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Experimenting with Coils for Crystal Sets
Was just wandering if anyone has used or experimented with television
IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David |
Experimenting with Coils for Crystal Sets
David wrote:
Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David I got beat up on my last reply offering comments....but, whatever. The small slug tuned HF coils in cans really have no place in crystal radio circuits. However, the long ferrite rod loopstick coils are quite good and are popular. I'll offer an invitation to http://theradioboard.com/rb/ if you aren't familiar with the site. You should be able to scrounge up some additonal suggestions there. -Bill |
Experimenting with Coils for Crystal Sets
David wrote:
Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David They'll have very low Q, and therefore not really be suitable for a crystal radio, where low-Q coils fight your ability to get good selectivity without burning up all your signal before it gets to the headphones. There's a whole bunch of _other_ cool things you can do with them, just not Xtal sets. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html |
Experimenting with Coils for Crystal Sets
On Apr 25, 11:51*pm, Tim Wescott wrote:
David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David They'll have very low Q, and therefore not really be suitable for a crystal radio, where low-Q coils fight your ability to get good selectivity without burning up all your signal before it gets to the headphones. There's a whole bunch of _other_ cool things you can do with them, just not Xtal sets. -- Tim Wescott Wescott Design Serviceshttp://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details athttp://www.wescottdesign.com/actfes/actfes.html I tinkered around a lot with crystal radios back when I was a kid. It seemed that the most important thing was matching the impedance of the radio to the antenna. This is much more critical on crystal set than a modern superhet The only ham in the county impressed me very much with what a crystal set could do if properly matched to a good antenna system. He had worked all the states and a lot of the countries using a crystal radio as his rx. I do think he kind of cheated though by finding the signal on his regular station rx and then listening for it on the crystal set. Jimmie |
Experimenting with Coils for Crystal Sets
David wrote:
Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Unfortunately, the *Q* of those coils will be much too low for effective use in crystal receivers. The best results I've had with "no power" receivers involved the use of FET detectors and /very/ High-Q coils. The inductors usually have to be large diameter air-cored, but you can get away with toroid or ferrite rod types if you can accept poorer results. It's quite illuminating to browse the offerings on the web. Bob |
Experimenting with Coils for Crystal Sets
Bob wrote:
David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Unfortunately, the *Q* of those coils will be much too low for effective use in crystal receivers. The best results I've had with "no power" receivers involved the use of FET detectors and /very/ High-Q coils. The inductors usually have to be large diameter air-cored, but you can get away with toroid or ferrite rod types if you can accept poorer results. It's quite illuminating to browse the offerings on the web. Bob How about making an Air Core Toroid. That can be made using a wax core. Then after winding it, some QDope or something in strips to hold the windings and then melting the wax. Or maybe a foam plastic core. Bill K7NOM |
Experimenting with Coils for Crystal Sets
On Apr 25, 10:51*pm, Tim Wescott wrote:
David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David They'll have very low Q, and therefore not really be suitable for a crystal radio, where low-Q coils fight your ability to get good selectivity without burning up all your signal before it gets to the headphones. There's a whole bunch of _other_ cool things you can do with them, just not Xtal sets. -- Tim Wescott Wescott Design Serviceshttp://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details athttp://www.wescottdesign.com/actfes/actfes.html What would be a good Q range for AM broadcast & Shortwave bands. Another thing is that a lot of these coils have only one coil on a form like an adjustable loopstick, maybe i'll never know unless i try them. Holding them next to each other they look alike. |
Experimenting with Coils for Crystal Sets
On Apr 26, 10:31*pm, Bill Janssen wrote:
