|
coils : turns shorted = quality ?
HI,
Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... regards |
coils : turns shorted = quality ?
"Tomylavitesse" wrote in message ... HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... regards Good question! I've always wondered about that too. In a former life as a marine electronics technician, the old marine "AM" transmitter antenna tuners uses shorted turns a lot. It always sort of went against the grain, I thought, but in practice, it seemed to work ok. What was I missing here? Old Chief Lynn, W7LTQ and some commercials with endorsements |
coils : turns shorted = quality ?
In article ,
coffelt2 wrote: Good question! I've always wondered about that too. In a former life as a marine electronics technician, the old marine "AM" transmitter antenna tuners uses shorted turns a lot. It always sort of went against the grain, I thought, but in practice, it seemed to work ok. What was I missing here? Old Chief Lynn, W7LTQ and some commercials with endorsements My understanding is that the presence of the shorted turns in the coil does lower the Q of the coil somewhat. If the loaded Q of the system/application is already low enough, the reduction in coil Q doesn't result in an unacceptable increase in losses. The alternative, in some cases, is to leave the unused turns open at one end. This isn't always possible, and when it is possible it can have an unfortunate side effect. Autotransformer action can cause the open end of the turns to develop a high voltage relative to the rest of the system, and there can be arcing to nearby points of lower potential. Could make a mess of the inside of your transmatch :-( -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
coils : turns shorted = quality ?
coffelt2 wrote:
"Tomylavitesse" wrote in message ... HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... regards Good question! I've always wondered about that too. In a former life as a marine electronics technician, the old marine "AM" transmitter antenna tuners uses shorted turns a lot. It always sort of went against the grain, I thought, but in practice, it seemed to work ok. What was I missing here? Old Chief Lynn, W7LTQ and some commercials with endorsements In many military antenna tuners, the variable inductor inside the tuner was open on one end. The coil was rotated such that the length of the coil winding was changed, or the position of the tap was changed to effect the tuning. There were also some tuners that spooled one end of the coil onto a grounded metal drum. The mechanism changed the effective length of the coil, but did not introduce any shorted turns. -- David masondg44 at comcast dot net |
coils : turns shorted = quality ?
Tomylavitesse wrote:
HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... regards If the shorted turns are connected to one end of the coil then the total shorted turns would appear as the final turn of the coil. In amateur transmitters the band switch often shorts turns from the cold end of the tank coil as one switches to higher frequency bands. The shorted turns simply are not in the circuit I guess. |
coils : turns shorted = quality ?
On 2009-09-24, Tomylavitesse wrote:
HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... The circuit to shorten turns in such environments has to be very low loss. Otherwise you will have lots of power lost in this short circiuted parts of the coil. And in PA / antenna environments you always have power. This is why switches are always "heavy duty" in PA and antenna switching units. If you do so, the current in this shortened turns is a complex current. And as you know, complex currents do not consume power. Only the real parts of current do. This is totally different from the situation e.g. in a power transformator. If you would shorten turns there, you would have power loss, because the resistant of the turns in power transformators has a much higher real part compared with rf coils in PA / antenna units. OK? |
coils : turns shorted = quality ?
rtfm wrote:
On 2009-09-24, Tomylavitesse wrote: HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... The circuit to shorten turns in such environments has to be very low loss. Otherwise you will have lots of power lost in this short circiuted parts of the coil. And in PA / antenna environments you always have power. This is why switches are always "heavy duty" in PA and antenna switching units. If you do so, the current in this shortened turns is a complex current. And as you know, complex currents do not consume power. Only the real parts of current do. This is totally different from the situation e.g. in a power transformator. If you would shorten turns there, you would have power loss, because the resistant of the turns in power transformators has a much higher real part compared with rf coils in PA / antenna units. OK? This seemed like an insightful response - but not one with universal appeal. Let me try embroidering on this theme a little more.... The leakage path for current in an insulated wire is strikingly different from the leakage path in a magnetic conductor - usually called a core or stamping. The leakage resistance can easily be 1000 megohms compared to the conductor's resistance of (say) 1 ohm. A ratio of a billion to one. The leakage path for cores and stampings is lucky to be a thousand times more "resistive" than the path through the core - if it's an iron stamping, an iron dust core, or a ferrite core. A ratio of a thousand to one. For air cored coils, the leakage path is lower still, so that the magnetic path does not couple all turns together at the best of times. If an end turn or two is shorted, the reactive current in the shorted turn pinches off most of the magnetic coupling from the remaining coil altogether, so the effect is not as dramatic as we might expect. Waddaya think of that? Brian W |
coils : turns shorted = quality ?
