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Magnetic Loops
On 10/20/2015 10:44 AM, amdx wrote:
On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? -- Rick |
#2
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Magnetic Loops
On 10/20/2015 1:56 PM, rickman wrote:
On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. Mikek |
#3
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Magnetic Loops
On 10/20/2015 9:21 PM, amdx wrote:
On 10/20/2015 1:56 PM, rickman wrote: On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. But the context was that a 256 turn loop was a bad thing. I'm trying to understand what that was about. I don't need to know when it is a good idea... well, I guess even that is interesting. But I think the way a 256 turn loop would be made for a WWVB receiver is around a piece of ferrite. But who knows, maybe a large loop of telephone cable would work well too. -- Rick |
#4
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Magnetic Loops
On 10/20/2015 10:35 PM, rickman wrote:
On 10/20/2015 9:21 PM, amdx wrote: On 10/20/2015 1:56 PM, rickman wrote: On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. But the context was that a 256 turn loop was a bad thing. I'm trying to understand what that was about. I don't need to know when it is a good idea... well, I guess even that is interesting. But I think the way a 256 turn loop would be made for a WWVB receiver is around a piece of ferrite. But who knows, maybe a large loop of telephone cable would work well too. It obviously works. It is not ideal because it would have a lot of interwinding capacitance. Also the interwinding capacitance is not a quality capacitance thus the Q is lowered. It could be built with space between wire and layers, and 256 solder connections is not a great idea when trying to insure high Q. As far as "bad idea", all it has to do is receive enough signal to keep the clock accurate, more than that is interesting, but useless. Mikek |
#5
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Magnetic Loops
On 10/21/2015 6:12 AM, amdx wrote:
On 10/20/2015 10:35 PM, rickman wrote: On 10/20/2015 9:21 PM, amdx wrote: On 10/20/2015 1:56 PM, rickman wrote: On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. But the context was that a 256 turn loop was a bad thing. I'm trying to understand what that was about. I don't need to know when it is a good idea... well, I guess even that is interesting. But I think the way a 256 turn loop would be made for a WWVB receiver is around a piece of ferrite. But who knows, maybe a large loop of telephone cable would work well too. It obviously works. It is not ideal because it would have a lot of interwinding capacitance. Also the interwinding capacitance is not a quality capacitance thus the Q is lowered. It could be built with space between wire and layers, and 256 solder connections is not a great idea when trying to insure high Q. As far as "bad idea", all it has to do is receive enough signal to keep the clock accurate, more than that is interesting, but useless. I haven't built a high Q antenna yet, but I am pretty sure people greatly exaggerate the significance of solder connections in the Q factor. Q is related to the losses. I am sure the solder connections will not significantly impact the dissipative resistance of the wire unless the turns are around a pencil. As to "It obviously works", that depends on many other factors. Sure, no doubt it will work a mile from the transmitter. What about along the US east coast in a metal building with many appliances around? There is working, and there is working well. The inter-winding capacitance is not a factor as long as the station can be tuned. -- Rick |
#6
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Magnetic Loops
On 10/21/2015 2:36 PM, rickman wrote:
On 10/21/2015 6:12 AM, amdx wrote: On 10/20/2015 10:35 PM, rickman wrote: On 10/20/2015 9:21 PM, amdx wrote: On 10/20/2015 1:56 PM, rickman wrote: On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. But the context was that a 256 turn loop was a bad thing. I'm trying to understand what that was about. I don't need to know when it is a good idea... well, I guess even that is interesting. But I think the way a 256 turn loop would be made for a WWVB receiver is around a piece of ferrite. But who knows, maybe a large loop of telephone cable would work well too. It obviously works. It is not ideal because it would have a lot of interwinding capacitance. Also the interwinding capacitance is not a quality capacitance thus the Q is lowered. It could be built with space between wire and layers, and 256 solder connections is not a great idea when trying to insure high Q. As far as "bad idea", all it has to do is receive enough signal to keep the clock accurate, more than that is interesting, but useless. I haven't built a high Q antenna yet, but I am pretty sure people greatly exaggerate the significance of solder connections in the Q factor. Q is related to the losses. I am sure the solder connections will not significantly impact the dissipative resistance of the wire unless the turns are around a pencil. On a large loop antenna, it is probably difficult to get an extremely high Q. So, the solder connections will have less of an effect than if it was higher. I made a loop with 1/4" copper pipe, about 2'x 2' with a vacuum variable. I measured it at about Q=800. Using a 240uh and assuming 1000kHz, That's about 1.88 ohms of loss, split between dissipation in materials, wire losses, connection losses and capacitor losses. If you had 0.12 ohms additional solder connection losses, Q