I do wonder about the effects of moisture and UV light on the RF characteristics of PVC pipe. Could this change the inductance or degrade the Q of coils wound on this material?
On another subject, relating to the use of contra-wound coils, check out this truly amazing 1-transistor reflex radio called the Macrohenrydyne built by Tom Polk: www.tompolk.com/radios/macrohenrydyne/macrohenrydyne.html
Be sure to listen to the audio recordings made with this set! Amazing!
This fellow is an overachiever. He also designed a specialized Theremin, and he is an avid collector of antique radios. Spend some time poking around his web site, and you will be well rewarded.
WEAK-AM
Classical Music and More!
I do wonder about the effects of moisture and UV light on the RF characteristics of PVC pipe. Could this change the inductance or degrade the Q of coils wound on this material?
On another subject, relating to the use of contra-wound coils, check out this truly amazing 1-transistor reflex radio called the Macrohenrydyne built by Tom Polk: www.tompolk.com/radios/macrohenrydyne/macrohenrydyne.html
Be sure to listen to the audio recordings made with this set! Amazing!
This fellow is an overachiever. He also designed a specialized Theremin, and he is an avid collector of antique radios. Spend some time poking around his web site, and you will be well rewarded.
WEAK-AM
Classical Music and More!
Tongue only mentions higher dielectric loss with PVC, causing lower Q, compared to when styrene is used. Since Tongue is concerned only with the design of crystal radios, he does not mention the results of exposure to the elements (UV and moisture). This should produce a further increase in dielectric loss.
In principle, dielectric loss should cause a small reduction in inductance because a lossy coil form behaves like a shorted turn closely coupled to the coil. But the resistance of the lossy coil form would be very high, so I would expect the inductance reduction to be nearly undetectable.
Tongue only mentions higher dielectric loss with PVC, causing lower Q, compared to when styrene is used. Since Tongue is concerned only with the design of crystal radios, he does not mention the results of exposure to the elements (UV and moisture). This should produce a further increase in dielectric loss.
In principle, dielectric loss should cause a small reduction in inductance because a lossy coil form behaves like a shorted turn closely coupled to the coil. But the resistance of the lossy coil form would be very high, so I would expect the inductance reduction to be nearly undetectable.
I've recently experimented with loading coils for use with an 8 foot , 1/2 inch diameter copper pipe on a roof mounted mast.
After trying various ferrite core loading coils vs. air core coils, I believe I'm going to settle on a space wound air coil (1/2-1 wire diameter spacing) on a polypropylene jar. Using a field strength meter, the space wound air coil has the edge over the ferrite coils. The range difference of the spaced air coil over a similar tight wound coil on a PVC form is significant. Once the geometry is set, I plan to replace the magnet wire with Litz.
I've not seen much discussion anywhere of using Litz for loading coils. But there's been plenty of experimentation and discussion of Litz coils. In an unloaded coil, properly configured Litz can double or triple the Q over magnet wire. Q around 1500 has recently been achieved for MW frequencies using Litz and ferrite: http://www.midnightscience.com/rapntap/topic.asp?whichpage=8&pagesize=15&forum_title=Crystal+Radio+Think+Tank+%2A%2A+Advanced+Forum&topic_title=Air+coils+enhanced+by+ferrite+%2E&forum_id=10&topic_id=4755
Although the contrawound coil is perfect for my Macrohenrydyne radio described above, at this time, I don't see much benefit to a contrawound loading coil.
Macrohenry
I've recently experimented with loading coils for use with an 8 foot , 1/2 inch diameter copper pipe on a roof mounted mast.
After trying various ferrite core loading coils vs. air core coils, I believe I'm going to settle on a space wound air coil (1/2-1 wire diameter spacing) on a polypropylene jar. Using a field strength meter, the space wound air coil has the edge over the ferrite coils. The range difference of the spaced air coil over a similar tight wound coil on a PVC form is significant. Once the geometry is set, I plan to replace the magnet wire with Litz.
I've not seen much discussion anywhere of using Litz for loading coils. But there's been plenty of experimentation and discussion of Litz coils. In an unloaded coil, properly configured Litz can double or triple the Q over magnet wire. Q around 1500 has recently been achieved for MW frequencies using Litz and ferrite: http://www.midnightscience.com/rapntap/topic.asp?whichpage=8&pagesize=15&forum_title=Crystal+Radio+Think+Tank+%2A%2A+Advanced+Forum&topic_title=Air+coils+enhanced+by+ferrite+%2E&forum_id=10&topic_id=4755
Although the contrawound coil is perfect for my Macrohenrydyne radio described above, at this time, I don't see much benefit to a contrawound loading coil.
Macrohenry
This is a very nicely presented study of a mission critical aspect of our hobby - very, very nice work.
Some of us really do enjoy trying to squeeze that extra nano-watt of performance out any way we can. Your study will provide a great jumping off point for further experimentation and covers a topic many of us need help to master.
Experimental broadcasting for a better tomorrow!