Bob wrote: David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Unfortunately, the *Q* of those coils will be much too low for effective use in crystal receivers. *The best results I've had with "no power" receivers involved the use of FET detectors and /very/ High-Q coils. *The inductors usually have to be large diameter air-cored, but you can get away with toroid or ferrite rod types if you can accept poorer results. *It's quite illuminating to browse the offerings on the web. Bob How about making an Air Core Toroid. That can be made using a wax core. Then after winding it, some QDope or something in strips to hold the windings and then melting the wax. Or maybe a foam plastic core. Bill K7NOM- Hide quoted text - - Show quoted text - Have used glue from a hot glue gun before, the coils will never move once the glue sets. |
Experimenting with Coils for Crystal Sets
Bill Janssen wrote:
How about making an Air Core Toroid. That can be made using a wax core. Then after winding it, some QDope or something in strips to hold the windings and then melting the wax. Or maybe a foam plastic core. Bill K7NOM There was a manufacturer in the 20s that used a coil like that. Thorola - they called them donut coils. Looks like they just wound them on a small form, maybe 1 inch, and then slid it off and bent it around afterwards into the form of a donut. Never heard any comment yeah or nay about their efficacy. -Bill M |
Experimenting with Coils for Crystal Sets
On Apr 27, 9:22*am, Bill M wrote:
Bill Janssen wrote: How about making an Air Core Toroid. That can be made using a wax core. Then after winding it, some QDope or something in strips to hold the windings and then melting the wax. Or maybe a foam plastic core. Bill K7NOM There was a manufacturer in the 20s that used a coil like that. *Thorola - they called them donut coils. *Looks like they just wound them on a small form, maybe 1 inch, and then slid it off and bent it around afterwards into the form of a donut. Never heard any comment yeah or nay about their efficacy. -Bill M I think the best one I ever made used an oatmeal box for a form. Big air core coils seems to be the way to go. I built a kit that used a pair of loopstick coils and the best I could ever do wiith it was a couple of local stations. I think it was a Heathkit. A good RF ground seemed to benifit the receiver more than the antenna. Back in the day my father had a service station with about two acres of concrete out front. I was able to connect into a ground point that hooked it up to all the wire and rebar in the concrete. Jimmie |
Experimenting with Coils for Crystal Sets
On Mon, 27 Apr 2009 05:32:07 -0700 (PDT), David
wrote: On Apr 25, 10:51*pm, Tim Wescott wrote: David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David They'll have very low Q, and therefore not really be suitable for a crystal radio, where low-Q coils fight your ability to get good selectivity without burning up all your signal before it gets to the headphones. There's a whole bunch of _other_ cool things you can do with them, just not Xtal sets. -- Tim Wescott Wescott Design Serviceshttp://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details athttp://www.wescottdesign.com/actfes/actfes.html What would be a good Q range for AM broadcast & Shortwave bands. If your _loaded_ Q is 100, the -3 dB bandwidth at 1 MHz would be 10 kHz (i.e. +/- 5 kHz from the carrier). The question is, what should the _un_loaded Q be ? If you have a full sized antenna, the signal strength would be sufficient even with an unloaded Q of just 100-200. At the middle of the HF band (10 MHz) a loaded Q of 1000 would be required for a single station bandwidth and quite large helical resonators would be required to get a usable unloaded Q without damping the resonant circuit Q too much. Paul OH3LWR |
Experimenting with Coils for Crystal Sets
Paul Keinanen wrote:
On Mon, 27 Apr 2009 05:32:07 -0700 (PDT), David wrote: On Apr 25, 10:51 pm, Tim Wescott wrote: David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David They'll have very low Q, and therefore not really be suitable for a crystal radio, where low-Q coils fight your ability to get good selectivity without burning up all your signal before it gets to the headphones. There's a whole bunch of _other_ cool things you can do with them, just not Xtal sets. -- Tim Wescott Wescott Design Serviceshttp://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details athttp://www.wescottdesign.com/actfes/actfes.html What would be a good Q range for AM broadcast & Shortwave bands. If your _loaded_ Q is 100, the -3 dB bandwidth at 1 MHz would be 10 kHz (i.e. +/- 5 kHz from the carrier). The question is, what should the _un_loaded Q be ? If you have a full sized antenna, the signal strength would be sufficient even with an unloaded Q of just 100-200. At the middle of the HF band (10 MHz) a loaded Q of 1000 would be required for a single