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[nanovnav2] CMCs - MORE (BAD) INFORMATION
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Dave,
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Have you tried teflon covered silver plated wire? Mike N2MS
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On 1/26/21 3:04 PM, n2msqrp wrote:
Dave,The silver doesn't help much for RF conductivity, but it is a LOT easier to solder to. I'm kind of surprised that magnet wire broke down. Most magnet wire is good for a kilovolt or so, because motors and transformers get hi-pot tested at 2-5kV. Decent insulated magnet wire Catalog here: AWG 12 copper bare is 0.0808 (nom) single build is 0.0825 (i.e. the insulation is 8.5 mils (0.2 mm) thick heavy build is 0.0842 is twice as thick. (0.4mm) most plastics have breakdown strength of 20 kV/mm or more, especially for thin layers. PTFE (Teflon) and Polyimide (Kapton) are a lot better (100 kV/mm) Polyester (Mylar) is in between (50kV/mm) Note that thinner layers have a higher breakdown voltage per mm (that is, you might find that a layer that's 0.1 mm and 1mm have almost the same breakdown voltage) Formvar is about 11kV as typically applied to wire. On 01/26/2021 3:30 PM David Eckhardt <davearea51a@...> wrote: |
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I was also surprised it broke down, but every CMC I wound with solid #12
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enameled wire broke down between 400 and 700 watts. I believe it has a lot to do with the impedances in which the CMCs are installed. My W/C is on 40-meters where I measure 1161 - j1110 at 7.0 MHz. My lowest impedance is at 2.0 MHz and measures 19 - j288. I have not tried or thought of using teflon coated silvered conductor. I need to find it and, if not too expensive, give it a try. I rewound the 400-31 single core with the DavisRF 'antenna' wire with no problems to at least 1.2 kW. It is certainly better at CM impedance than the 43 material especially on 160 and 75 meters but 43 is a bit better on the higher bands - as expected. Out of all the CMCs I've wound and tested, I have three that I can use, all wound with the DavisRF #14 stranded and insulated antenna wire: 1) 240-31, 15 turns, 2) 300-43, 16 turns, and 3) 400-31, 21 turns. Those remaining do not perform well under DM loss (rejection) or phase balance (¦Ð-radians out of phase) and/or amplitude balance on the DM side. I'm going shopping for Teflon coated silvered solid conductor. Dave - W?LEV On Tue, Jan 26, 2021 at 11:26 PM Jim Lux <jim@...> wrote:
On 1/26/21 3:04 PM, n2msqrp wrote:Dave,The silver doesn't help much for RF conductivity, but it is a LOT easier --
*Dave - W?LEV* *Just Let Darwin Work* |
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I have found that using regular magnet wire with teflon tubing slid over
the wire will easily handle 2KW. I have wound a number of 4:1 transformers as well as 49:1 transformers for EFHW using this method. All the balun designs use a similar scheme; most likely due to the cost and difficulty in obtaining silvered wire. Ken WB6MMV On Wed, Jan 27, 2021 at 2:10 PM David Eckhardt <davearea51a@...> wrote: I was also surprised it broke down, but every CMC I wound with solid #12 |
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Because of the temperatures involved in the fabrication process solder would melt on plated stranded wire so teflon wire is either available silver plated or bare copper wire.