This is a very nicely presented study of a mission critical aspect of our hobby - very, very nice work.
Some of us really do enjoy trying to squeeze that extra nano-watt of performance out any way we can. Your study will provide a great jumping off point for further experimentation and covers a topic many of us need help to master.
Experimental broadcasting for a better tomorrow!
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.
53 turns of 54/44 Litz wire were close-wound on a styrofoam cylinder. The coil diameter is 2.88 inches and the length/diameter ratio is .434. The low-frequency inductance is 242 uH. The inductance at 1680 kHz is 299 uH. The measured self-capacitance is 7 pF. The measured Q is 382, and the equivalent series resistance of the coil is 8.3 ohms.
The self-capacitance calculated by the Medhurst method is 3.8 pF. Combining the Medhurst and Butterworth methods gives a calculated Q of 558.
I performed an experiment to try to improve the Q of the close-wound loading coil wound with Litz wire. I used a twisted pair of Litz wires connected together at the ends. The idea was to reduce the proximity effect by reducing the magnetic coupling between adjacent turns. Unfortunately, there would still be a proximity effect between the two wires of the twisted pair itself. I did not know what would happen, because I have not seen this configuration described before. I am not able to calculate a Q because Butterworth does not have formulas for a winding made up of a twisted pair of Litz wires. More recently, formulas have been developed for calculating the Q of "bundled" insulated solid wires, which are several insulated wires twisted together. Bundled wires are more uniform than Litz wires, and, therefore, different formulas apply. I have not been able to find formulas related to bundled Litz wires.
For the twisted pair of Litz wire coil, the-low frequency inductance is 273 uH. The diameter of the 49 turn winding on a styrofoam cylinder is 4.5 inches and the length is 4.12 inches. The inductance at 1610 kHz is 325 uH, and the Q is 436. This is slightly higher than the Q of 382 obtained with the Litz wire coil without the twisted pair winding. The effective series resistance of the coil is 7.6 ohms.
The measured self-capacitance is 5.7 pF. The length/diameter ratio is .916. The self-capacitance obtained with the Medhurst method is 5.1 pF. As I already mentioned, the Butterworth method cannot be used to calculate the Q of this type of coil.
Although the Q improvement is only slight, it might be worth using a twisted pair of Litz wire, because the highest obtainable Q for the loading coil should be used.
I performed an experiment to try to improve the Q of the close-wound loading coil wound with Litz wire. I used a twisted pair of Litz wires connected together at the ends. The idea was to reduce the proximity effect by reducing the magnetic coupling between adjacent turns. Unfortunately, there would still be a proximity effect between the two wires of the twisted pair itself. I did not know what would happen, because I have not seen this configuration described before. I am not able to calculate a Q because Butterworth does not have formulas for a winding made up of a twisted pair of Litz wires. More recently, formulas have been developed for calculating the Q of "bundled" insulated solid wires, which are several insulated wires twisted together. Bundled wires are more uniform than Litz wires, and, therefore, different formulas apply. I have not been able to find formulas related to bundled Litz wires.
For the twisted pair of Litz wire coil, the-low frequency inductance is 273 uH. The diameter of the 49 turn winding on a styrofoam cylinder is 4.5 inches and the length is 4.12 inches. The inductance at 1610 kHz is 325 uH, and the Q is 436. This is slightly higher than the Q of 382 obtained with the Litz wire coil without the twisted pair winding. The effective series resistance of the coil is 7.6 ohms.
The measured self-capacitance is 5.7 pF. The length/diameter ratio is .916. The self-capacitance obtained with the Medhurst method is 5.1 pF. As I already mentioned, the Butterworth method cannot be used to calculate the Q of this type of coil.
Although the Q improvement is only slight, it might be worth using a twisted pair of Litz wire, because the highest obtainable Q for the loading coil should be used.
I thought that this coil, which is 29 turns of #16 wire with a diameter of 10 inches, and a coil length/diameter ratio of .45, would have a good possibility of having a high Q, since there was adequate spacing between turns, resulting in only a little bit of proximity effect. Actually. the measured Q was only moderate. What was particularly surprising was that placing this coil on a styrofoam cylinder a little less than 10 inches in diameter and 6 inches long caused the Q to increase appreciably. I expected the Q to decrease, rather than increase, when placing it on the styrofoam form. Any dielectric losses resulting from the use of the styrofoam should have decreased the Q.
Originally, I made this coil by winding 29 turns of # 16 wire on the styrofoam cylinder, holding the coil in place with polyethylene tape. I measured a fairly high Q, but it was not as high as the calculated Q using the combination of the Butterworth and Medhurst methods. So, I carefully removed the coil little-by-little from the styrofoam form, retaining the original coil shape, supporting the coil by adding four plastic ribs cut from tie-wraps, and fastening the ribs in place with polyethylene tape. Now I had an air-core coil with the minimum amount of supporting structure to retain the original shape of the coil. I was expecting to get a yet-higher Q measurement because I now was no longer using the styrofoam form along with whatever dielectric loss it might have had. But, when I measured the Q of what was now an air-core coil, the Q was actually much lower than when the same coil was mounted on the styrofoam form.