station bandwidth and quite large helical resonators would be required to get a usable unloaded Q without damping the resonant circuit Q too much. Paul OH3LWR What Paul didn't mention is that the ratio between the unloaded and loaded Q is pretty much the ratio between the energy coming from the antenna and the energy that's wasted in the tank. I.e. if your unloaded Q equals your loaded Q, then you're using up _all_ the energy in the tank, and there's none left over for your headphones. If your unloaded Q is 200 and your loaded is the 100 that you'd want for AM reception, you're using as much energy heating up your tank circuit as you are using to drive your headphones. So insanely efficient tank circuits are a necessity for a crystal set. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html |
Experimenting with Coils for Crystal Sets
On Mon, 27 Apr 2009 13:43:41 -0700, Tim Wescott
wrote: Paul Keinanen wrote: On Mon, 27 Apr 2009 05:32:07 -0700 (PDT), David wrote: On Apr 25, 10:51 pm, Tim Wescott wrote: David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David They'll have very low Q, and therefore not really be suitable for a crystal radio, where low-Q coils fight your ability to get good selectivity without burning up all your signal before it gets to the headphones. There's a whole bunch of _other_ cool things you can do with them, just not Xtal sets. -- Tim Wescott Wescott Design Serviceshttp://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details athttp://www.wescottdesign.com/actfes/actfes.html What would be a good Q range for AM broadcast & Shortwave bands. If your _loaded_ Q is 100, the -3 dB bandwidth at 1 MHz would be 10 kHz (i.e. +/- 5 kHz from the carrier). The question is, what should the _un_loaded Q be ? If you have a full sized antenna, the signal strength would be sufficient even with an unloaded Q of just 100-200. At the middle of the HF band (10 MHz) a loaded Q of 1000 would be required for a single station bandwidth and quite large helical resonators would be required to get a usable unloaded Q without damping the resonant circuit Q too much. Paul OH3LWR What Paul didn't mention is that the ratio between the unloaded and loaded Q is pretty much the ratio between the energy coming from the antenna and the energy that's wasted in the tank. I.e. if your unloaded Q equals your loaded Q, then you're using up _all_ the energy in the tank, and there's none left over for your headphones. If your unloaded Q is 200 and your loaded is the 100 that you'd want for AM reception, you're using as much energy heating up your tank circuit as you are using to drive your headphones. I fully agree with this. With a Qu/Ql ratio 2:1 you end up with only 6 dB insertion loss. However, the antenna capture area and hence the captured power (for a constant field strength and hence constant power density) is inversely proportional to the square of frequency (-6 dB/octave). While a 1/2 wavelength dipole might be sufficient to feed a crystal detector at 1 MHz, a 1/2 wave dipole at 10 MHz will unfortunately produce only 1/100 the power. Paul OH3LWR |
Experimenting with Coils for Crystal Sets
On Apr 27, 10:30*am, Paul Keinanen wrote:
On Mon, 27 Apr 2009 05:32:07 -0700 (PDT), David wrote: On Apr 25, 10:51*pm, Tim Wescott wrote: David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David They'll have very low Q, and therefore not really be suitable for a crystal radio, where low-Q coils fight your ability to get good selectivity without burning up all your signal before it gets to the headphones. There's a whole bunch of _other_ cool things you can do with them, just not Xtal sets. -- Tim Wescott Wescott Design Serviceshttp://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details athttp://www.wescottdesign.com/actfes/actfes.html What would be a good Q range for AM broadcast & Shortwave bands. If your _loaded_ Q is 100, the -3 dB bandwidth at 1 MHz would be 10 kHz (i.e. +/- 5 kHz from the carrier). The question is, what should the _un_loaded Q be ? If you have a full sized antenna, the signal strength would be sufficient even with an unloaded Q of just 100-200. At the middle of the HF band (10 MHz) a loaded Q of 1000 would be required for a single station bandwidth and quite large helical resonators would be required to get a usable unloaded Q without damping the resonant circuit Q too much. Paul OH3LWR To put some numbers on what Paul suggested: If I want an unloaded coil Q of 2000 at 10MHz, I'd expect to need a coil about 5 inches (13cm) diameter and 10 inches (26cm) long--or similar. That assumes no loss to radiation. If I shield it to make a helical resonator, the Q will actually be lowered slightly, though such a large coil may have enough loss to radiation that it would be a wash