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My subject line is only slightly misleading. I am reporting coil measurements published in 1925, about a decade before what were known as the “Phono Oscillator Rules” were established by the FCC, and many decades before the NBS became the NIST. What makes these measurements related to present-day Part 15 AM is that they were for three different radio coils, each of which had a low-frequency inductance of 291 uH (making the inductance over 300 uH at the operating frequency because of the self-capacitance of the coils), which makes them possible candidates for use as Part 15 AM loading coils. The winding dimensions and wire type of each coil are different, making a comparison between different kinds of coils possible.
That due care was used in the measurements is assured by the fact that they were made by employees of a government agency that exists for the purpose of making accurate measurements. That the measurements were made a long time ago should not detract from their credibility. Very accurate measurements have been made for a long time without the use of modern technology. My favorite example of this fact is the measurement of the conductivity of pure water as 5 uS/m at 25 degrees C in 1894 by Kohlrausch. Kohlrausch’s experiment has not been improved upon in 114 years. The conductivity of very pure distilled water is about 16 times this level because water is in equilibtium with the small amount of carbon dioxide in the air, and this forms carbonic acid in the water, greatly increasing its conductivity. The experimental difficulties of removing the carbon dioxide from the water were enormous, and Kohlrausch’s results have been the standard even to this day. So, it is very possible for experiments made a long time ago to have been accurate.
The data about the coils is contained in Figure 49 in Section 2 of Terman’s Radio Engineers’ Handbook (1943). Figure 49 contains Q measurements of many coils of different inductances reported by several investigators. The data I am reporting in this post is by August Hund and H.B. DeGroot in Bureau of Standards Technical Paper 298. It reports the Q of three single-layer coils with the same inductance, wound with 32/38 Litz wire, # 16 wire, and # 28 wire.
I compared the Qs reported by Hund and DeGroot with calculated Qs using a method in Terman’s Handbook that was developed by Butterworth. Butterworth’s method calculates the skin effect and the proximity effect, but it tends to greatly overestimate the Q, partially because it doesn’t consider the self-capacitance of the coil. Terman’s Handbook does not give a method for determining the self-capacitance of coils. I use a technique developed by R. G. Medhurst in Wireless Engineering, Vol. 24, p. 35, 1947. Medhurst’s method tends to give lower values of self-capacitance than I have measured, but that is because self-capacitance is not a property of only the coil itself, but also the environment in which the coil is located. Because of that, different measurement methods result in different values of self-capacitance. I use Medhurst’s results when comparing actual measurements with calculations, because the Medhurst method is a standard.
The 32/38 Litz wire coil is 3.2 inches in diameter, and has a length/diameter ratio of .73, and 65 turns. At 1.5 MHz, the measured Q is 260 and the calculated Q is 356.
The #16 solid wire coil is 6.4 inches in diameter, and has a length/diameter ratio of .41, and 40 turns. At 1.5 MHz, the measured Q is 150 and the calculated Q is 236.
The # 28 solid wire coil is 3.2 inches in diameter, and has a length/diameter ratio of .39, and 55 turns. At 1.5 MHz, the measured Q is 200, and the calculated Q is 321.
What we see from the above is that neither the measured nor calculated Qs are very high, the measured Qs are lower than the calculated Qs, and the sequence of low, middle and high Q coils is the same whether the Qs are measured or calculated.
An unexpected result is that the Q of the # 28 solid wire coil is higher than for the #16 solid wire coil. This is because the # 16 wire coil is closer wound than the # 28 wire coil, resulting in more of a proximity effect for the # 16 wire coil. Some spacing is needed between turns. As expected, the Litz wire coil has the highest Q.
Terman says that Q is maximum if the length/diameter ratio is about .5, but this rule is not very critical. The coil should just not be very long or very short. The wire diameter should be somewhat less than what is needed to fill the available space. We see this from the results above with the # 16 and # 28 solid wire coils, in which the larger diameter wire gives less Q than the smaller diameter wire. Adding an external tuning capacitor to the coil increases the self-capacitance of the coil itself, reducing Q.
I will be reporting some of my own measurements of various coils and comparing my measurements with the calculated results. I thought it best to precede reports of my own measurements with the published authoritative results in this post.