"brian whatcott" wrote in message ... rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... The circuit to shorten turns in such environments has to be very low loss. Otherwise you will have lots of power lost in this short circiuted parts of the coil. And in PA / antenna environments you always have power. This is why switches are always "heavy duty" in PA and antenna switching units. If you do so, the current in this shortened turns is a complex current. And as you know, complex currents do not consume power. Only the real parts of current do. This is totally different from the situation e.g. in a power transformator. If you would shorten turns there, you would have power loss, because the resistant of the turns in power transformators has a much higher real part compared with rf coils in PA / antenna units. OK? This seemed like an insightful response - but not one with universal appeal. Let me try embroidering on this theme a little more.... The leakage path for current in an insulated wire is strikingly different from the leakage path in a magnetic conductor - usually called a core or stamping. The leakage resistance can easily be 1000 megohms compared to the conductor's resistance of (say) 1 ohm. A ratio of a billion to one. The leakage path for cores and stampings is lucky to be a thousand times more "resistive" than the path through the core - if it's an iron stamping, an iron dust core, or a ferrite core. A ratio of a thousand to one. For air cored coils, the leakage path is lower still, so that the magnetic path does not couple all turns together at the best of times. If an end turn or two is shorted, the reactive current in the shorted turn pinches off most of the magnetic coupling from the remaining coil altogether, so the effect is not as dramatic as we might expect. Waddaya think of that? Brian W Wooo hoooooo, now we're cookin' Old Chief Lynn |
coils : turns shorted = quality ?
On 2009-09-27, brian whatcott wrote:
... The leakage path for current in an insulated wire is strikingly different from the leakage path in a magnetic conductor - usually called a core or stamping. The leakage resistance can easily be 1000 megohms compared to the conductor's resistance of (say) 1 ohm. A ratio of a billion to one. The leakage path for cores and stampings is lucky to be a thousand times more "resistive" than the path through the core - if it's an iron stamping, an iron dust core, or a ferrite core. A ratio of a thousand to one. For air cored coils, the leakage path is lower still, so that the magnetic path does not couple all turns together at the best of times. If an end turn or two is shorted, the reactive current in the shorted turn pinches off most of the magnetic coupling from the remaining coil altogether, so the effect is not as dramatic as we might expect. Waddaya think of that? Thanks for this posting. You are totally right. I tried to explain thinks a little more easier. And if you try to reduce complexity in an explanation, you have to simplyfy. Looks like I simplyfied too much. My fault. |
coils : turns shorted = quality ?
Hi,
Thanks everybody. I just found your answers, so I will work with them. I've also found some other info on my old handbook ARRL. Thanks again Raph |
coils : turns shorted = quality ?
On 2009-09-27, Tomylavitesse wrote:
Hi, Thanks everybody. I just found your answers, so I will work with them. I've also found some other info on my old handbook ARRL. Worlds best HAM tech source! |
coils : turns shorted = quality ?
"rtfm" wrote in message ... On 2009-09-27, brian whatcott wrote: ... The leakage path for current in an insulated wire is strikingly different from the leakage path in a magnetic conductor - usually called a core or stamping. The leakage resistance can easily be 1000 megohms compared to the conductor's resistance of (say) 1 ohm. A ratio of a billion to one. The leakage path for cores and stampings is lucky to be a thousand times more "resistive" than the path through the core - if it's an iron stamping, an iron dust core, or a ferrite core. A ratio of a thousand to one. For air cored coils, the leakage path is lower still, so that the magnetic path does not couple all turns together at the best of times. If an end turn or two is shorted, the reactive current in the shorted turn pinches off most of the magnetic coupling from the remaining coil altogether, so the effect is not as dramatic as we might expect. Waddaya think of that? Thanks for this posting. You are totally right. I tried to explain thinks a little more easier. And if you try to reduce complexity in an explanation, you have to simplyfy. Looks like I simplyfied too much. My fault. No, no, no..... your explanation was right on the mark! Many of us thoroughly enjoyed your reduction of imaginary numbers and such into a down-to-earth, homebrew response. Well done! Old Chief Lynn |
coils : turns shorted = quality ?
