To start Ebay has plenty, look for mak1939. He also carries the R40C1 toroids and rods. You can easily get Qs over 1000 with his toroid and litz. Here is mak's Ebay site, The second thing to try is spacing your wire one wire width apart, that will increase your Q. I have 4 inch diameter coil 4-7/8 inch long using # 18 wire and spaced one wire width apart that has a Q of 410 at 1Mhz. I also have a coil on a 6" styrene pipe coupler using 660/46 litz wire at 10 turns per inch. It has a peak Q of 1450 at 800kHz and 1400 at 1Mhz. This was measured with a Boonton 260 Q meter with an external meter and using the 3 db method. But, admittedly high Q measurements are finicky, changing orientation affects Q and an am radio signal near your test frequency will affect it and getting within 2 ft will lower Q. You might like this where I tested multiple 6" styrene core coils with wire spaced between 7TPI and 12TPI to see where the highest Q was. ?????????????????????? Mikek On 10/20/2021 10:17 AM, Andrew Kurtz via groups.io wrote: Thanks, Manfred, this is incredibly helpful to a totally novice crystal radio builder! I see that low permeability is best for my application, which greatly reduces the ¡°advantage¡± of needing very few turns compared to an air core. As a hobbyist, none of the factors important to a design engineer matter to me except my single goal: get the highest Q possible in my AM tuning circuit so I can get selectivity and separate the various stations that overlap. My current best is 41 turns on a Clorox bottle with Q = 130 or so. I believe the only way to improve this would probably (?) not involve an iron or ferrite core, but would use litz wire. Litz wire seems absolutely unavailable! Anyone know where to find some?
Andy
On Oct 20, 2021, at 11:03 AM, Manfred Mornhinweg <manfred@...> wrote:
Engineering is the art of finding workable compromises. Let's leave perfection to the philosophers.
When a electronic engineer needs an inductor in a circuit, it would be really rare that the requirements are: "Absolutely possible highest Q, and all other factors are irrelevant". Usually the situation is much harder: The name of the game is finding the most convenient way to implement that inductor, balancing all the many requirements: Inductance, Q (or loss factor, more commonly used by engineers), AC current handling, DC current handling, size, shape, weight, cost, ease of manufacturing, availability, accuracy, stability (relative to temperature, aging, etc), stray fields (self-shielding), and several more. The Q very often doesn't need to be as high as possible. It just needs to be high enough! For example, if one makes a lowpass filter for a low-power application, that works at a loaded Q of unity, the difference in performance between using very poor coils having a Q of 30, or very good ones having a Q of 300, is completely irrelevant! Current handling also doesn't need to be high, stability might not need to be great, and so on. So there is a lot of room to optimize those coils for size and cost, rather than for Q. Instead when building a lowpass filter for a high power transmitter, Q becomes important simply because it will define how much power the coil will have to dissipate as heat. Also AC current handling will need to be high. Stability and accuracy still don't need to be brilliant. So the engineer will choose very different coils here, large enough and and with high enough Q, that they won't overheat and won't run into nonlinearity problems. They will be much larger and much more expensive.
But in either case, a coil wound on a magnetic core will very often be a better choice than one having an air coil. The engineer has the freedom to choose the best option.
Why a coil on a core is often better: It tends to be much smaller. Often it's lighter too. The Q in some cases might actually be better than for an air core coil. This happens when the savings in copper loss, gained by shrinking the coil, are greater than the core losses introduced.
Of course there are cases where cores are a bad idea. For example, when someone is building a VFO. An extremely important requirement here is stability, and most core materials result in poorer stability than air-cored coils. Q is also important, because it has a large effect on the phase noise produced by the VFO. So one would typically choose an air cored coil, and make it pretty large, and wind it on the most stable former one can find. But if space is restricted, a coil wound on one of the more stable powdered iron material toroids might work better than a miniaturized air coil.
Using a mystery toroid and measuring very poor performance at a given frequency, is no reason to declare that all toroidal cores are bad, and that air-core coils are generally and always better! There are hundreds, if not thousands of different core materials available. Each of those has its own characteristics, and they include a two-dimensional permeability versus frequency curve, which tells the frequency range in which that material provides high Q, among other things.
In the example given many posts ago, my calculations end up with that mystery toroid having a permeability of roughly 6000. So it must be one of those extra high permeability ferrites, used mainly for noise suppression in EMI filtering chokes. This material can give good Q only on low frequencies, in the audio range. In fact, only in the lower audio range!
If anyone wants to try building ferrite-cored coils that beat air core coils in Q, in the megahertz range, I suggest trying toroids made from the lowest permeability grades of ferrite available. They commonly have either a permability of either 20 or 40. A fair comparison might be to take such a toroid, wind 15 turns of very thick wire on it, measure what inductance it gives, then wind an air coil of about the same size, designed to have the same inductance (many more turns, necessarily much thinner wire). Then compare the Q curves over frequency for both coils, and see which wins in which frequency range. I would not be surprised if there is a wide range in which the ferrite-cored coil is better.
Other fair comparisons could be between such a ferrite coil, and an air coil weighing the same, even if it's larger, or costing the same. In those cases air coils might end up better positioned. And for homebrewers, a very important point influencing decisions is that ferrite cores of specific materials need to be bought, usually online, with a waiting time. It's often far more attractive to wind an air coil, and be done with it! And cheaper too.
But measuring a ferrite outside its intended operation frequency range is unfair! The trade union of ferrite cores will bitterly complain, and mount a riot! :-)
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