Hello all,
Just some thoughts on the subject of coils. In the paragraphs below I have enclosed some phrases in quotes which you might want to web search for more information. I have also given some teaser information in case you want to pursue the subjects.
The inductance of a coil will change if a metal is introduced into the core. It will increase with ferrous material and decrease with non-ferrous such as brass. Radio techs know about a "Magic Wand", a plastic rod with a brass tip on one end and a ferrite slug on the other. This is really nifty for tuning VHF coils since it tells whether the L needs to go up or down (squeeze or stretch the coil).
When a material is inserted in a coil it introduces hysteresis and eddy current "Core Losses". This is why air cores are preferred for low loss coils and why a non-air core must use a core material designed to minimize the loss.
Slug tuned coils for AM frequencies have many turns in a small volume to achieve the needed inductance. The number of turns required means the resistance of the coil is high compared to a large coil. The "Skin Effect" at RF frequencies also increases the loss. "Litz Wire" is used to reduce the skin effect.
I have used a "Roller Inductor" for my ham work. They are $$$ but some might be interested.
We can tolerate coil losses in a receiver since the loss in signal can be recovered by amplifier gain, but for transmitting on a limited power budget (100 mW.) the only option is to minimize the coil loss rather than increasing the power. This is why we don't worry about small high L lossy coils in receivers but should in transmitters.
Maybe the keywords and brief paragraphs above will help guide you in some reading.
Neil
I don't have a copy of the Panaxis publication, so I don't know what led him to this conclusion about tapped coils for 50 ohm matching of Part 15 AM antennas.
But no software is needed to verify the statements I made in my last post -- just standard equations available in many engineering textbooks, as shown here:
The first equation calculates the reflection coefficient of a load impedance with reference to a standard impedance value.
The second equation uses the value calculated by the first equation to find the resulting SWR.
If this is done using 0.2 ohms resistance, 50 ohms reactance, and a 50 ohm standard impedance value (as in my last post), it will illustrate my point.
//
Since I have both the publication referred to and the practical experience, which indicates the approach is quite successful, I would highly recommend that others give the tapped inductor a try. Very little expense, and a whole lot of fun.
And it is the fun and experimentation, rather than the software modeling and mathematical calculations, that makes experimental broadcasting so enjoyable.
After all, no one ever broadcast with a formula or an engineering text book, now, did they?
🙂
Experimental broadcasting for a better tomorrow!
Hi all,
This is a very interesting thread.
Air core coils or coils wound on non-magnetic forms have the lowest loss. The coil needs to be much larger than a coil wound on an iron powder form to get the same inductance.
Larger diameter solid wire has less skin effect loss. Litz wire (many small bundled insulated wires) has much less skin effect loss and can be a smaller gauge.
Toroids are better than cylindrical cores because the magnetic field is mostly confined to the core rather than spilling off the ends of a cylinder. There is less interaction with nearby conductors.
Toroid cores are available in ferrite and iron powder. Forget ferrite, it’s for very low frequencies (switching supplies). Iron powder cores are tailored to various RF frequency ranges. Material #1 and #15 are the only two practical for the AM band. The link posted previously, http://www.amidoncorp.com/aai_ironpowdercores.htm is a great resource for selecting a toroid of the appropriate material and diameter. An even better resource is http://micrometals.com/. Request their free catalog and their free "RF Applications" book. They also have a great downloadable toroid calculator program: Toroid.zip, and they will send you a couple free samples on request.
You first need to know your target inductance, which you can calculate from the formulas (posted previously here by others) to resonate with the antenna capacitance (300uH to 350uH is in the ballpark). Then run toroid.exe. You will quickly see the effect of the toroid material (1 or 15), the diameter of the core and the wire size. You should go with only a single layer coil. Multi-layers introduce a lot of inter-winding capacitance, which isn't good. You will find that #24 or #26 wire is about as big as you can go without spilling into a second layer. The "RF Applications" book shows Q charts with regular wire and Litz wire. Litz is far superior given the #24-#26 gauge restriction.
A low loss coil and a good low resistance ground producess a very sharp tuning peak. A coil must have taps to get the right inductance to resonate. The antenna must have a means to fine tune the capacitance. A capacitor to ground in parallel with the antenna should be restricted to about 10 pf or less. A parallel tuning capacitor shunts some of the signal away from the antenna. A length-tunable antenna is superior. Without a parallel cap to ground, all of the signal goes to the antenna.
A high Q coil simply means that it is more efficient and has less resistive loss. High Q is good.
Bottom line:
- Large air-core or non-magnetic-core coils have the highest Q (highest efficiency)
- Iron powder toroids using the proper core material wound with Litz wire are the next best
- Ferrite cores are a NO NO.
- Larger wire diameter reduces skin effect loss.