On Sep 26, 8:25*pm, brian whatcott wrote:
rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... The circuit to shorten turns in such environments has to be very low loss. Otherwise you will have lots of power lost in this short circiuted parts of the coil. And in PA / antenna environments you always have power. This is why switches are always "heavy duty" in PA and antenna switching units. If you do so, the current in this shortened turns is a complex current. And as you know, complex currents do not consume power. Only the real parts of current do. This is totally different from the situation e.g. in a power transformator. If you would shorten turns there, you would have power loss, because the resistant of the turns in power transformators has a much higher real part compared with rf coils in PA / antenna units. OK? This seemed like an insightful response - but not one with universal appeal. Let me try embroidering on this theme a little more.... The leakage path for current in an insulated wire is strikingly different from the leakage path in a magnetic conductor - usually called a core or stamping. The leakage resistance can easily be 1000 megohms compared to the conductor's resistance of (say) 1 ohm. A ratio of a billion to one. The leakage path for cores and stampings is lucky to be a thousand times more "resistive" than the path through the core - if it's an iron stamping, an iron dust core, or a ferrite core. A ratio of a thousand to one. For air cored coils, the leakage path is lower still, so that the magnetic path does not couple all turns together at the best of times. If an end turn or two is shorted, the reactive current in the shorted turn pinches off most of the magnetic coupling from the remaining coil altogether, so the effect is not as dramatic as we might expect. Waddaya think of that? Brian W It's actually not too difficult to come up with a reasonably accurate model that you can then analyze...for example, you can put it into Spice and look at the losses, the lowering of Q, etc. What I've done to build the model is to consider that the shorted turns are one coil and the remainder of the turns are another. Before those turns are shorted, each of those two coils (that happen to form one coil) has a self-inductance, and when they are put end-to-end to form the one coil, they then also have mutual inductance. You can use your favorite coil inductance calculator to find the inductance of each of the two individual coils, and the net single coil composed of those two. Those three inductances let you calculate the mutual inductance (or the coupling coefficient), and that's the core of the model. You can add in the RF resistance of each coil: there are various ways to get a good estimate of the Qu, and from that you can get the RF resistance at the operating frequency. If this is too muddy, I could offer a specific example... Cheers, Tom |
coils : turns shorted = quality ?
On Sun, 27 Sep 2009 22:28:33 -0700 (PDT), K7ITM wrote:
On Sep 26, 8:25*pm, brian whatcott wrote: rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, OK? snip As well, it would be interesting to see the effect of stray capacitance, as the reason for shorting unused turns is to reduce the problem of stray capacitance and the inductance having unwanted parasitic resonances in the system, That is why shorting unused sections is normally advocated. Peter Dettmann |
coils : turns shorted = quality ?
Peter Dettmann wrote:
On Sun, 27 Sep 2009 22:28:33 -0700 (PDT), K7ITM wrote: On Sep 26, 8:25 pm, brian whatcott wrote: rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, OK? snip As well, it would be interesting to see the effect of stray capacitance, as the reason for shorting unused turns is to reduce the problem of stray capacitance and the inductance having unwanted parasitic resonances in the system, That is why shorting unused sections is normally advocated. Peter Dettmann Well, its the better of the options but the Q of the 'desired' coil still takes a hit because of that nearby lump of copper. It works out ok in practice, though. -Bill |
coils : turns shorted = quality ?