between turning it into a helical resonator or leaving it in free air. Of course, with the shielding, it's less susceptible to changes in the environment around it. Anyway, the shield for a helical resonator should be a couple times the diameter of the coil, so it's a rather large arrangement anyway! If you have very sensitive earphones (and sensitive ears!) and a good antenna, what may matter more than keeping the loss down to an absolute minimum is getting rid of interfering signals. A single- tuned circuit with 10kHz -3dB bandwidth offers only 20dB attenuation of a signal 50kHz away, and a signal 200kHz away is attenuated only about 32dB (assuming I didn't mess up my mental arithmetic). That's not a lot if you live in a metropolitan area with several stations nearby, and you want to hear the ones from far away. You can make the tuner with two or even three tuned circuits that are properly coupled, and get much better attenuation of those unwanted signals. But more tuned circuits means more loss in the tuner, too, even if you use high Q coils. It's a really good idea to check out what others have done to advance the art of winding high Q coils for AM broadcast band frequencies, and of the circuits to use those coils to best advantage. Cheers, Tom |
Experimenting with Coils for Crystal Sets
K7ITM wrote:
To put some numbers on what Paul suggested: If I want an unloaded coil Q of 2000 at 10MHz, I stopped here. I'd expect to need a.... No way you'll ever see 2000 Q at 10 Mcs. Can't happen. Some guys are approaching 2000 at 1 MC BCB but there's lots of expensive hoops to jump through to reach that point. Simply cannot be had at 10 Mcs. 200-300 on a good SW coil is about all that can be achieved. If you have very sensitive earphones (and sensitive ears!) and a good antenna, what may matter more than keeping the loss down to an absolute minimum is getting rid of interfering signals. Thats Q. = It's a really good idea to check out what others have done to advance the art of winding high Q coils for AM broadcast band frequencies, and of the circuits to use those coils to best advantage. Indeed. But that agreed to there's no method to make the same numbers and techniques work at 10 Mcs. You really cannot achieve the numbers on SW as you can BCB given the 10x frequency difference. |
Experimenting with Coils for Crystal Sets
On Mon, 27 Apr 2009 16:34:23 -0700 (PDT), K7ITM wrote:
If you have very sensitive earphones (and sensitive ears!) and a good antenna, what may matter more than keeping the loss down to an absolute minimum is getting rid of interfering signals. A single- tuned circuit with 10kHz -3dB bandwidth offers only 20dB attenuation of a signal 50kHz away, and a signal 200kHz away is attenuated only about 32dB (assuming I didn't mess up my mental arithmetic). That's not a lot if you live in a metropolitan area with several stations nearby, and you want to hear the ones from far away. You can make the tuner with two or even three tuned circuits that are properly coupled, and get much better attenuation of those unwanted signals. But more tuned circuits means more loss in the tuner, too, even if you use high Q coils. Unless you live very near an international SW broadcasters, how do you expect to get any usable amplitude modulated signal in the middle of the SW band for your crystal set ? To make the situation even worse, the power delivered by a matched dipole at 10 MHz is only 1/100 (-20 dB) of the power delivered by a matched dipole on 1 MHz due to the antenna capture area. Paul OH3LWR |
Experimenting with Coils for Crystal Sets
On Apr 27, 10:59*pm, Paul Keinanen wrote:
On Mon, 27 Apr 2009 16:34:23 -0700 (PDT), K7ITM wrote: If you have very sensitive earphones (and sensitive ears!) and a good antenna, what may matter more than keeping the loss down to an absolute minimum is getting rid of interfering signals. *A single- tuned circuit with 10kHz -3dB bandwidth offers only 20dB attenuation of a signal 50kHz away, and a signal 200kHz away is attenuated only about 32dB (assuming I didn't mess up my mental arithmetic). *That's not a lot if you live in a metropolitan area with several stations nearby, and you want to hear the ones from far away. *You can make the tuner with two or even three tuned circuits that are properly coupled, and get much better attenuation of those unwanted signals. *But more tuned circuits means more loss in the tuner, too, even if you use high Q coils. Unless you live very near an international SW broadcasters, how do you expect to get any usable amplitude modulated signal in the middle of the SW band for your crystal set ? To make the situation even worse, the power delivered by a matched dipole at 10 MHz is only 1/100 (-20 dB) of the power delivered by a matched dipole on 1 MHz due to the antenna capture area. Paul OH3LWR Actually, I was thinking in the paragraph you quoted more of MW broadcast, but since you mention SW: I have it on good authority that European SW broadcasters put 7MHz signals into the East coast of the US at levels up to close to 0dBm into a receiver's antenna terminals when the skip is right. That's using an antenna with a bit of gain over a dipole, but nothing fantastic. (That level gave me a goal for strong-signal handling for an HF receiver I recently put into production.) Cheers, Tom |
