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[Articles Home]  [Add Article]  

All Toroids Are Not Created Equal

Tom Thompson (W0IVJ) on April 14, 2007
View comments about this article!

I have used ferrite material to successfully attenuate common mode signals for years . Common mode currents are those currents that flow in the same direction on a wire bundle, as opposed to the currents that flow in opposite directions on wire pairs to form a complete circuit. The differential currents cause a net flux of zero in the core and consequently are not affected by the core. However, the other day I discovered that I had misunderstood what was really happening. I knew that if you wound a cable on a toroid made of ferrite material that you could get an attenuation of the common mode signals on that cable, but I always thought it was due to just the impedance from the increased inductance that the ferrite provided. Yes, I had read the specification sheet and noted that the cores provided loss, but it never clicked...inductance does not provide loss; resistance does...duh. Well, I recently acquired a nice little piece of test equipment, the AIM 4170 impedance measuring device developed by Bob Clunn, W5BIG, and sold at Array Solutions. Without Bob's nice design, none of these measurements would have been possible on my budget.

I had wound some wire on an FT-114-77 ferrite core and was looking at its impedance. There was a resistive component and a capacitive component that looked so strange to me that I began to wonder if my measurements were correct. The purpose of this paper is to share with you some additional understanding of ferrites that I acquired from these measurements. I hope to show that:

The impedance of a wire passing through or wound around a ferrite core can be inductive which is associated with permeability, resistive, and even capacitive which is associated with self resonance. All three of these parameters are frequency dependent in a ferrite.

The maximum attenuation of a ferrite core is not where the inductance is maximum, but instead is where the magnitude of the impedance is maximum. This impedance peak may even be where the impedance has no inductive component nor capacitive component i.e. at resonance.

The frequencies that you are trying to attenuate determine the core that you should use.



In order to make sense of these measurements, an understanding of complex impedance is necessary. Those of you who are conversant in the complex number representation of impedance should skip the next section unless you want to read it and keep me honest so that I don't mislead some one :)


Complex Impedance in a Nutshell

Let me explain the basics of complex impedance a little so the following graphs will make sense to those of you who might be new to the field. Impedance is generally made up of two components, resistance and reactance. These two components are vector components i.e. they have a magnitude and a direction. We will define these vectors on an X-Y grid. Resistance will be a line going from zero to some value on the X-axis. The direction of resistance is always from zero to the right and the length of the line represents the value of the resistance. Reactance will be a line that is perpendicular to the resistance line i.e. it is a line that either goes from zero up if it is an inductive reactance or zero down if it is a capacitive reactance. Again the length of the line indicates the magnitude of the inductance. These lines fit our definition of a vector in that they have both magnitude and a direction. We will define the resistance as R and the reactance as X. Since the reactance is in a different direction from the resistance, we will place a J in front of the X to indicate this. Now we have an expression R+JX if there is a resistance in series with an inductive reactance or R-JX if the resistance is in series with a capacitive reactance. XC is used to designate a capacitive reactance and it is related to the value of the capacitor by XC=1/(2*PI*f*C) where PI = 3.14159... , f is the frequency in Hz, and C is the capacitance in Farads. XL is used to designate an inductive reactance and is related to the value of the inductor by XL=2*PI*f*L where PI=3.14159..., f is the frequency in Hz, and L is the inductance in Henries. This representation of impedance is shown graphically in Figure 1.

Since the resistance and reactance vectors form a 90 degree right angle, the magnitude of the sum of the two, R+JX which we will call Zmag , can be found by taking the square root of the sum of the squares i.e. Zmag = SQRT(R^2 + X^2). Since Z is a vector and Zmag is its magnitude, we need some way to specify which way it is pointing. The convention for this is to measure the angle from the X-axis or in this case the R-axis. If you remember any trigonometry at all, recall that the angle that the vector R+JX makes with the horizontal axis is given by the inverse tangent of X/R. Let us assume that R = 3 ohms and XL= 4 ohms. Then Zmag = SQRT( 9+16) which is 5. The angle, or phase angle theta, as it is typically called, is the inverse tangent of 4/3=1.333 which is 53.13 degrees. We can now say that the impedance is 5 at an angle of 53 degrees. Since theta is positive, we know the reactive part of the impedance is inductive. If X had been negative the magnitude would have remained the same, but theta would have been the inverse tangent of -4/3=-1.333 which is -53.13 degrees. We would now say that the impedance is 5 at an angle of -53 degrees and we would immediately know that the reactive part of the impedance is capacitive because of the negative phase angle. If the impedance is purely reactive i.e. the resistive part of the impedance is zero or very, very small, then the R vector will be zero or very, very short. This results in a phase angle of 90 degrees if the reactance is inductive or a phase angle of -90 degrees if the reactance is capacitive.