On Sep 27, 10:28*pm, K7ITM wrote:
On Sep 26, 8:25*pm, brian whatcott wrote: rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, Often I can see coils where some turns are shorted in order to modify there value. I wonder whether these shorted turns would not be seen as a short cicuit and could decrease the quality of that coil. How can I have an idea of the lost of quality without complex equipment ? I plan to build this kind of coil for a shortened antenna... The circuit to shorten turns in such environments has to be very low loss. Otherwise you will have lots of power lost in this short circiuted parts of the coil. And in PA / antenna environments you always have power. This is why switches are always "heavy duty" in PA and antenna switching units. If you do so, the current in this shortened turns is a complex current. And as you know, complex currents do not consume power. Only the real parts of current do. This is totally different from the situation e.g. in a power transformator. If you would shorten turns there, you would have power loss, because the resistant of the turns in power transformators has a much higher real part compared with rf coils in PA / antenna units. OK? This seemed like an insightful response - but not one with universal appeal. Let me try embroidering on this theme a little more.... The leakage path for current in an insulated wire is strikingly different from the leakage path in a magnetic conductor - usually called a core or stamping. The leakage resistance can easily be 1000 megohms compared to the conductor's resistance of (say) 1 ohm. A ratio of a billion to one. The leakage path for cores and stampings is lucky to be a thousand times more "resistive" than the path through the core - if it's an iron stamping, an iron dust core, or a ferrite core. A ratio of a thousand to one. For air cored coils, the leakage path is lower still, so that the magnetic path does not couple all turns together at the best of times. If an end turn or two is shorted, the reactive current in the shorted turn pinches off most of the magnetic coupling from the remaining coil altogether, so the effect is not as dramatic as we might expect. Waddaya think of that? Brian W It's actually not too difficult to come up with a reasonably accurate model that you can then analyze...for example, you can put it into Spice and look at the losses, the lowering of Q, etc. What I've done to build the model is to consider that the shorted turns are one coil and the remainder of the turns are another. *Before those turns are shorted, each of those two coils (that happen to form one coil) has a self-inductance, and when they are put end-to-end to form the one coil, they then also have mutual inductance. *You can use your favorite coil inductance calculator to find the inductance of each of the two individual coils, and the net single coil composed of those two. *Those three inductances let you calculate the mutual inductance (or the coupling coefficient), and that's the core of the model. *You can add in the RF resistance of each coil: *there are various ways to get a good estimate of the Qu, and from that you can get the RF resistance at the operating frequency. If this is too muddy, I could offer a specific example... Cheers, Tom So, just for fun I ran an analysis this way... I asked the Hamwaves on-line inductance calculator about three coils, all 2.5mm wire, 5mm pitch, 75mm mean coil diameter, default copper wire. You can put effective shunt capacitances into the model too, but I didn't in this case. The test frequency is 10.0MHz. These are the "lumped equivalent" values. L1: 13 turns 65mm long -- 9.272uH, 0.491 ohms, Qu=1187 L2: 17 turns 85mm long -- 13.191uH, 0.646 ohms, Qu=1283 L3 (L1 and L2 end to end): 30t, 150mm -- 26.600uH, 0.880 ohms, Qu=1899 (My impression is that the RF resistance and Qu calculation yields Qu that's a bit too high, but that's what the calculator says...) From these, I calculate the mutual inductance. L1+L2+2*M = L3, so M=2.0685uH. Coupling coefficient equals M/sqrt(L1*L2), or 0.187. I put L1 and L2 and their respective RF resistances in series in Spice, with a resistance representing a shorting switch across L2. Note that this model, with the switch open, will give an RF resistance simply equal to the sum of the RF resistances of the two coils, which isn't the same as the Hamwaves calculator gives for L3. The results are kind of interesting. It doesn't take much shunt resistance in the switch to lower the Q (increase the net RF resistance). You want to be sure your switch is really OPEN when the coil isn't shorted. You can actually stand quite a bit of resistance (a good fraction of an ohm) when it's shorted without really bad effects. Here are the results I got, again at 10.0MHz: Effective net R(switch) L(net) series resistance ohms uH ohms Qu -------- -------- -------- -------- 1e9 26.598 1.138 1469 1e8 26.598 1.146 1458 1e7 26.598 1.229 1360 1e6 26.598 2.056 813 1e5 26.597 10.32 162 1e4 26.476 92.4 18.0 1e3 19.405 545 2.24 1e2 9.201 132.4 4.37 10 8.950 13.886 40.5 1.0 8.947 1.845 305 0.5 8.948 1.175 478 0.2 8.947 0.774 726 0.1 8.947 0.640 878 0.05 8.947 0.573 981 0.02 8.947 0.533 1054 0.01 8.947 0.520 1081 Cheers, Tom |
coils : turns shorted = quality ?
