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#11
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Coil Dope
JB wrote:
I seem to recall reading that Styrene had the best dielectric properties of the plastics. I did some similar experiments building 1/4 wave spikes in a BNC connector for 2m HTs and found that glues would heat up from the RF. Clear epoxy worked best and JB weld, Pipe dope, Silicone sealers and Hot glue were poor performers. I still have some fiberglass tape that I use for torroids. Of common plastics, Teflon, polyethylene, styrene, and polypropylene have the lowest RF loss. However, others can be perfectly adequate for some applications. Roy Lewallen, W7EL |
#12
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Coil Dope
"Roy Lewallen" wrote in message
treetonline... JB wrote: I seem to recall reading that Styrene had the best dielectric properties of the plastics. I did some similar experiments building 1/4 wave spikes in a BNC connector for 2m HTs and found that glues would heat up from the RF. Clear epoxy worked best and JB weld, Pipe dope, Silicone sealers and Hot glue were poor performers. I still have some fiberglass tape that I use for torroids. Of common plastics, Teflon, polyethylene, styrene, and polypropylene have the lowest RF loss. However, others can be perfectly adequate for some applications. Roy Lewallen, W7EL Actually made good working ladder line from bell wire from the Science dept. and plastic spoons from the lunchroom for the school radio club station. The ugliest, goofiest looking projects always seem to work the best. |
#13
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Coil Dope
Having read Roy's impressive measurement results, I would like to
interject a word of caution about the use of RTV's in electronic applications. As Roy states, many RTV sealants and adhesives use a curing process that releases acetic acid (vinegar). These should be used with care in electronic applications as the acetic acid can, and does, cause corrosion on metallic surfaces in the vicinity of the curing RTV. Coils constructed with a medium to heavy gauge of wire are unlikely to suffer from this but any inductors employing fine gauge wire can fail due to corroded (broken) turns resulting from contact with the acetic acid. The effect is not immediately apparent and can take months or years before a problem arises. Application of acid curing RTV should also be avoided to any component in an assembled electronic device that has switch contacts or plug and socket contacts in the immediate vicinity, for the same reason. Electronic grade RTVs do not use an acid curing process and have been developed to avoid the above problems. Derek (ex Dow Corning employee) In message tonline, Roy Lewallen writes Dr. Barry L. Ornitz wrote: . . . Roy Lewallen, W7EL, dipped a number of coils in various materials (RTV silicone, epoxy cement, Q-dope, hot melt adhesive, etc.) a while back and then measured their losses with a Q-meter. If Roy can find his old article, perhaps he can post it again. Great memory, Barry! It was posted on Dec. 16, 1998. A copy of the original posting follows. I didn't do any other experiments as I said I would, and I've gotten very little confirming or contradictory feedback. I recall you and some other folks posting some very good information about RTV, the general thrust of which was that there's a very wide range of formulations, so results might vary a lot from what I measured. --------------- Spurred by recent comments and questions on this newsgroup, I made some inductors last weekend and measured them. Results follow. Toroidal Inductor Measurements Roy Lewallen, W7EL December, 1998 Test Equipment Inductance: GR 1606-A impedance meter. Stray series impedance was removed by initial calibration. Stray shunt capacitance was separately measured and removed mathematically. Repeatability is within about +/- 2%. No attempt was made to establish accuracy. Q: A home-made fixture was used. This is simply an air-variable capacitor which the inductor is connected across. Coupling into and out of the fixture with signal generator and oscilloscope is done with very small capacitors. The Q is calculated from the center frequency and 3 dB bandwidth. Use of frequency counter, built-in oscilloscope voltmeter, and switchable 3 dB pad allow repeatability of about +/- 2%. Checks against a commercial Q meter show agreement within a few percent. All measurements were made at 10 MHz. Experiment 1: Coatings Several identical inductors were fabricated, then coated with various compounds. Inductance and Q were measured before and after coating. The inductors were wound on Micrometals T-50-6 cores (Carbonyl SF material, relative permeability 8.5) with 25 turns of #22 wire. This just fits on a single layer. Ind. # Uncoated Coated Coating L(uH) Q L(uH) Q 1 2.54 281 2.54 284 None (control) 2 2.52 286 2.54 218 Duco cement 3 2.51 267 2.52 265 Standard RTV 4 2.52 280 5 2.54 279 2.55 255 Clear hot melt glue 6 2.48 268 2.55 164 "Sealing tape" 7 2.51 283 2.51 262 Paraffin 8 2.55 262 9 2.56 281 2.59 278 GE Silicone II 10 2.50 283 11 2.51 277 Notes (by inductor number): Unless otherwise noted, coatings covered the entire core and winding, extending more than a wire diameter beyond the outside of the wire, and the center of the core was filled. Inductances within about 0.3 uH and Q's within less than about 5 should be considered equal. 