- Litz wire has much lower skin effect loss than solid wire of the same diameter.
- The best antenna will have a tapped coil and a length-adjustable antenna.
Why all the fuss? The answer is simple: we want to get as much of that meager 100mW as possible to the antenna.
Lots of good information in your post, Phil. Just wanted to expand on one point in it...
A high Q coil simply means that it is more efficient and has less resistive loss. High Q is good.
But if antenna system Q is too high, it will reduce the amplitude of the AM sidebands that are radiated, and the audio at the receiver can sound quite muffled -- because the higher audio frequencies have been filtered out. High Q also makes the antenna system more difficult to tune to, and maintain system resonance, and makes the antenna match more sensitive to its mounting environment. So as in many things, a compromise may be useful.
Probably the Q of the complete antenna system should not exceed that which allows a 3 dB r-f bandwidth of about 10 kHz. This will support a 3 dB audio bandwidth of about 5 kHz at the receiver. Most modern AM receivers don't have much audio response beyond that, anyway. Simple math leads to the answer.
Q = Loading Coil Reactance in ohms / Antenna System Resistance in ohms*
RF Bandwidth = Tx Frequency in kilohertz / Q
*The biggest elements of the antenna system resistance are the radiation resistance of the radiating structure, the DC resistance of the loading coil, and the resistance in the r-f ground system.
Here is an example calculation for a ground-mounted radiator on 1500 kHz and an r-f ground resistance of 10 ohms. By other means the radiation resistance was first calculated to be 0.09 ohms, the coil reactance at resonance to be 4,000 ohms, and the coil resistance to be 4 ohms.
Q = 4,000 / (0.09 + 4 + 10) = 284, approx.
so RF bandwidth = 1500 / 284 = 5.3 kHz, approx, and the audio bandwidth at the receiver would be 5.3 / 2 = 2.65 kHz, which would sound rather poor (about like an old telephone).
A 10 kHz r-f bandwidth for this system would need a Q of 1,500 / 10 = 150. Assuming we can't easily change the coil and ground resistance, we would need to add a series resistor to the system to produce that Q. The total resistance value needed would be 4,000 / 150 = 27 ohms, approx, so the value of the added resistor itself would be 27 - 14.09 = 13 ohms, approx.
Of course, the added resistance reduces the amount of radiated power, but that is the compromise it takes for this setup to get good audio fidelity at the receiver.
The r-f resistance of the ground system will be an unknown in most Part 15 AM systems. It is not just the resistance of the conductor(s) leading to a buried r-f ground, but includes the resistance to r-f in the path through the earth TO the buried r-f ground conductor(s). That r-f path through the earth is probably 99% of the total resistance in the r-f ground system, and is difficult to measure/estimate.
Most r-f grounds used with Part 15 AM antenna systems have higher resistance than in this example, so getting adequate r-f bandwidth may not be an issue. But this information might be useful if your Part 15 AM audio has poor high-frequency response in most receivers.
//
- Ferrite cores are a NO NO.
Great post, especially the info about Ferrite vs Iron Core. I had been a bit sloppy earlier, at one point even posted an incorrect link which was quickly fixed, and need to make sure that's really brought to the forefront.
As the late Phil Hartman would have said in his SNL Frankenstein character
Ferrite baaaaaaaaad
Iron Core gooooooooood
🙂
Experimental broadcasting for a better tomorrow!
Excellent answers everyone, and just the sorts of things I'd been wondering about. Being more of a dabbler than anything else, I referred to the cores in am radios as being "ferrite" without checking any references to see what they're actually made of.
I'd just gotten back onto these ideas a bit from the talk about the talking house using a moving core inductor for tuning, and also from taking a look at some of the "dx" style crystal radios some folks make. I'd been toying with the idea of talking my daughter into tackling a crystal radio project (or doing one with me), and was thinking of the possibilities for building some interest in the switch to am. A friend of mine who is well outside the range of my current little FM rig is being very hopeful of AM, since he has an old floor model RCA Victor (IIRC) reciever that seems to pick up well. So some of the parts and techniques from crystal radios, like variometer tuning had been running through my head.
But in any case, great answers to the questions!
Daniel
I've been through the SStran system. I built the antenna with the taps and all but I've found that the true "Sweet Spot" on the coil is hard to find. There are 2 loops between each tap and I've tapped between taps (lol..wanna dance) and found a better spot with a higher target voltage, better than just sliding the extension in and out and changing taps..