K7ITM wrote:
So, just for fun I ran an analysis this way... I asked the Hamwaves on-line inductance calculator about three coils, all 2.5mm wire, 5mm pitch, 75mm mean coil diameter, default copper wire. You can put effective shunt capacitances into the model too, but I didn't in this case. The test frequency is 10.0MHz. These are the "lumped equivalent" values. L1: 13 turns 65mm long -- 9.272uH, 0.491 ohms, Qu=1187 L2: 17 turns 85mm long -- 13.191uH, 0.646 ohms, Qu=1283 L3 (L1 and L2 end to end): 30t, 150mm -- 26.600uH, 0.880 ohms, Qu=1899 (My impression is that the RF resistance and Qu calculation yields Qu that's a bit too high, but that's what the calculator says...) From these, I calculate the mutual inductance. L1+L2+2*M = L3, so M=2.0685uH. Coupling coefficient equals M/sqrt(L1*L2), or 0.187. I put L1 and L2 and their respective RF resistances in series in Spice, with a resistance representing a shorting switch across L2. Note that this model, with the switch open, will give an RF resistance simply equal to the sum of the RF resistances of the two coils, which isn't the same as the Hamwaves calculator gives for L3. The results are kind of interesting. It doesn't take much shunt resistance in the switch to lower the Q (increase the net RF resistance). You want to be sure your switch is really OPEN when the coil isn't shorted. You can actually stand quite a bit of resistance (a good fraction of an ohm) when it's shorted without really bad effects. Here are the results I got, again at 10.0MHz: Effective net R(switch) L(net) series resistance ohms uH ohms Qu -------- -------- -------- -------- 1e9 26.598 1.138 1469 1e8 26.598 1.146 1458 1e7 26.598 1.229 1360 1e6 26.598 2.056 813 1e5 26.597 10.32 162 1e4 26.476 92.4 18.0 1e3 19.405 545 2.24 1e2 9.201 132.4 4.37 10 8.950 13.886 40.5 1.0 8.947 1.845 305 0.5 8.948 1.175 478 0.2 8.947 0.774 726 0.1 8.947 0.640 878 0.05 8.947 0.573 981 0.02 8.947 0.533 1054 0.01 8.947 0.520 1081 Cheers, Tom Good post! Brian W |
coils : turns shorted = quality ?
On Mon, 28 Sep 2009 19:33:12 -0400, Bill M
wrote: Peter Dettmann wrote: On Sun, 27 Sep 2009 22:28:33 -0700 (PDT), K7ITM wrote: On Sep 26, 8:25 pm, brian whatcott wrote: rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, OK? snip As well, it would be interesting to see the effect of stray capacitance, as the reason for shorting unused turns is to reduce the problem of stray capacitance and the inductance having unwanted parasitic resonances in the system, That is why shorting unused sections is normally advocated. Peter Dettmann Well, its the better of the options but the Q of the 'desired' coil still takes a hit because of that nearby lump of copper. It works out ok in practice, though. -Bill Exactly Bill, and the whole thing really is a matter of just what is the needed, or acceptable Q. If the Q is satisfactory with the shorted turns, then that is the way to make your design (keeping in mind the disadvantages of open circuit unused turns). However should you need the highest obtainable Q, then a single isolated coil is called for which has the needed turns. Peter |
coils : turns shorted = quality ?
On Sep 30, 3:46*am, Peter Dettmann wrote:
On Mon, 28 Sep 2009 19:33:12 -0400, Bill M wrote: Peter Dettmann wrote: On Sun, 27 Sep 2009 22:28:33 -0700 (PDT), K7ITM wrote: On Sep 26, 8:25 pm, brian whatcott wrote: rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, OK? snip As well, it would be interesting to see the effect of stray capacitance, as the reason for shorting unused turns is to reduce the problem of stray capacitance and the inductance having unwanted parasitic resonances in the system, * *That is why shorting unused sections is normally advocated. Peter Dettmann Well, its the better of the options but the Q of the 'desired' coil still takes a hit because of that nearby lump of copper. It works out ok in practice, though. -Bill Exactly Bill, and the whole thing really is a matter of just what is the needed, or acceptable Q. * If the Q is satisfactory with the shorted turns, then that is the way to make your design (keeping in mind the disadvantages of open circuit unused turns). However should you need the highest obtainable Q, then a single isolated coil is called for which has the needed turns. Peter- Hide quoted text - - Show quoted text - All this very interesting; because many, many, (like 50+) years ago I built a signal generator, that didn't work! Nice job in an ex- Admiralty copper lined wooden box! Came across a complete set of coils with a multi position switch which covered a wide range of frequencies. Tested it out breadboard fashion and it worked fine. Then built it into a double layer chassis constructed from two thick and therefore potentially rigid and frequency stable pieces of Aluminum! But what I had done was dismount a set of coils from a plastic frame and mounted them in holes drilled into the two opposite panels of aluminum. Nice job mechanically but what must have hap pend is the Al. panels provided shorted turns in the same plane as the winding of the coils and reduced their Q to where the (tube, inductive feedback) circuit wouldn't work. Never got time to work on it because I then emigrated to North America and the unfinished item is long gone. Had a nice Muirhead slow motion drive on it too! Live and learn! |
coils : turns shorted = quality ?