Experimenting with Coils for Crystal Sets
On Apr 27, 8:02*pm, Bill M wrote:
K7ITM wrote: To put some numbers on what Paul suggested: If I want an unloaded coil Q of 2000 at 10MHz, I stopped here. I'd expect to need a.... No way you'll ever see 2000 Q at 10 Mcs. Can't happen. Some guys are approaching 2000 at 1 MC BCB but there's lots of expensive hoops to jump through to reach that point. *Simply cannot be had at 10 Mcs. *200-300 on a good SW coil is about all that can be achieved. .... Interesting comment. In the filters I build for test fixtures, I use air-core coils that are about 1" diameter and 1" long, and they give me Qu in excess of 300 at 10MHz. For what I do, I don't need Qu up in the thousands, and don't have room for really big coils, but can you give me a reason I shouldn't expect Qu to scale linearly with size up to the point where radiation losses become significant? Can you tell me why I should think that the inductance calculator at http://hamwaves.com/antennas/inductance.html is not giving me accurate results when I put in, say, D=130mm, n=20, l=260mm, d=7mm, and f=10MHz? It agrees with other independent ways I have to estimate the Qu of the ~1 inch coils I build, and those coils measure within engineering tolerance of the estimates. When you go to very large coils like this, you have to be careful about the self-resonance becoming too low, but in the example above, it's (barely) OK at 10MHz. (Actually, you better be careful about the self-resonance of any coil...) Fewer turns of larger diameter "wire" (tubing: cheaper, and easier to work with) can yield about the same Qu and a considerably higher self-resonance. Cheers, Tom |
Experimenting with Coils for Crystal Sets
K7ITM wrote:
On Apr 27, 8:02 pm, Bill M wrote: K7ITM wrote: To put some numbers on what Paul suggested: If I want an unloaded coil Q of 2000 at 10MHz, I stopped here. I'd expect to need a.... No way you'll ever see 2000 Q at 10 Mcs. Can't happen. Some guys are approaching 2000 at 1 MC BCB but there's lots of expensive hoops to jump through to reach that point. Simply cannot be had at 10 Mcs. 200-300 on a good SW coil is about all that can be achieved. ... Interesting comment. In the filters I build for test fixtures, I use air-core coils that are about 1" diameter and 1" long, and they give me Qu in excess of 300 at 10MHz. For what I do, I don't need Qu up in the thousands, and don't have room for really big coils, but can you give me a reason I shouldn't expect Qu to scale linearly with size up to the point where radiation losses become significant? Can you tell me why I should think that the inductance calculator at http://hamwaves.com/antennas/inductance.html is not giving me accurate results when I put in, say, D=130mm, n=20, l=260mm, d=7mm, and f=10MHz? It agrees with other independent ways I have to estimate the Qu of the ~1 inch coils I build, and those coils measure within engineering tolerance of the estimates. Self-capacitance (ultimately self-resonance) is always a contributing factor that prevents coils from achieving their maximum theoretical Q. Estimating coils whose Q is going to fall in the 200 range for other reasons is relatively easy but that doesn't mean you can scale upwards proportionately. I build air coils in the 3 inch range along the lines of what is pictured here, http://www.sparkbench.com/homebrew/grebe/cr18.html They only *measure* in the 250 range. Silver-plated wire could certainly improve a coil of this size but no way would you achieve numbers like 2000. -Bill |
Experimenting with Coils for Crystal Sets
On Mon, 27 Apr 2009 16:34:23 -0700 (PDT), K7ITM wrote:
If I want an unloaded coil Q of 2000 at 10MHz, I'd expect to need a coil about 5 inches (13cm) diameter and 10 inches (26cm) long--or similar. That assumes no loss to radiation. If I shield it to make a helical resonator, the Q will actually be lowered slightly, though such a large coil may have enough loss to radiation that it would be a wash between turning it into a helical resonator or leaving it in free air. Of course, with the shielding, it's less susceptible to changes in the environment around it. Anyway, the shield for a helical resonator should be a couple times the diameter of the coil, so it's a rather large arrangement anyway! 1/4 wave filters used in repeater duplexers have quite high unloaded-Q and are available to at least down to 50 MHz. Of course these are long, but in order to reduce total shape, these might be bent into U-shape. To go into even lower frequencies, the resonator can be bent into a helix. At least the design nomogram in old ARRL handbooks seem to indicate that with sufficient size very high unloaded-Q could be achieved (up to several thousands). Of course the helix itself and the inside of the resonator may have to be silver plated. Paul OH3LWR |
