Antenna Loading Coil

I have found that a lower frequency can help the internal tuner match different Antenna lengths and sometimes that moves me into a little quieter area on the band. I am currently using the supplied antenna and internal tuner, but working on 1270 KHz to be in a quieter place.

Of course, we need to be careful about our compliance because when we don't use the certified transmitter as approved we move into field strength enforcement.

Still in all, lots of fun experimenting and it's not like we're knocking out TV sets with our 100 MW AM units 🙂

Experimental broadcasting for a better tomorrow!

Yes, thank you, Joe, for "passing this information along to anyone who may make use of it." Your post was very useful to me. I am constructing a Part 15 transmitter with higher efficiency than is available with commercial transmitters. I think that the kind of antenna tuning circuit used in the SSTRAN will work well in my system. The SSTRAN uses a tapped loading coil to tune out the capacitive reactance of the antenna, and to provides the inductance for an "L" network for tuning for maximum output to the resistive component of the antenna circuit. I think, however, that it is very inconvenient to use taps. It would be much easier to use a continuously variable inductor. I tried using a slug-tuned inductor using a commercial coil form, but the tuning range of the inductor was too small. I then made my own slug-tuned inductor by using a ferrite antenna rod as the slug. There was plenty of tuning range, but the core loss caused by the ferrite rod was too high. The equivalent series resistance of my inductor was even higher than the expected ground resistance of the antenna system.

I have a variable-inductance loopstick coil of the kind that I think you are using. It is about a quarter inch in diameter, and has a 1 1/8 inch ferrite rod attached to a long brass screw. The screw is used to adjust the position of the ferrite rod with respect to the winding on the coil form. If I hadn't read your post, I would have never thought of testing this loopstick. It seemed to me that its dimensions were too small to give good performance. However, the results were much better than I expected. The inductance tuning range was 31.6 uH to 287 uH. What surprised me the most was how low the equivalent series resistance of the coil was. At minimum inductance, the resistance was only 3.14 ohms. The DC resistance of the winding is 1.7 ohms. As expected, the equivalent series resistance increased as inductance was increased, because of increased core loss. However, the equivalent series resistance at maximum inductance is only 12.3 ohms, which is quite small, considering how small the dimensions of the inductor are. It's too bad that these loopsticks are hard to find these days. I'd sure like to have some more of them, in order to be able to build more transmitters.

The maximum inductance of 287 uH, while high, is not high enough for all antenna tuning situations. It would be useful to add an inductor in series with the loopstick inductance. A situation where the required loading coil inductance is less than 100 uH is very unlikely, so I made a 71 uH inductor by filling an Amidon T-106-2 iron powder coil form with a single layer of #24 magnet wire. Combined with the loopstick, this coil produced a minimum inductance of slightly over 100 uH. The equivalent series resistance of this inductor was 2.3 ohms. Filling the same kind of Amidon toroid with a single layer of #28 magnet wire produced an inductance of 212 uH. This inductor, however, has a high series resistance of 23.3 ohms. It's surprising that the loopstick actually has a lower equivalent series resistance for the same inductance than the supposedly low-loss toroid. I also tested the Amidon T-106-3 coil form, which has higher permeability than the T-106-2, and gives more inductance for the same number of turns. This coil form gave excessively high equivalent series resistances.

Funny, I had a loopstick from my very first part 15 XMTR project, a three-transistor AM unit built from scratch based on the schematic in James Cunningham's Low Power Broadcasting book

I never thought to use it elsewhere because I thought the series resistance would be too high 🙂 I picked mine up at an old ham radio store and used it to substitute for a fixed value inductor I couldn't find.

So, would it be more effecient to use several T-106-2s in series, wound with heavier gauge wire instead of using a single T-106-2 wound with the very fine wire needed to get the required inductance? It looks like it might get the series resistance of the entire assembly down, while still gaining the size and stability advantage over the air coil option.

I have five T-106-2s and I was thinking of trying perhaps two, with the second one featuring large step tuning taps then to the loopstick for final fine tuning.

Experimental broadcasting for a better tomorrow!