1. Inductor Q was measured several times over several days to establish repeatability. Results were 280, 281, 283, 286, 284. (The apparent trend is interesting, but not conclusive.) 2. Devcon Duco(R) Cement, allowed to dry for 24 hours. Although a generous coating was applied, the dried coating was less than a wire diameter in thickness, and the center of the core wasn't filled. 3. Dap Dow Corning 100% Silicone Sealant, Clear, allowed to cure for 24 hours. This is standard acetic-acid (vinegar) curing RTV. 5. Stanley All Purpose GlueSticks, claimed set time 25-30 seconds. These are nearly clear, translucent white, and not tan or brown colored. 6. This was some stuff I got surplus. It's a black, sticky, rubbery compound something like Coax Seal, but may be of entirely different composition. It's in the form of a thick tape. It's soluble in naphtha (and the solution dyes everything black it gets on), but acetone doesn't touch it. 7. Household canning paraffin. (I don't know what it's called in Britain, but I'm not referring to the liquid -- kerosene to us -- you call paraffin. This is a common wax made from petroleum.) 9. GE Silicone II Household Glue & Seal, Clear. This is a non-acetic-acid curing RTV. Allowed to cure for two days. Still just a little soft even after this much curing. 11. This is the same core as #6. After the coated #6 was measured, the core was cleaned, the winding cut off, the core further cleaned, then a new winding put on. Comments: I had made some measurements years ago, but couldn't locate the results. I recall that standard RTV was poor (lowered Q noticeably) but that an industrial non-acetic-acid curing RTV was good. The results with standard RTV in this test were striking. Either a) the formulation of standard RTV has changed over the years, b) there are major differences among brands, or c) my memory is faulty. I recall from my earlier tests that epoxy was quite poor, but this has to be qualified after the experience with RTV. There's a huge number of different types of epoxy, and some may be much worse than others. I might test some in the future, but didn't during this test. I intend to test Q-dope in the future, but didn't have any on hand. Conclusions: Of the materials tested, both types of RTV stand out as having a negligible effect on inductor Q. Hot melt glue and paraffin have a small enough effect that they should be tolerable for many applications. Duco cement seriously degrades Q, even in a much thinner layer than the other coatings. The "sealing tape", tested out of curiosity, shows just how great a degradation can be caused by a poor coating. None of the coatings made much of a change in apparent inductance. This implies that the reduction in Q is due primarily to dielectric loss rather than simple increase in capacitance due to the material's dielectric constant. Note the difference in inductance and Q between inductors 6 and 11, which were wound on the same core. Apparently physical differences in the windings (perhaps such as tightness and conformance to the core, or uniformity of turn spacing) are a major contributor to differences between inductors. All the cores used in the test were ordered at the same time, so they may have come from the same batch and have relatively little variation. On the basis of just the comparison between numbers 6 and 11, it's entirely possible that most of the variation between inductors in this experiment is due to winding differences. The variation might be less if smaller wire with less stiffness is used. Experiment 2: Turn Spacing I believe it's well established that even a partial second layer can greatly reduce the Q of a toroidal inductor. But I had recently heard that optimum Q is achieved when the first layer isn't quite full, but rather has about a 30 degree gap in the winding, to reduce the capacitance between winding ends. To test this, I wound 23 turns of #22 wire on a T-50-6 core, and measured the Q with the turns pushed close together to make a gap (of about 30 degrees), and then spread to evenly distribute the turns completely around the core. Measured 10 MHz Q's were 272 and 284, respectively. (Inductances were 2.21 and 2.11 uH.) This one test doesn't by any means exhaust all the possibilities of core geometries, permeabilities, number of turns, and wire size, all of which may play a role. But if there's any advantage to leaving a gap, I believe it would be a small one. And in at least one case, it's slightly better not to. Another test was run using 10 turns of the same size wire on the same core. With the turns pushed together (the winding covering less than half the core), Q was 213, L was 841 nH. With the turns spread evenly around the core, Q was 209 but the L had dropped to 505 nH. I reasoned that a