equal." Yes that`s the unity power factor point. There`s a rule that when the circuit Q`s not less than 10, fo=1/2pi on the sq.rt. of LC. For lower Qs, the calculation is more laborious. I sure miss my ARRL Lightning Coil Calculator! Best regards, Richard Harrison, KB5WZI |
#13
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Resontate frequency of parallel L/C
Tom Donaly wrote:
My calculator needs fixing. When I divide 100 uH by 100 pF and take the square root, I end up with the number 1000. Where did I go wrong? The actual formula is 1/[2pi*SQRT(L*C)] -- 73, Cecil http://www.w5dxp.com |
#14
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Resontate frequency of parallel L/C
On 18 Nov, 14:11, Cecil Moore wrote:
Tom Donaly wrote: My calculator needs fixing. When I divide 100 uH by 100 pF and take the square root, I end up with the number 1000. Where did I go wrong? The actual formula is 1/[2pi*SQRT(L*C)] -- 73, Cecil http://www.w5dxp.com I see that somebody intimated a 1000 ohm resistive impedance. My antenna on 160 is about half of that! The question I have now is how can we relate the radiation with respect to that high resistive impedance? Regards Art |
#15
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Resontate frequency of parallel L/C
On 18 nov, 13:11, Dave wrote:
What is the resonate frequency of this network, as determined between the top and bottom of what I have drawn? I don't know how well the drawing will come out, but it consists of: 100 uH in series with 1000 Ohms. 100 pF in series with 1000 Ohms The two two networks above are in parallel i.e. | | ! -----!----- | | | | L C | | | | R R | | | | ------------ | | | hello Dave, Normallly the resonant frequency of circuit is the frequency where Zin is real. The problem with this circuit is that Z is real everywhere and Q will be zero. So in my opinion it is useless to define a resonant frequency for this circuit. The only other option you have is to find the frequency where Im(current left leg) = -Im(current right leg), 1.600 MHz. Best regards, Wim PA3DJS www.tetech.nl |
#16
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Resontate frequency of parallel L/C
On Sun, 18 Nov 2007 21:45:54 GMT, "Tom Donaly"
wrote: My calculator needs fixing. When I divide 100 uH by 100 pF and take the square root, I end up with the number 1000. Where did I go wrong? Hi Tom, You didn't, I misread micro for nanohenry. No resonance as specified. 73's Richard Clark, KB7QHC |
#17
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Resontate frequency of parallel L/C
Art wrote:
"The question I now have is how can we relate the radiation with respact to that high resistive impedance?" Efficiency = radiation resistance / radiation resistance + loss resistance Best regards, Richard Harrison, KB5WZI |
#18
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Resontate frequency of parallel L/C
Cecil Moore wrote:
Dave wrote: What is the resonate frequency of this network, as determined between the top and bottom of what I have drawn? I don't know how well the drawing will come out, but it consists of: 100 uH in series with 1000 Ohms. 100 pF in series with 1000 Ohms The two two networks above are in parallel Since the two resistances are equal, seems to me the resonant frequency would be where the two reactances are equal. Where the 100 uH line crosses the 100 pf line on the reactance chart in the ARRL Handbook is in the ballpark of 1.591549431 MHz. :-) But it does not resonate at 1.591549431 MHz - or anywhere else for that matter. The impedance is 1000 Ohms, purely resistive, at any frequency. |
#19
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Resontate frequency of parallel L/C
Cecil Moore wrote:
Tom Donaly wrote: My calculator needs fixing. When I divide 100 uH by 100 pF and take the square root, I end up with the number 1000. Where did I go wrong? The actual formula is 1/[2pi*SQRT(L*C)] Not in this case. All you have to do, Cecil, is take your formula, above, find the frequency you think is the resonant frequency, and then use it to find the impedance across the circuit. Now, try some other frequency. You can prove that this circuit can be replaced by a 1000 ohm resistor for all frequencies, using network analysis, but that's a little more difficult. 73, Tom Donaly, KA6RUH |
#20
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Resontate frequency of parallel L/C
Dave wrote:
Roy Lewallen wrote: Is this by any chance an exam question? No, it is not. I was shown it by a lecturer of mine more than 10 years ago. The result is quite interesting. With the given values, it's a constant-impedance network. I've used one many times in time domain circuit designs. Its impedance is a constant real value of 1000 ohms at all frequencies. Since "resonance" implies a single frequency (at which the reactance is zero), this circuit isn't resonant at any frequency. The circuit is often used in time domain applications (e.g., oscilloscopes) where it's sometimes necessary to provide a constant impedance load but you're stuck with a capacitive device input impedance. In that situation, the C is the input C of the device. However, the transfer function isn't flat with frequency-- you end up with a single pole lowpass rolloff, dictated by the R and C values. For anyone who cares about such matters, "resonate" is a verb, "resonant" is the adjective, and "resonance" the noun. A resonant circuit resonates at resonance. Roy Lewallen, W7EL |
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