SCWIS,

The term "loopstick" can refer to any coil wound on a ferrite rod. It can be either a fixed or variable inductor. We are discussing the popular variable inductor in this thread, which was the basis for many published hobby projects many years ago. However, your diagram shows a fixed inductor, L1, tuned by a parallel capacitor. As you explained in your post, when you built the circuit in the diagram, you replaced the fixed coil shown with a variable loopstick. I'm glad you still have this inductor. It is a real gem, and very useful.

As I stated in my previous post, I measured an equivalent series resistance of 12.3 ohms when my loopstick was adjusted to the maximum inductance of 287 uH. I also stated in my post that that a 71 uH coil I made with an Amidon T-106-2 toroid has an equivalent series resistance of 2.3 ohms. Four of these coils in series would have nearly the same inductance as the maximum inductance of my loopstick, and have an equivalent series resistance of 9.2 ohms. This is slightly less than the loopstick resistance. So, there is an efficiency advantage to using the Amidon coils in series, but not as much as would be expected, considering how tiny the loopstick is. Whoever designed the loopstick had really put a lot of knowledge and effort into it. Until now, I had wrongly considered the loopstick to be trivial.

The circuit diagram in your post is very interesting. It uses the combination loading coil and "L" network design used in the SSTRAN. This kind of tuning network for short antennas was featured in the Mobile Antennas section of several editions of the ARRL Antenna Book. It seems to have originated from a 1951 QST article. It looks like the designer of your circuit intended to make an efficient class C amplifier, but, it seems to me that there would not be enough RF drive to the final transistor stage to get respectable efficiency. Thanks to some posts about some efficiency tests performed by Neil late last year, the efficiency of Part 15 transmitters has become a particular interest of mine. I contributed several posts about this subject on the "New products under proposal" thread initiated by Wilcom Labs, which is

http://part15.us/node/1280

My reason for wanting to look into using a loopstick was to get a continuous-tuning inductor for matching the output stage of a Part 15 AM transmitter to a 3 meter antenna. I also want to get a continuous-tuning capacitor at the input to this tuning network. The capacitance required is rather large. The present approach is to select a large value fixed capacitor (several hundred pF) in parallel with a trimmer capacitor. I think it is inconvenient to select any fixed capacitor. Fortunately, I have a four-gang air-variable capacitor with a total capacitance of 728 pF. I connected a 100 pF air-variable capacitor parallel to the larger capacitor to serve as a trimmer. Unfortunately, air-variable capacitors have become about as scarce as variable-inductance loopsticks. I will be hard-pressed to find the components I need If I try to build any more transmitters.

Glad to see a discussion going on using the loop stick for antenna tuning. The loop stick I am using I am almost sure is a “SuperX” Loop stick I bought back in the 70s for making crystal radios.. With the long wire antenna (actually a short wire) I am using, I can tune to resonance the upper part of the AM band using the Talking House AM transmitter, and other part 15 AM transmitters I have. The loop stick inductor doesn’t have enough inductance to tune below about 900 KHZ using the length of wire attached to it that I am using for an antenna, but works well above that. As most of us know, these loop sticks can be found from time to time on eBay, as well as in rocket crystal radios. Some times there are rocket crystal radios on eBay that are so damaged that no one wants them, but usually the coil in them is still good so this would be a good source for starters. I am sure the inductance and other factors vary with the different coils so it’s buying and try. All I can say is I was amazed at the tuning and efficiency of the loop stick coil I tried.
I also use the same loop stick and antenna, to tune the long wire antenna to resonance for crystal radio projects, however for some reason, I can’t use the inductor in series with the out side antenna to the input on the crystal sets to get the antenna to resonance , what I do is actually ground one end of the loop stick, then tap off on the top side of the loop stick where the long wire attaches, just T off that spot to the antenna coil on the crystal receivers, then tune the loop stick for max output. This really works well for short wire antennas. I did try a much larger fixed loop stick antenna that I salvaged from a large transistor radio, not sure of the inductance of it, I put this in series with the crystal radio with out grounding the loop stick in any way, and that worked too, I assume the fixed larger antenna loop stick just had more inductance. So just want to say im very happy that my “discovery” is working for others, I am also going to try putting some inductance in series on the one I am using to see if I can tune my short wire antenna to the lower frequencies. Glad to be part of the group.
Radio Joe

On another thread, I said that I suspected that real Part 15 AM installations perform much worse than their owners think they do, for several reasons. One of the reasons I mentioned is that the losses in the loading coils are much higher than several published sources (such as The Low Power AM Broadcasters Handbook) say they are. The thread also mentions an online coil calculation utility program that also gives overly optimistic values of coil loss. I also mentioned my measurements on the B&W type 3062 Air Inductor which showed that the coil loss was much higher than expected for that type of coil.