The Measurements

The following graphs were made using the AIM 4170 mentioned above. I first connected a 12 inch wire to the fixture forming a loop from the center pin of the connector to the common, and ran a scan from 0.1 MHz to 30 Mhz. I set the cursor at about 30 MHz, so the values listed to the right of the curve represent the values at this frequency. This is a busy curve, so bear with me as I point out some of the features. The color code on the curves is:

Black: Reactance
Tan: Resistance
Green: Zmagnitude
Purple: Phase angle (theta)

The horizontal axis is frequency and it runs from 0.1 MHz to about 30 MHz. Figure 2 is the plot from the 12 inch wire in the fixture.

Figure 2: Single 12 Inch Wire. (click herefor a higher resolution graph)

Observe from the curves and the listings on the right of Figure 2 that theta is 90 degrees and R=0 which indicates a pure inductance or R+JX is equal to J60.44 ohms at 29.736 MHz and the inductance is 0.324 uH. Since there is no resistive component, Zmag = 60.44 ohms which is due entirely to the inductance. The curves behave exactly like you would expect. The inductive reactance is increasing linearly with frequency, there is no resistance, and theta stays at 90 degrees. Zmag, which is in green, actually is the same as the black curve which is the inductive reactance.



Figure 3 is the same 12 inch wire with a snap on ferrite over the wire which would be considered one turn.

Figure 3: Single Turn of a 12 Inch Wire with a Snap-On Ferrite of Unknown Material. (click here for a higher resolution graph)

Now you can see that there is a resistive and an inductive reactance component to the impedance. If you look at the values on the right for a frequency of 29.736 MHz, you will see that the resistance, Rs (the s stands for series resistance), equals 147 ohms and the reactance, Xs (the s stands for series reactance), equals 162 ohms. Solving for Zmag which is equal to the square root of the sum of the squares of the resistance and reactance, we get the value 218. Since the reactance is just slightly larger than the resistance, we would expect theta to be slightly larger than 45 degrees. And indeed it is, Theta = 48 degrees.

Now what does all this mean? Well, first of all, the impedance now has a resistive component, which must come from the core since there was no resistive component in Figure 2. Also, The reactance has increased from 60.44 ohms to 161.575 ohms and Zmag has gone from 60.444 to 218.368, a factor of more than 3.6. You can see from this plot that we have a complex relationship here. The inductive reactance is no longer linear as it flattens out as frequency increases. There is a resistive component and theta is no longer 90 degrees.



The next plot, Figure 4, shows the same parameters with the turns doubled from one to two.


Figure 4: Two Turns of a 12 Inch Wire with a Snap-On Ferrite of Unknown Material. (click here for a higher resolution graph)

One would expect the inductance and hence the inductive reactance to increase by a factor of four since the turns were doubled and the increase is a function of turns increase squared. That is not what happened. The inductive reactance is starting to fold over as can be seen by the black curve. However, the resistance increase was close to a factor of four. The important thing is that Zmag, which is responsible for the attenuation, is increasing. If you ever wondered why some of these baluns that are wound on ferrite were destroyed by heat, here is your answer. Remember any RF current, at the specified frequency, through the wire would produce a power equal to the resistance times the current squared. That power will generate heat in the core. Let us say that we had an rms current at 30 MHz of 250 ma through our single wire (Fig 3). When we clamp the snap-on core onto the wire, the resistance is about 147 ohms which means that about about 9 watts will be dissipated in the core. This core is 0.75 x 0.75 x 1.25 inches long. I would say it would not be long for this world. That is why these ferrite snap-ons are good for common mode currents which are generally small but should not be used for a differential current like a decoupling choke where the RF current could be large unless the resistive part is small enough not to cause a power heating problem.


One interesting feature in Figure 4 is the inductive reactance starting to turn over. Out of curiosity, I extended the sweep range to 170 MHz to see where that was going. Figure 5 shows the results of that frequency sweep extension.

Figure 5: Extended Frequency Scan Two Turns of a 12 Inch Wire With a Snap-On Ferrite of Unknown Material. (click here for a higher resolution graph)

As one can see from Figure 5, theta goes through zero and becomes negative at 53.506 MHz. The reactance part of the impedance becomes capacitive at this point and Zmag turns over and starts becoming less. The resistance component is decreasing, and the capacitive reactance is increasing. However, the resistance continues to decrease after the capacitive reactance levels off. Since Zmag is a function of both these quantities, it decreases. Now, one might speculate that the capacitance between those two turns of wire is what caused this resonance condition. The maximum reactance over the whole frequency range occurs at 20 MHz and is about 400 ohms, which translates to an inductance of about 3.2 uH. The capacitance needed to resonate that inductance at 53 MHz, is about 2.8 pF. If I place two wires adjacent to each other as they might be placed as they pass through the snap-on core, the capacitance between the two wires is about 2.5 pF. So, I have concluded that it is the inter-winding capacitance of the two wires that cause the inductance to turn over, decrease, and finally become zero at the resonant point.

So what does all this mean. Traditionally one thinks of a core material as a concentrator of magnetic field lines. In other words, the core brings the field lines closer together which raises the inductance. The amount that this is done is reflected in the permeability of the core. Air has a permeability of one. A type 43 material ferrite has an initial permeability of 850. The interesting thing about ferrite permeability is that it is not a constant, but instead a complex quantity consisting of resistance and reactance. I am certainly no expert on magnetics, so for a good discussion on this subject, click HERE.