Experimenting with Coils for Crystal Sets
On Apr 25, 7:23*pm, David wrote:
Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David Wonder if an adjustable coil loopstick and variable capacitor preselector between the antenna and the crystal set would improve reception for distant stations on the broadcast and high frequency bands? David |
Experimenting with Coils for Crystal Sets
David wrote:
On Apr 25, 7:23 pm, David wrote: Was just wandering if anyone has used or experimented with television IF, Video and Detector coils, most are slug tuned coils that have a few uh to several hundred uh, some are sheilded some are not, i have about 500 that i bought years ago, a lot of them look very close too the old loopstick type coils, looking for ideas. Thanks David Wonder if an adjustable coil loopstick and variable capacitor preselector between the antenna and the crystal set would improve reception for distant stations on the broadcast and high frequency bands? David Nope, you have the same problem with loaded vs. unloaded Q and efficiency. You can get there from here, you just can't do it with that loopstick. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html |
Experimenting with Coils for Crystal Sets
On Apr 28, 5:20*am, Bill M wrote:
K7ITM wrote: On Apr 27, 8:02 pm, Bill M wrote: K7ITM wrote: To put some numbers on what Paul suggested: If I want an unloaded coil Q of 2000 at 10MHz, I stopped here. I'd expect to need a.... No way you'll ever see 2000 Q at 10 Mcs. Can't happen. Some guys are approaching 2000 at 1 MC BCB but there's lots of expensive hoops to jump through to reach that point. *Simply cannot be had at 10 Mcs. *200-300 on a good SW coil is about all that can be achieved. ... Interesting comment. *In the filters I build for test fixtures, I use air-core coils that are about 1" diameter and 1" long, and they give me Qu in excess of 300 at 10MHz. *For what I do, I don't need Qu up in the thousands, and don't have room for really big coils, but can you give me a reason I shouldn't expect Qu to scale linearly with size up to the point where radiation losses become significant? Can you tell me why I should think that the inductance calculator at http://hamwaves.com/antennas/inductance.htmlis not giving me accurate results when I put in, say, D=130mm, n=20, l=260mm, d=7mm, and f=10MHz? *It agrees with other independent ways I have to estimate the Qu of the ~1 inch coils I build, and those coils measure within engineering tolerance of the estimates. Self-capacitance (ultimately self-resonance) is always a contributing factor that prevents coils from achieving their maximum theoretical Q. Estimating coils whose Q is going to fall in the 200 range for other reasons is relatively easy but that doesn't mean you can scale upwards proportionately. I build air coils in the 3 inch range along the lines of what is pictured here, *http://www.sparkbench.com/homebrew/grebe/cr18.html They only *measure* in the 250 range. *Silver-plated wire could certainly improve a coil of this size but no way would you achieve numbers like 2000. -Bill OK, I was curious. Was I way off-base, or is it reasonable to think that you can get a 10MHz Qu considerably higher than 250 (and possibly up in the stratosphere above 1000)? I'm not going to spend the time, effort and money to build a seriously large coil as the theory suggests I'd need, but I did wind a somewhat smaller one... I wound 15 turns of #10 AWG (2.55mm) bare copper with about 2.25 inches ID and 3 inches long. I resonated it with 3 * 12pF C0G capacitors; it resonates at 9.088MHz. I coupled an output to an analyzer through 1pF tapped one turn up from the "cold" end, and loosely coupled an input from the analyzer's source using a one-turn loop spaced away from the coil. My back-of-the-envelope calculation says such a coil with air insulation should have a Qu around 740 at 9MHz. In a tank circuit, the finite Q of the capacitors will lower the tank Q below that value. What I actually measure is a 3dB bandwidth of 15.87kHz, for a tank Q of 572. OK, so that's a bit lower than I might have expected. BUT--this coil is wound on a length of black ABS drain pipe, which is an absolutely terrible thing to use as a coil form if you're trying to get the highest possible Q. (My plan was originally to take the coil off the form after I wound it, but it had too much of a mind of its own about what shape it was going to assume. I've wound some smaller self-supporting coils, but this one didn't work that way.) I'm convinced by this little 'speriment that I could build an LC tank resonant at 10MHz with a tank Q above 1000 with no trouble--and probably _well_ above 1000 if I used really high Q capacitors and just enough low-loss solid insulation to keep the coil turns properly spaced. I don't need one at the moment, but if I ever do, I sure won't be afraid to try it. Cheers, Tom |