I repeated this test by first measuring the coil as an air-core coil, and I then fitted the coil over the styrofoam cylinder again. Once again, the Q was significantly higher when the coil was on the styrofoam form rather than in the air.
I don't know why this unexpected situation occurred. Although a lot has been written about air cores or magnetic cores, not much has been written about coils with dielectric cores. Dielectric cores don't affect the magnetic field much, but they do affect the electric field. It is the magnetic field that generates the eddy currents in the windings of conductors that produce the skin effect and the proximity effect, which affect the Q. At first glance, it appears that a lossless dielectric core has little to do with Q, but my measurements indicate that that is not the case. It looks like the effects of the use of a dielectric core for coils require further investigation to properly understand them.
The air-core coil has a low-frequency inductance of 236 uH. The effective inductance at 1630 kHz is measured to be 317 uH. The self-capacitance measured to be 10.3 pF. The measured Q is 363. The equivalent series resistance of the coil is 8.96 ohms. The self-capacitance calculated by the Medhurst method is 12.6 pF. This is one of the rare instances where the Medhurst calculated self-capacitance is higher than the measured self-capacitance.
The Q calculated by the combination of the Butterworth and the Medhurst methods is 607.
With the styrofoam coil form, the measured Q is 510. The measured self-capacitance 13.3 pF, which is slightly larger than when the air-core is used. Usually, a larger self-capacitance results in a lower Q, but not in this case. The low-frequency inductance is the same as for the air-core.
The Q of loading coils in an actual installation are likely to be lower than reported in this thread, because the nearby conductive objects, like the antenna, transmitter, and ground plane, will probably reduce Q significantly. Ground losses will probably be bigger than coil losses, but the loading coil loss will still be a big contributor to the inefficiency of a Part 15 AM system.
I thought that this coil, which is 29 turns of #16 wire with a diameter of 10 inches, and a coil length/diameter ratio of .45, would have a good possibility of having a high Q, since there was adequate spacing between turns, resulting in only a little bit of proximity effect. Actually. the measured Q was only moderate. What was particularly surprising was that placing this coil on a styrofoam cylinder a little less than 10 inches in diameter and 6 inches long caused the Q to increase appreciably. I expected the Q to decrease, rather than increase, when placing it on the styrofoam form. Any dielectric losses resulting from the use of the styrofoam should have decreased the Q.
Originally, I made this coil by winding 29 turns of # 16 wire on the styrofoam cylinder, holding the coil in place with polyethylene tape. I measured a fairly high Q, but it was not as high as the calculated Q using the combination of the Butterworth and Medhurst methods. So, I carefully removed the coil little-by-little from the styrofoam form, retaining the original coil shape, supporting the coil by adding four plastic ribs cut from tie-wraps, and fastening the ribs in place with polyethylene tape. Now I had an air-core coil with the minimum amount of supporting structure to retain the original shape of the coil. I was expecting to get a yet-higher Q measurement because I now was no longer using the styrofoam form along with whatever dielectric loss it might have had. But, when I measured the Q of what was now an air-core coil, the Q was actually much lower than when the same coil was mounted on the styrofoam form.
I repeated this test by first measuring the coil as an air-core coil, and I then fitted the coil over the styrofoam cylinder again. Once again, the Q was significantly higher when the coil was on the styrofoam form rather than in the air.
I don't know why this unexpected situation occurred. Although a lot has been written about air cores or magnetic cores, not much has been written about coils with dielectric cores. Dielectric cores don't affect the magnetic field much, but they do affect the electric field. It is the magnetic field that generates the eddy currents in the windings of conductors that produce the skin effect and the proximity effect, which affect the Q. At first glance, it appears that a lossless dielectric core has little to do with Q, but my measurements indicate that that is not the case. It looks like the effects of the use of a dielectric core for coils require further investigation to properly understand them.
The air-core coil has a low-frequency inductance of 236 uH. The effective inductance at 1630 kHz is measured to be 317 uH. The self-capacitance measured to be 10.3 pF. The measured Q is 363. The equivalent series resistance of the coil is 8.96 ohms. The self-capacitance calculated by the Medhurst method is 12.6 pF. This is one of the rare instances where the Medhurst calculated self-capacitance is higher than the measured self-capacitance.
The Q calculated by the combination of the Butterworth and the Medhurst methods is 607.
With the styrofoam coil form, the measured Q is 510. The measured self-capacitance 13.3 pF, which is slightly larger than when the air-core is used. Usually, a larger self-capacitance results in a lower Q, but not in this case. The low-frequency inductance is the same as for the air-core.
The Q of loading coils in an actual installation are likely to be lower than reported in this thread, because the nearby conductive objects, like the antenna, transmitter, and ground plane, will probably reduce Q significantly. Ground losses will probably be bigger than coil losses, but the loading coil loss will still be a big contributor to the inefficiency of a Part 15 AM system.