I was working at 1.620khz. and we know the coil is larger than needed at this frequency. I used a piece of aluminum flashing, 6" wide, rolled it into a circle as to fit inside of the pvc the coil is wound on. I stepped my tap connection down two taps from where I had my best settings prior to inserting the flashing. I left the adjustment in the copper pipe portion at the best setting also. I then used a piece of wood to bump the flashing up into the coil.. Very slowly while watching the vom and the field strength meter. I found the peak very easily and didn't need to change the copper pipe adjustment at all. I did try to move it to find a better voltage but I couldn't. If I moved it any, I could still recover a good setting just by moving the flashing in the coil, to a point. If the copper pipe was too far out of adjustment, I couldn't regain the good voltages. I just pulled the flashing out, went to my sweetest tap point prior to messing everything up, re-adjusted the copper pipe back to the best setting, moved my tap point down 2 taps, put the flashing back into the coil and re-peaked again. I gained almost a volt by doing this and the bandwidth was fine..
Have I found a new use for beer cans...?
Have I found a new use for beer cans...?
At the very least, another great reason to empty them {8-D=
Seriously, very helpful info, and a great addition to the other fine tuning approaches.
Experimental broadcasting for a better tomorrow!
So should we be putting in taps at 90 degree angles all the way around? Or should every AMT3000 kit arrive with a 6-pack? (Just for tuning mind you)
Should the coil be made with a smaller form?
What about all the wire that is left after you solder the tap, should it be trimmed off? Doesn't it radiate too?
So should we be putting in taps at 90 degree angles all the way around? Or should every AMT3000 kit arrive with a 6-pack? (Just for tuning mind you)
Might help in the tuning process but not while putting the kit togather.. 🙂
I haven't done it but putting taps on both sides will make it more flexible. I built the coil as instructed but scraped a few spots on the opposte side to connect to and I got better voltage readings. (Between the taps, so to say) Or, instead of installing the taps every 2 turns, Make it every turn. It can be done..
Just think outside of the box.. It's fun!
I used a piece of aluminum flashing, 6" wide, rolled it into a circle as to fit inside of the pvc the coil is wound on.
Metal inserted into the coil will definitely affect the tuning. With non-magnetic aluminum, the inductance will be lowered and at the same time, the interwinding capacitance will be increased. The result depends on the combined effect of the two. A metallic coil insert will increase losses due to eddy currents induced in the insert. The effect is measurable, but without precise measuring instruments, you may not notice the loss in the signal.
The SSTRAN antenna has taps every 2 turns. This gets you in the ballpark. The final fine-tuning to resonance is accomplished by adjusting the antenna length. The "impreciseness" of the coil taps is exactly why the antenna length adjustment is required. Everyone should note the importance of moving your hands away from the antenna each time you make a length adjustment. Your body capacitance will have a big effect.
The SSTRAN antenna is designed for minimum losses. Inserting any metallic tuning device into the coil, whether magnetic or non-magnetic will increase losses. On the capacitance side, adding a small variable capacitor in parallel with the antenna or adding a very large variable capacitor in series with the antenna will eliminate the need for length adjustment but will increase losses.
I certainly don't want to discourage experimentation. I just want everyone to be aware of the physical realities.
So if we have more taps, like one every turn, then can we get to where the antenna length adjustment is less important? That way once the antenna length is close, it colud be soldered to prevent corrosion and all final tuning could be at the coil. You could even pull the coil apart and use the extra wire to forms taps around the required point every quarter turn. Maybe a quarter turn is far too close, but every turn or every half turn is certainly withing the realm of workable.
What about the extra wire at the end of the coil, doesn't it effect things as much as another antenna would? It is an unterminated conductor, so it should radiate.
What about the extra wire at the end of the coil, doesn't it effect things as much as another antenna would? It is an unterminated conductor, so it should radiate.
I agree. It seemed that by using the flashing "Killed" the extra windings and it was possible to tune the antenna below the radiation field (physically) and the tuning was more precise..
The original coil design could be redesigned to include an aluminum covered "Piston" that slides easily inside of the pvc connected to a non inductive rod to move it with. Make the taps on every turn, Find the best peak possible, (keeping the piston outside of the windings) Fine tune with the piston and a field strength meter.. Well below the antenna.. And lock it down..
The coil form would need to be a little longer at the bottom and the last wrap would need to be secured on the outside so not to get in the way of the piston. A way to lock the piston in position after tuning in the bottom end cap of the coil form..
So if we have more taps, like one every turn, then can we get to where the antenna length adjustment is less important?
The radiator length adjustment keeps the antenna frequency agile for those who choose whatever frequency with maxium radiator length. You don't want to give up radiator length in the coil..
I also had better luck by adjusting the tuning cap on the board to the position of least capacitance..
Infinite tapping points sound like the idea solution, which could be provided by a a coil wound with uninsulated wire and a bit of space between each turn, like the Texas Bug Catcher:
It doesn't look like the coil forms are sold separately, but the general idea seems pretty easy to reproduce. It looks like four disks with serrated rods equidistant around the circumference.
Experimental broadcasting for a better tomorrow!