more fair comparison would be with a winding of about the same inductance as the original close-spaced one. This required 15 turns when distributed around the core, and resulted in a Q of 257 and L of 924 uH. Here again, the Q is best when the turns are evenly distributed. Note that type 6 powdered iron has a very low relative permeability (8.5), so results might be different with higher-permeability materials. However, this is the material I usually use for high-Q inductors at HF, so I'm most interested in how it's affected. Experiment 3: "Regressive" Winding In the past, I've found what I thought was a moderate improvement in Q by "regressively" winding an inductor. To do this, you wind half the turns in the normal manner. Then you pass the wire through the hole, but to the opposite side of the inductor (with it ending up beside the first turn), then completing the winding from the vicinity of the first turn back toward the origination of the crossover. The result is an inductor with the two leads coming from points directly opposite each other. Stray capacitance is allegedly reduced by keeping the ends of the winding apart. An inductor wound in this manner with 25 turns of #22 wire measured Q = 285, L = 2.48 uH. This Q is on the high side, and the L on the low side, of the uncoated inductors measured in Experiment 1. I didn't try comparing with a standard winding on the same core, since the same number of turns wound on the same core at different times (e.g., inductors 6 and 11 in Experiment 1) were shown to come out differently from each other. My conclusion is that any Q improvement due to "regressive" winding is slight. Another claim for "regressive" winding is that it eliminates the "single turn" effect of toroidal inductors. A normal toroid will couple into its surroundings as though it consists of a single turn the size of the core. In the "regressively" wound inductor, there are two half-turns in opposite directions, so coupling should be reduced. I haven't tested this in any way, but it may be an argument in favor of the method. The relatively long wire of the crossover turn would contribute to coupling, however. I'll undertake more experiments and measurements as time permits. I'd love to hear from anyone who has made either supporting or contradictory quantitative measurements. Roy Lewallen, W7EL -- Derek R. Smith. |
#14
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Coil Dope
I have seen hams use silicon sealant to fill a UHF connector before
screwing it together. The idea is to keep out water. (The connection is also taped or sealed with Coax seal). Is this a good idea? Bob W8ERD |
#15
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Coil Dope
JB wrote:
Actually made good working ladder line from bell wire from the Science dept. and plastic spoons from the lunchroom for the school radio club station. The ugliest, goofiest looking projects always seem to work the best. Sweet! Any pictures? - 73 de Mike N3LI - |
#16
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Coil Dope
"Bob Dixon" wrote in message
... I have seen hams use silicon sealant to fill a UHF connector before screwing it together. The idea is to keep out water. (The connection is also taped or sealed with Coax seal). Is this a good idea? Bob W8ERD NO! The silicone will impair the connection, the dielectric, makes a fumbling mess and usually causes the tape to come off. Just tightly wrap with electrical tape and tie off the loose end. It will outlast the coax. Unless the antenna itself is prone to leak into the connector. |
#17
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Coil Dope
Probably not a good idea.
Silicone Grease would be much less harmful to the connector and be much easier to undo, when the time comes. Derek In message , Bob Dixon writes I have seen hams use silicon sealant to fill a UHF connector before screwing it together. The idea is to keep out water. (The connection is also taped or sealed with Coax seal). Is this a good idea? Bob W8ERD -- Derek R. Smith. |
#18
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Coil Dope
On Feb 11, 12:40*pm, Bob Dixon wrote:
I have seen hams use silicon sealant to fill a UHF connector before screwing it together. The idea is to keep out water. (The connection is also taped or sealed with Coax seal). Is this a good idea? Bob W8ERD I like to grease the threads with silicon dielectric grease to keep water from wicking up the threads but I have seen people pack N connectors full of it before screwing them together and cant say it did any harm. I am am a big fan of wiping down a connector with Deoxit before puting it together. I heard that Marvel Mystery Oil works as well as Deoxit. Has anyone else ever heard of this? Jimmie |
#19
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Coil Dope
On Feb 11, 7:04*am, "Derek R. Smith"