I am posting here because I may have found a way to calculate the actual coil loss of the B&W coil I mentioned. My calculated coil loss was almost the same as my measured coil loss. That these two numbers match each other is almost certainly a coincidence, because I have usually had poor results when comparing coil calculations with measurements. I am, nevertheless, encouraged. I think that coil calculations that are consistently between one half and twice the measuremnts are useful, and this result causes me to think that such accuracy may be possible.

The B&W 3062 coil is two inches in diameter and ten inches long. It has 160 turns of #16 wire. The measured inductance at 1 MHz is 326 uH, and the equivalent series resistance was measured to be 10 ohms. 326 uH is a useful Part 15 AM loading coil inductance, but the low-frequency inductance was only 235 uH. This indicates that the coil capacitance is quite high, about 30 pF. I measured the DC resistance of the coil to be .4 ohms. Using the length of the wire in the coil, and the resistance of #16 wire per 1000 feet, I calculated the DC resistance to be .346 ohms. Using Equation 94 in Section 2 of Terman's Radio Engineers' Handbook (1943), this coil operated at 1MHz has a series resistance that is 15.1 times the DC resistance. This calculates to be 5.2 ohms. which is less than the 10 ohms of series resistance actually measured. The 30 pF of coil capacitance reduces the Q of the coil and increases the equivalent series resistance. It happens that this series resistance is close to 10 ohms, which is the measured series resistance.

Equation 94 gives the coil loss due to the skin effect and the proximity effect. In most coils, the proximity effect is appreciably higher than the skin effect. By increasing the spacing between turns, the proximity effect is reduced, but this causes other coil losses to increase because the wire must then be longer and/or thinner.

Adding the reduction of Q due to the coil capacitance gives the total series resistance. The combination of skin effect, proximity effect, and coil capacitance cause the loss resistance of the coil to be high.

So, what can be done to reduce coil losses? It seems that reducing one kind of loss increases another kind of loss. For example, copper losses can be reduced by winding the coil on a ferrite core. However, this produces core losses. Litz wire is good for reducing skin effect at the lower frequencies, but its effectiveness is not very good at the upper end of the AM BCB. Twisting a bundle of insulated wires eliminates the proximity effect between adjacent turns of the bundle, but there is still a proximity effect between the wires inside the bundle. (A twisted bundle of wires is not the same as Litz wire.) Making a coil with a Q above several hundred is still an open problem in electrical engineering. High Q has been obtained with mechanical resonators, such as tuning forks and piezoelectric crystals, and with resonant cavities, but not with coils. Some improvements in coils may be possible with further research.

Ermi,

Check out this article by Ben Tongue:

http://www.bentongue.com/xtalset/22St4bXS/22St4bXS.html

In particular, see Table 2.

There appears to be a lot of commonality between some of the techniques used to realize high performance crystal set design and some of the requirements for Part 15 operation. It would be a good idea to look into this further.

WEAK-AM
Classical Music and More!

I did not mean to suggest that the Q of the coil I described in my post(which is just over 200) is the best that can be done. The point I wanted to make is that the Q available with coils is lower than what is desired.

In the Low Power AM Broadcasters Handbook, a loading coil loss resistance of 2 ohms is assumed. The inductive reactance required at the high end of the AM BCB is about 3000 ohms. (Interestingly, elevating the antenna greatly increases the radiation resistance, but does not significantly reduce the required inductive reactance.) For 2 ohms of loading coil loss, the required Q of the coil is 3000/2 = 1500. This high a Q is just not available with coils. The HP(Agilent) model 4342A Q meter has a maximum reading of 1000. The manufacturer clearly did not expect the need for a measuring the Q of a coil higher than that.

Having said that, I was truly impressed with the measured Q of 620 on Table 2 in the linked article. There are calculated Qs higher than that in the article, but calculated Qs are often overestimates. This high Q reading caused me to read some other articles in the series, specifically, # 0, # 26, and # 29. The descriptions of "contrawound" coils were particularly interesting. It may be that improved loading coil loss can be obtained by using contrawound coils. Even with this higher Q, the loading coil loss would be about 5 ohms.