would drop to 753 from 800. As to "It obviously works", that depends on many other factors. Sure, no doubt it will work a mile from the transmitter. What about along the US east coast in a metal building with many appliances around? There is working, and there is working well. The inter-winding capacitance is not a factor as long as the station can be tuned. It is my pet theory, that interwinding capacitance will lower Q. It causes displacement current which causes more current flow between turns, also, the interwinding capacitance causes capacitive proximity effects. (vs magnetic proximity effect) I suppose a neat experiment would be to find two materials with equal losses but one having much higher permittivity. Then test Q with one material placed between turns, pull that out and install the higher permittivity material and retest Q. But, I could be all wrong on the subject. Check out this guys site, has some nice loops. http://makearadio.com/loops/ Mikek |
#7
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Magnetic Loops
On 10/21/2015 8:41 PM, amdx wrote:
On 10/21/2015 2:36 PM, rickman wrote: On 10/21/2015 6:12 AM, amdx wrote: On 10/20/2015 10:35 PM, rickman wrote: On 10/20/2015 9:21 PM, amdx wrote: On 10/20/2015 1:56 PM, rickman wrote: On 10/20/2015 10:44 AM, amdx wrote: On 10/19/2015 10:53 PM, rickman wrote: On 10/19/2015 3:50 PM, bilou wrote: "rickman" wrote in message ... On 10/19/2015 3:34 AM, Brian Howie wrote: How does the coil affect the tuning range of the cap? A cap is limited by the ratio of the minimum to maximum capacitance. The ratio of frequency is limited to the same ratio. In a multiturn loop you get huge capacitance between turns. For a given variable capacitor it appears in parallel. The Q of that big coil might be higher but as you need to add fixed capacitors to the variable one to get useful tuning range you loose almost what you gain. I sort of lost the thought here. If you up the inductance of the loop, it lowers the required tuning capacitance, so why would fixed capacitors be needed? Are you saying the parasitic capacitance of the loop is enough to significantly reduce the tuning range of the variable cap? Maybe, but there are construction methods that minimize the parasitic capacitance of multi-turn loops. Wide spacing is important. I've seen spiral loops wound on wooden frames that look like God's Eyes, very attractive. I saw descriptions using a 128 pairs telephone cable and spending several days to wire it as a 256 turns loop. A bad idea IMHO. I'm not sure what problem you would be trying to solve by using a 256 turn loop. There are middle grounds... Often a 60kHz WWVB time receiver. So why would that be a "bad idea"? Ahh, you ask "what problem you would be trying to solve" I should clarify, a resonant antenna for 60kHz, and that requires a large inductance. Or at least that is one approach. But the context was that a 256 turn loop was a bad thing. I'm trying to understand what that was about. I don't need to know when it is a good idea... well, I guess even that is interesting. But I think the way a 256 turn loop would be made for a WWVB receiver is around a piece of ferrite. But who knows, maybe a large loop of telephone cable would work well too. It obviously works. It is not ideal because it would have a lot of interwinding capacitance. Also the interwinding capacitance is not a quality capacitance thus the Q is lowered. It could be built with space between wire and layers, and 256 solder connections is not a great idea when trying to insure high Q. As far as "bad idea", all it has to do is receive enough signal to keep the clock accurate, more than that is interesting, but useless. I haven't built a high Q antenna yet, but I am pretty sure people greatly exaggerate the significance of solder connections in the Q factor. Q is related to the losses. I am sure the solder connections will not significantly impact the dissipative resistance of the wire unless the turns are around a pencil. On a large loop antenna, it is probably difficult to get an extremely high Q. So, the solder connections will have less of an effect than if it was higher. I made a loop with 1/4" copper pipe, about 2'x 2' with a vacuum variable. I measured it at about Q=800. Using a 240uh and assuming 1000kHz, That's about 1.88 ohms of loss, split between dissipation in materials, wire losses, connection losses and capacitor losses. If you had 0.12 ohms additional solder connection losses, Q would drop to 753 from 800. Why would you assume 120 mOhms of resistance from soldered connections? Have you measured this somewhere? As to "It obviously works", that depends on many other factors. Sure, no doubt it will work a mile from the transmitter. What about along the US east coast in a metal building with many appliances around? There is working, and there is working well. The inter-winding capacitance is not a factor as long as the station can be tuned. It is my pet theory, that interwinding capacitance will lower Q. It causes displacement current which causes more current flow between turns, also, the interwinding capacitance causes capacitive proximity effects. (vs magnetic proximity effect) I suppose a neat experiment would be to find two materials with equal losses but one having much higher permittivity. Then test Q with one material placed between turns, pull that out and install the higher permittivity material and retest Q. But, I could be all wrong on the subject. Check out this guys site, has some nice loops. http://makearadio.com/loops/ Yeah, not bad. -- Rick |
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