wrote: Having read Roy's impressive measurement results, I would like to interject a word of caution about the use of RTV's in electronic applications. As Roy states, many RTV sealants and adhesives use a curing process that releases acetic acid (vinegar). These should be used with care in electronic applications as the acetic acid can, and does, cause corrosion on metallic surfaces in the vicinity of the curing RTV. Coils constructed with a medium to heavy gauge of wire are unlikely to suffer from this but any inductors employing fine gauge wire can fail due to corroded (broken) turns resulting from contact with the acetic acid. The effect is not immediately apparent and can take months or years before a problem arises. Application of acid curing RTV should also be avoided to any component in an assembled electronic device that has switch contacts or plug and socket contacts in the immediate vicinity, for the same reason. Electronic grade RTVs do not use an acid curing process and have been developed to avoid the above problems. Derek (ex Dow Corning employee) In message tonline, Roy Lewallen writes Dr. Barry L. Ornitz wrote: . . . Roy Lewallen, W7EL, dipped a number of coils in various materials (RTV *silicone, epoxy cement, Q-dope, hot melt adhesive, etc.) a while back *and then measured their losses with a Q-meter. *If Roy can find his old *article, perhaps he can post it again. Great memory, Barry! It was posted on Dec. 16, 1998. A copy of the original posting follows. I didn't do any other experiments as I said I would, and I've gotten very little confirming or contradictory feedback. I recall you and some other folks posting some very good information about RTV, the general thrust of which was that there's a very wide range of formulations, so results might vary a lot from what I measured. --------------- Spurred by recent comments and questions on this newsgroup, I made some inductors last weekend and measured them. Results follow. * * * * * *Toroidal Inductor Measurements * * * * * * * * *Roy Lewallen, W7EL * * * * * * * * * *December, 1998 * * * * * * * * * *Test Equipment Inductance: GR 1606-A impedance meter. Stray series impedance was removed by initial calibration. Stray shunt capacitance was separately measured and removed mathematically. Repeatability is within about +/- 2%. No attempt was made to establish accuracy. Q: A home-made fixture was used. This is simply an air-variable capacitor which the inductor is connected across. Coupling into and out of the fixture with signal generator and oscilloscope is done with very small capacitors. The Q is calculated from the center frequency and 3 dB bandwidth. Use of frequency counter, built-in oscilloscope voltmeter, and switchable 3 dB pad allow repeatability of about +/- 2%. Checks against a commercial Q meter show agreement within a few percent. All measurements were made at 10 MHz. * * * * * * * Experiment 1: Coatings Several identical inductors were fabricated, then coated with various compounds. Inductance and Q were measured before and after coating. The inductors were wound on Micrometals T-50-6 cores (Carbonyl SF material, relative permeability 8.5) with 25 turns of #22 wire. This just fits on a single layer. Ind. # * Uncoated * * * *Coated * * * * Coating * * * L(uH) * Q * * * L(uH) * Q 1 * * *2.54 * *281 * * 2.54 * *284 * * None (control) 2 * * *2.52 * *286 * * 2.54 * *218 * * Duco cement 3 * * *2.51 * *267 * * 2.52 * *265 * * Standard RTV 4 * * *2.52 * *280 5 * * *2.54 * *279 * * 2.55 * *255 * * Clear hot melt glue 6 * * *2.48 * *268 * * 2.55 * *164 * * "Sealing tape" 7 * * *2.51 * *283 * * 2.51 * *262 * * Paraffin 8 * * *2.55 * *262 9 * * *2.56 * *281 * * 2.59 * *278 * * GE Silicone II 10 * * 2.50 * *283 11 * * 2.51 * *277 Notes (by inductor number): Unless otherwise noted, coatings covered the entire core and winding, extending more than a wire diameter beyond the outside of the wire, and the center of the core was filled. Inductances within about 0.3 uH and Q's within less than about 5 should be considered equal. 1. Inductor Q was measured several times over several days to establish repeatability. Results were 280, 281, 283, 286, 284. (The apparent trend is interesting, but not conclusive.) 2. Devcon Duco(R) Cement, allowed to dry for 24 hours. Although a generous coating was applied, the dried coating was less than a wire diameter in thickness, and the center of the core wasn't filled. 3. Dap Dow Corning 100% Silicone Sealant, Clear, allowed to cure for 24 hours. This is standard acetic-acid (vinegar) curing RTV. 5. Stanley All Purpose GlueSticks, claimed set time 25-30 seconds. These are nearly clear, translucent white, and not tan or brown colored. 6. This was some stuff I got surplus. It's a black, sticky, rubbery compound something like Coax Seal, but may be of entirely different composition. It's in the form of a thick tape. It's soluble in naphtha (and the solution dyes everything black it gets on), but acetone doesn't touch it. 7. Household canning paraffin. (I don't know what it's called in Britain, but I'm not referring to the liquid -- kerosene to us -- you call paraffin. This is a common wax made from petroleum.) 9. GE Silicone II Household Glue & Seal, Clear. This is a non-acetic-acid curing RTV. Allowed to cure for two days. Still just a little soft even after this much curing. 