I found out that Ben H. Tongue, the author of the article about crystal radios linked by WEAK-AM, is a famous man. He, along with Isaac S. Blonder, founded Blonder-Tongue Laboratories, Inc. in 1950. Blonder-Tongue originally made wideband RF preamps and antennas. I have not heard about Blonder-Tongue for years, but the company is still very much in business. They continue to manufacture RF products. Ben has been awarded about 30 patents.

Blonder and Tongue sold their company in 1989, but both continued to work for the company as consultants. Ben Tongue must be about 90 now (judging by the fact that his company was founded 58 years ago). The true radio experts who are still around are getting on in years. Trainotti, in comparison, might be considered to be a spring chicken at 73.

I found out that Ben H. Tongue, the author of the article about crystal radios linked by WEAK-AM, is a famous man. He, along with Isaac S. Blonder, founded Blonder-Tongue Laboratories, Inc. in 1950. Blonder-Tongue originally made wideband RF preamps and antennas. I have not heard about Blonder-Tongue for years, but the company is still very much in business. They continue to manufacture RF products. Ben has been awarded about 30 patents.

Blonder and Tongue sold their company in 1989, but both continued to work for the company as consultants. Ben Tongue must be about 90 now (judging by the fact that his company was founded 58 years ago). The true radio experts who are still around are getting on in years. Trainotti, in comparison, might be considered to be a spring chicken at 73.

Publication #26 on www.bentongue.com has a technique for measuring the Q of a coil if a Q meter is not available. Tongue's method consists of connecting the output of a signal generator with an accurate frequency indication to a small coil. This coil is loosely magnetically coupled to the cold end of the coil under test. The hot end of the coil is connected to an oscilloscope through a 1X (not 10 X) probe. The probe is not directly connected to the coil under test, but clipped to an insulated wire connected to the hot end of the coil. This provides very loose capacitive coupling between the coil and the probe. The capacitor used to resonate with the coil is set for peak amplitude, at the desired frequency, on the oscilloscope. Then the signal generator frequency is adjusted to find the bandwidth over which the oscilloscope amplitude is down by a factor of .707 from the peak amplitude at resonace. This is the 3 dB bandwidth. Dividing the resonant frequency by the 3 dB bandwidth gives the Q. Using this method, I got a Q indication that is about 37 % higher than what I got for the B&W type 3062 air inductor when measuring Q with an HP(Agilent) 4342A Q Meter. The self-capacitance of the coil was less than half of what I measured with the Q meter, and the equivalent series resistance was 33 % lower. Using a grounded sheet of aluminum over the workbench caused the Q to decrease and the self-capacitance of the coil to increase, making the results closer to what was obtained with the Q meter. It looks like the metal cover of the Q meter is responsible for the lower Q reading (compared to when using Tongue's method) and the higher self-capacitance reading.

The self-capacitance of a coil has two components. The smaller component is the capacitance between the turns, which forms part the capacitive portion of a transmission line that exists along the length of the coil. The larger component is the capacitance between the coil and the ground. This second capacitance is caused not only by the coil itself, but its surroundings. So, since the self-capacitance of a coil reduces its Q, it is possible to get better Q readings with a simple bench set-up than with an expensive piece of test equipment. The large metal case of the Q meter seems to interfere with the Q reading.

It turns out that Tongue's "contrawound" coil winding technique is not applicable to loading coils because it involves connecting two coils in parallel which are wound in different directions. This method results in a coil assembly that is too large and cumbersome to be conveniently used as a Part 15 AM loading coil.

I tried the Tongue method of measuring Q without a Q meter by using a 10X probe directly connected to the hot side of the coil instead of a 1X probe loosely capacitively coupled. With the 10X probe, the Q reading was only 147 compared to 276 with the 1X probe. The reduction of Q caused by the 10X probe is equivalent to adding about a 600 k ohm load in parallel with the coil. The DC load by the 10X probe is specified to be 10 M ohms, but, around 1 MHz, the actual load is about 17 times lower.