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It used to be available for a decent price at ham fests years ago. I don't knoe know since the proce of all surplus wire has risen. Mike N2MS On 01/27/2021 4:09 PM David Eckhardt <davearea51a@...> wrote: I have not tried or thought of using teflon coated silvered conductor. I |
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On 1/27/21 4:58 PM, n2msqrp wrote:
Because of the temperatures involved in the fabrication process solder would melt on plated stranded wire so teflon wire is either available silver plated or bare copper wire.$650/for 100ft from Digikey - Alpha 5859/12 WH005 MIL-W-16878/4 Type E $434/100ft for AT121925 WH005 (19/25 strand copper silver coated) you could probably find a less expensive insulation that works as well, but "dollars/ft" is what you're looking at. I'll bet a lot of the cost is NOT the copper cost.. So running a 4 wire twist (2 in parallel) might actually be cheaper. That would be AWG 15 AWG16 is about 129/100ft and TFE (not PTFE) insulation, still 600V, still good to 200C. and you can buy these things by the foot (even from Digikey). On 01/27/2021 4:09 PM David Eckhardt <davearea51a@...> wrote: |
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I found silver coated PTFE insulated stranded wire. It's a bit pricey, but
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not out of reach. The huge CMC I had decades ago was wound with #12 solid conductor. I had slipped PTFE tubing over it before I wound it on the toroid. I no longer have that tubing and need to find a source, but I'm now running low on #12 solid wire. So, I may order some of the #12 Flex-weave antenna wire from DavisRF. I know the #14 of similar design does OK at power. Ideally, I'd rewind several of the cores that had my enameled #12 wire with #12 solid conductor with PTFE tubing slipped over the wire. I'll make my decision tomorrow......... Dave - W?LEV On Wed, Jan 27, 2021 at 9:25 PM Doc <kjenkins5623@...> wrote:
I have found that using regular magnet wire with teflon tubing slid over --
*Dave - W?LEV* *Just Let Darwin Work* |
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PTFE tubing from AliExpress:
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$4 for 10m, ID 0.7mm OD 1.3mm will be ~100¦¸ for a parallel pair. On Thu, 28 Jan 2021 at 02:12, Jim Lux <jim@...> wrote:
On 1/27/21 4:58 PM, n2msqrp wrote:Because of the temperatures involved in the fabrication process solderwould melt on plated stranded wire so teflon wire is either available |
My W/C is on 40-meters where I measure 1161 - j1110 at 7.0 MHz.700W on 1161? is a current of 0.776A. The impedance magnitude is 1606?, and so the voltage is 1246V RMS, which is 1762V peak. At 1500W it would be about 50% higher. That's what you have at the input of the CMC, which is where I understand you are measuring. Along the length of the winding the voltage could get higher or lower. By Murphy's Law it will get higher, of course... Even if the insulation of the magnet wire could take a million volt, it wouldn't be enough. This is because of Corona Effect. Consider a cut view through your bifiliar line: Two round copper wires, with a thin film of insulation around each, in direct contact, and with a wedge of air penetrating between the wires on each side. The problem is this wedge of air. With the insulation material having a dielectric constant of about 4 times that of air, the electric field gradient in the insulation is about 4 times lower than in the air. So, at a place where the air wedge has a suitable thickness, most of the voltage will appear across the air, rather than across the insulation thickness. Now it happens that air won't break down below roughly 300V, no matter how thin the air layer is. But at voltages above that level it can break down. When the air breaks down, microscopic arcing happens, and most of the voltage moves into the insulation layers, limiting the arcing. But it is there, right at the surface of the insulation, and will erode the insulation over time. Sooner or later it will fail. Depending on how strong the effect it, it might fail after several weeks, or after several milliseconds, or anywhere in between. So a magnet wire pair in direct contact can certainly work at somewhat more than 300V, because some of the voltage is dropping in the insulation, but not a huge lot more. Even if it has a very good insulation. The insulation must be thick enough, and its dielectric constant low enough, to never exceed the breakdown voltage of the air wedge. With insulated wires in direct contact, that's about 300V in the air, and if there is a significant spacing it gets higher. With enough spacing, of course, the air alone provides enough insulation. A better parallel transmission line would have the dielectric material molded around both conductors, filling the space between them, eliminating the air wedges. A thicker insulation helps a lot, and an insulation having the lowest possible dielectric constant also helps. This is why teflon is so good: It does have a low dielectric constant, so more of the field appears in the teflon and less in the air. An alternative is inserting a magnet-wire-wound CMC in a can of some suitable oil, and make sure no tiny air bubbles remain lodged between the wires. The downside of oil, of course, is that it increases the stray capacitance... My lowest impedance is at 2.0 MHz and measures 19 - j2881500W on 19? is 8.89A. The magnitude of that impedance is 288.6?. So the voltage is 2566V RMS, or 3629V peak. And very likely you could have even higher voltage spots along the line in your CMC! You could measure the impedances with two different line lengths, both lines being of the same construction, for example using two of your CMCs having very different winding length. Then you can calculate what the highest voltage will be. But even without doing this, it's crystal clear that a CMC used on several bands in a high-SWR line, tuned/matched between the CMC and the transceiver, needs to be able to handle really high voltage, and pretty high current. Manfred |
BryonB
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