11. This is the same core as #6. After the coated #6 was measured, the core was cleaned, the winding cut off, the core further cleaned, then a new winding put on. Comments: I had made some measurements years ago, but couldn't locate the results. I recall that standard RTV was poor (lowered Q noticeably) but that an industrial non-acetic-acid curing RTV was good. The results with standard RTV in this test were striking. Either a) the formulation of standard RTV has changed over the years, b) there are major differences among brands, or c) my memory is faulty. I recall from my earlier tests that epoxy was quite poor, but this has to be qualified after the experience with RTV. There's a huge number of different types of epoxy, and some may be much worse than others. I might test some in the future, but didn't during this test. I intend to test Q-dope in the future, but didn't have any on hand. Conclusions: Of the materials tested, both types of RTV stand out as having a negligible effect on inductor Q. Hot melt glue and paraffin have a small enough effect that they should be tolerable for many applications. Duco cement seriously degrades Q, even in a much thinner layer than the other coatings. The "sealing tape", tested out of curiosity, shows just how great a degradation can be caused by a poor coating. None of the coatings made much of a change in apparent inductance. This implies that the reduction in Q is due primarily to dielectric loss rather than simple increase in capacitance due to the material's dielectric constant. Note the difference in inductance and Q between inductors 6 and 11, which were wound on the same core. Apparently physical differences in the windings (perhaps such as tightness and conformance to the core, or uniformity of turn spacing) are a major contributor to differences between inductors. All the cores used in the test were ordered at the same time, so they may have come from the same batch and have relatively little variation. On the basis of just the comparison between numbers 6 and 11, it's entirely possible that most of the variation between inductors in this experiment is due to winding differences. The variation might be less if smaller wire with less stiffness is used. * * * * * * * Experiment 2: Turn Spacing I believe it's well established that even a partial second layer can greatly reduce the Q of a toroidal inductor. But I had recently heard that optimum Q is achieved when the first layer isn't quite full, but rather has about a 30 degree gap in the winding, to reduce the capacitance between winding ends. To test this, I wound 23 turns of #22 wire on a T-50-6 core, and measured the Q with the turns pushed close together to make a gap (of about 30 degrees), and then spread to evenly distribute the turns completely around the core. Measured 10 MHz Q's were 272 and 284, respectively. (Inductances were 2.21 and 2.11 uH.) This one test doesn't by any means exhaust all the possibilities of core geometries, permeabilities, number of turns, and wire size, all of which may play a role. But if there's any advantage to leaving a gap, I believe it would be a small one. And in at least one case, it's slightly better not to. Another test was run using 10 turns of the same size wire on the same core. With the turns pushed together (the winding covering less than half the core), Q was 213, L was 841 nH. With the turns spread evenly around the core, Q was 209 but the L had dropped to 505 nH. I reasoned that a more fair comparison would be with a winding of about the same inductance as the original close-spaced one. This required 15 turns when distributed around the core, and resulted in a Q of 257 and L of 924 uH. Here again, the Q is best when the turns are evenly distributed. Note that type 6 powdered iron has a very low relative permeability (8.5), so results might be different with higher-permeability materials. However, this is the material I usually use for high-Q inductors at HF, so I'm most interested in how it's affected. * * * * * * * Experiment 3: "Regressive" Winding In the past, I've found what I thought was a moderate improvement in Q by "regressively" winding an inductor. To do this, you wind half the turns in the normal manner. Then you pass the wire through the hole, but to the opposite side of the ... read more »- Hide quoted text - - Show quoted text - Ive see the center conductor of RG214 dissolved through with RTV. The corrosion on the associated Al/Cu joint was likewise disgusting. Jimmie |
#20
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Coil Dope
JIMMIE wrote:
Ive see the center conductor of RG214 dissolved through with RTV. The corrosion on the associated Al/Cu joint was likewise disgusting. Jimmie And I've used the acetic acid-curing variety many times on bare copper (wire and PC board traces) and aluminum, and not a hint of corrosion when it was cut off long after. Besides the wide variety of formulations, there may be other factors at work causing the corrosiveness to vary so much. Roy Lewallen, W7EL |
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