Experimenting with Coils for Crystal Sets
K7ITM wrote:
On Apr 28, 5:20 am, Bill M wrote: K7ITM wrote: On Apr 27, 8:02 pm, Bill M wrote: K7ITM wrote: To put some numbers on what Paul suggested: If I want an unloaded coil Q of 2000 at 10MHz, I stopped here. I'd expect to need a.... No way you'll ever see 2000 Q at 10 Mcs. Can't happen. Some guys are approaching 2000 at 1 MC BCB but there's lots of expensive hoops to jump through to reach that point. Simply cannot be had at 10 Mcs. 200-300 on a good SW coil is about all that can be achieved. ... Interesting comment. In the filters I build for test fixtures, I use air-core coils that are about 1" diameter and 1" long, and they give me Qu in excess of 300 at 10MHz. For what I do, I don't need Qu up in the thousands, and don't have room for really big coils, but can you give me a reason I shouldn't expect Qu to scale linearly with size up to the point where radiation losses become significant? Can you tell me why I should think that the inductance calculator at http://hamwaves.com/antennas/inductance.htmlis not giving me accurate results when I put in, say, D=130mm, n=20, l=260mm, d=7mm, and f=10MHz? It agrees with other independent ways I have to estimate the Qu of the ~1 inch coils I build, and those coils measure within engineering tolerance of the estimates. Self-capacitance (ultimately self-resonance) is always a contributing factor that prevents coils from achieving their maximum theoretical Q. Estimating coils whose Q is going to fall in the 200 range for other reasons is relatively easy but that doesn't mean you can scale upwards proportionately. I build air coils in the 3 inch range along the lines of what is pictured here, http://www.sparkbench.com/homebrew/grebe/cr18.html They only *measure* in the 250 range. Silver-plated wire could certainly improve a coil of this size but no way would you achieve numbers like 2000. -Bill OK, I was curious. Was I way off-base, or is it reasonable to think that you can get a 10MHz Qu considerably higher than 250 (and possibly up in the stratosphere above 1000)? I'm not going to spend the time, effort and money to build a seriously large coil as the theory suggests I'd need, but I did wind a somewhat smaller one... I wound 15 turns of #10 AWG (2.55mm) bare copper with about 2.25 inches ID and 3 inches long. I resonated it with 3 * 12pF C0G capacitors; it resonates at 9.088MHz. I coupled an output to an analyzer through 1pF tapped one turn up from the "cold" end, and loosely coupled an input from the analyzer's source using a one-turn loop spaced away from the coil. My back-of-the-envelope calculation says such a coil with air insulation should have a Qu around 740 at 9MHz. In a tank circuit, the finite Q of the capacitors will lower the tank Q below that value. What I actually measure is a 3dB bandwidth of 15.87kHz, for a tank Q of 572. OK, so that's a bit lower than I might have expected. BUT--this coil is wound on a length of black ABS drain pipe, which is an absolutely terrible thing to use as a coil form if you're trying to get the highest possible Q. (My plan was originally to take the coil off the form after I wound it, but it had too much of a mind of its own about what shape it was going to assume. I've wound some smaller self-supporting coils, but this one didn't work that way.) I'm convinced by this little 'speriment that I could build an LC tank resonant at 10MHz with a tank Q above 1000 with no trouble--and probably _well_ above 1000 if I used really high Q capacitors and just enough low-loss solid insulation to keep the coil turns properly spaced. I don't need one at the moment, but if I ever do, I sure won't be afraid to try it. Cheers, Tom Ok, I admit to lowballing my practical estimates. I don't want to be argumentative about it but it the steps of increase become incrementally more difficult to obtain as you approach higher levels of Q. Never any harm in trying to make the best possible coil for the application. Good luck with your projects! -Bill |
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