Tongue warns against using PVC pipe as a coil form, and recommends styrene for high Q. (The B&W coil I tested has no coil form other than thin plastic supports for holding the windings in place.) He also prefers Litz wire over solid wire. Litz wire is good, if you can get it, but it loses a lot of its effectiveness at the upper end of the AM BCB. The 660/46 Litz wire used by Tongue has the DC resistance of #18 solid wire. For Part 15 AM, it might be a tossup between using 660/46 Litz wire and, say, #16 solid wire, maintaining about a wire diameter spacing between turns to reduce the proximity effect.

Publication #26 on www.bentongue.com has a technique for measuring the Q of a coil if a Q meter is not available. Tongue's method consists of connecting the output of a signal generator with an accurate frequency indication to a small coil. This coil is loosely magnetically coupled to the cold end of the coil under test. The hot end of the coil is connected to an oscilloscope through a 1X (not 10 X) probe. The probe is not directly connected to the coil under test, but clipped to an insulated wire connected to the hot end of the coil. This provides very loose capacitive coupling between the coil and the probe. The capacitor used to resonate with the coil is set for peak amplitude, at the desired frequency, on the oscilloscope. Then the signal generator frequency is adjusted to find the bandwidth over which the oscilloscope amplitude is down by a factor of .707 from the peak amplitude at resonace. This is the 3 dB bandwidth. Dividing the resonant frequency by the 3 dB bandwidth gives the Q. Using this method, I got a Q indication that is about 37 % higher than what I got for the B&W type 3062 air inductor when measuring Q with an HP(Agilent) 4342A Q Meter. The self-capacitance of the coil was less than half of what I measured with the Q meter, and the equivalent series resistance was 33 % lower. Using a grounded sheet of aluminum over the workbench caused the Q to decrease and the self-capacitance of the coil to increase, making the results closer to what was obtained with the Q meter. It looks like the metal cover of the Q meter is responsible for the lower Q reading (compared to when using Tongue's method) and the higher self-capacitance reading.

The self-capacitance of a coil has two components. The smaller component is the capacitance between the turns, which forms part the capacitive portion of a transmission line that exists along the length of the coil. The larger component is the capacitance between the coil and the ground. This second capacitance is caused not only by the coil itself, but its surroundings. So, since the self-capacitance of a coil reduces its Q, it is possible to get better Q readings with a simple bench set-up than with an expensive piece of test equipment. The large metal case of the Q meter seems to interfere with the Q reading.

It turns out that Tongue's "contrawound" coil winding technique is not applicable to loading coils because it involves connecting two coils in parallel which are wound in different directions. This method results in a coil assembly that is too large and cumbersome to be conveniently used as a Part 15 AM loading coil.

I tried the Tongue method of measuring Q without a Q meter by using a 10X probe directly connected to the hot side of the coil instead of a 1X probe loosely capacitively coupled. With the 10X probe, the Q reading was only 147 compared to 276 with the 1X probe. The reduction of Q caused by the 10X probe is equivalent to adding about a 600 k ohm load in parallel with the coil. The DC load by the 10X probe is specified to be 10 M ohms, but, around 1 MHz, the actual load is about 17 times lower.

Tongue warns against using PVC pipe as a coil form, and recommends styrene for high Q. (The B&W coil I tested has no coil form other than thin plastic supports for holding the windings in place.) He also prefers Litz wire over solid wire. Litz wire is good, if you can get it, but it loses a lot of its effectiveness at the upper end of the AM BCB. The 660/46 Litz wire used by Tongue has the DC resistance of #18 solid wire. For Part 15 AM, it might be a tossup between using 660/46 Litz wire and, say, #16 solid wire, maintaining about a wire diameter spacing between turns to reduce the proximity effect.

"Tongue warns against using PVC pipe as a coil form, and recommends styrene for high Q. "

Thank you for passing that along - I had always wondered why the Hams on 160 Meters (right above AM BCB) always used air coils with the absolute minimum of coil form material and the open coil spacing - now I know!

Experimental broadcasting for a better tomorrow!

"Tongue warns against using PVC pipe as a coil form, and recommends styrene for high Q. "

Thank you for passing that along - I had always wondered why the Hams on 160 Meters (right above AM BCB) always used air coils with the absolute minimum of coil form material and the open coil spacing - now I know!

Experimental broadcasting for a better tomorrow!