On Oct 8, 8:17*am, stan wrote:
On Sep 30, 3:46*am, Peter Dettmann wrote: On Mon, 28 Sep 2009 19:33:12 -0400, Bill M wrote: Peter Dettmann wrote: On Sun, 27 Sep 2009 22:28:33 -0700 (PDT), K7ITM wrote: On Sep 26, 8:25 pm, brian whatcott wrote: rtfm wrote: On 2009-09-24, Tomylavitesse wrote: HI, OK? snip As well, it would be interesting to see the effect of stray capacitance, as the reason for shorting unused turns is to reduce the problem of stray capacitance and the inductance having unwanted parasitic resonances in the system, * *That is why shorting unused sections is normally advocated. Peter Dettmann Well, its the better of the options but the Q of the 'desired' coil still takes a hit because of that nearby lump of copper. It works out ok in practice, though. -Bill Exactly Bill, and the whole thing really is a matter of just what is the needed, or acceptable Q. * If the Q is satisfactory with the shorted turns, then that is the way to make your design (keeping in mind the disadvantages of open circuit unused turns). However should you need the highest obtainable Q, then a single isolated coil is called for which has the needed turns. Peter- Hide quoted text - - Show quoted text - All this very interesting; because many, many, (like 50+) years ago I built a signal generator, that didn't work! Nice job in an ex- Admiralty copper lined wooden box! Came across a complete set of coils with a multi position switch which covered a wide range of frequencies. Tested it out breadboard fashion and it worked fine. Then built it into a double layer chassis constructed from two thick and therefore potentially rigid and frequency stable pieces of Aluminum! But what I had done was dismount a set of coils from a plastic frame and mounted them in holes drilled into the two opposite panels of aluminum. Nice job mechanically but what must have hap pend is the Al. panels provided shorted turns in the same plane as the winding of the coils and reduced their Q to where the (tube, inductive feedback) circuit wouldn't work. Never got time to work on it because I then emigrated to North America and the unfinished item is long gone. Had a nice Muirhead slow motion drive on it too! Live and learn! In at least some editions of "Reference Data for Radio Engineers," there's a graph of how much the inductance of a solenoid coil is lowered by being placed inside a shield. If the shield isn't too close to the coil, the percentage decrease in inductance is rather low (the coil isn't affected very much). The graph doesn't tell you how much the Q is lowered, but I did some comparisons of Q estimates from a couple different solenoid coil programs I have and Q estimates for helical resonators, and came to the conclusion that you can account for most of the lowering of Q by assuming the RF resistance remains constant and the inductance is lowered, per the R.D.R.E. graph (attributed to RCA). That assumes a highly-conducting shield, I'm sure. The observation helped dispel the "magic" aura of helical resonators: their Q is actually lower than the Qu of the coil they contain, if that coil is in free air. (A big advantage, of course, is that they are fully shielded.) Cheers, Tom |
coils : turns shorted = quality ?
K7ITM wrote:
In at least some editions of "Reference Data for Radio Engineers," there's a graph of how much the inductance of a solenoid coil is lowered by being placed inside a shield. If the shield isn't too close to the coil, the percentage decrease in inductance is rather low (the coil isn't affected very much). The graph doesn't tell you how much the Q is lowered, but I did some comparisons of Q estimates from a couple different solenoid coil programs I have and Q estimates for helical resonators, and came to the conclusion that you can account for most of the lowering of Q by assuming the RF resistance remains constant and the inductance is lowered, per the R.D.R.E. graph (attributed to RCA). That assumes a highly-conducting shield, I'm sure. The observation helped dispel the "magic" aura of helical resonators: their Q is actually lower than the Qu of the coil they contain, if that coil is in free air. (A big advantage, of course, is that they are fully shielded.) This gets to be a jello-y area if you follow the clues far enough. The coil's Q, or better said, the circuit's Q has a lot to do with how much spacing is required. Old timey Rule Of Thumb suggests one diameter spacing away from enclosures but thats typically relative to lower Q coils which are common rote. I make that comparison against high-Q coils in dx crystal sets where 12-18 inch separation isn't uncommon. The textbook Q formula doesn't address this...nor is it a necessarily a biggie because most of the coils that we encounter in the situation presented in the original post aren't of high enough Q so as to present any problem. But, it needs to be reminded that coil Q is definitely subject to its surroundings. Re-tweaking the inductance value is easy enough but you cannot recoup the loss of Q. It always depends on the specific application. -Bill |
| All times are GMT +1. The time now is 07:34 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com