In contrast, to the ferrite core plots above, I scanned a T-80-2, powdered iron core, with 19 turns. That scan is shown below in Figure 6.

Figure 6: Nineteen Turns on a T-80-2 Powdered Iron Core. (click here for a higher resolution graph)

As you can see from Figure 6, the resistance remains zero and the self-resonace is very sharp. Theta switches from +90 degrees on the low frequency side of resonance to -90 degrees on the high frequency side of resonance just as you would expect for an air inductor. This behavior for the powdered iron core is very different from the ferrite cores shown above.



Figure 7 below shows the jig I used to actually determine the attenuation one might expect from a given ferrite inductor.

Figure 7: Test Jig for Measuring Attenuation of Cores

In order to determine the actual attenuating capability of a ferrite core, I wound several FT-114-43 and FT-114-77 cores and placed them in the jig shown in Figure 7. In order to determine a reference level, I substituted a short for Z unknown and recorded the reading. When Z unknown is of some value that causes an attenuation of > 20 db, then the output of the 10 db pad is essentially open so the output rises by 6 db. To compensate for this, 6 db of attenuation should be added to every reading. When one does this, one can calculate the expected attenuation by using the measured Zmag from the AIM 4170. The expected attenuation factor in db is given by :
Attenuation = 20* Log((Zmag + 50)/50). This will yield a result within a few db of the measured values.



Figure 8 shows an impedance plot for a 1 mH choke consisting of 30 turns on an FT-114-77 core. Figure 9 is an expanded view showing the parameters over the 40 meter band.


Figure 8: 30 Turns on an FT-114-77 Core. (click here for a higher resolution graph)

Figure 9: 30 Turns on an FT-114-77 Core Expanded View for 40 Meters. (click here for a higher resolution graph)

When you first look at Figure 8, it looks terrible. But let's take a closer look. First of all, the reactive part of the impedance is capacitive from the start, but the capacitance is small so the reactance is still pretty large for the 40 meter band where I am using this core. Also, the resistance appears in series with this reactance. In fact, Figure 9 shows that the impedance is almost flat over the 40 meter band with a Zmag = 5570 ohms at 7220 kHz. The attenuation in our jig shown in Figure 7 should be 20*Log((5570+50)/50) = 41 db. When I placed it in the jig, I measured 43 db. I would have done a similar plot for an FT-114-43 core, but the only one I had on hand is in a halogen light in a neighboring building. Nevertheless, the plot would have been similar with a little more attenuation at the higher frequencies.


The curves shown in Figure 10 are the results of attenuation measurements of both FT-114-43 and FT-114-77 cores on all the HF ham bands.



Figure 10: Attenuation of FT-114-77 and FT-114-43 Cores vs Frequency. (click here for a higher resolution graph)

As you can see from Figure 10, more turns do not always mean more attenuation as seen in the blue and tan curves. The literature has always recommended a type 77 material ferrite for the lower HF bands and a type 43 ferrite for the upper HF bands, and the red and black curves confirm that. And, not surprisingly, a 43 core in series with a 77 core yields good attenuation across the whole HF band as seen by the green curve. Figure 10 gives a quantitative result for what we have known qualitatively for years.



One of the most common uses of ferrites in the ham community consists of a series of ferrite beads slipped over the coaxial feed line of an antenna in the form of a choke balun. These ferrite beads attenuate the RF current that may be flowing on the outside of the coax due to an unbalanced feed line feeding a balanced antenna or radiation of the antenna onto the feed line. Figure 11 shows a plot of a piece of wire with seven SB-1020-43 beads slipped over it. These beads are made from type 43 material and will slip over RG-8 type coax.




Figure 11: Attenuation of Seven SB-1020-43 Ferrite Beads. (click here for a higher resolution graph)

As you can see from the plot in Figure 11, the attenuation on 160 meters is mainly due to the inductive reactance part of the impedance, whereas the attenuation at 10 meters is mainly due to the resistive part of the impedance. If we assume the minimum impedance is 4 X 50 = 200 ohms, seven beads is just adequate for the 160 meter band. From our experience from previous measurements, if we slipped a few more beads on the coax, we would see that the reactive part of the impedance would be capacitive at the 10 meters frequency, but Zmag would still be large and give plenty of attenuation.


Conclusions
I hope that this analysis has been interesting and has explained some of the strange results you may have gotten when using ferrites in the past. Ferrites are wonderful materials that I feel we as hams will be using more and more. With antennas being forced down lower, the chances of interference to less and less robust consumer electronics are increasing. A common mode choke using either ferrite snap-ons or ferrite cores can eliminate a lot of this interference. Also, many new lighting plans include halogen lights with switching power supplies that generate interference at high frequencies. Common mode chokes may be useful in these situations, also. Many consumer devices such as plasma TVs will radiate into our HF bands. Common mode chokes on leads emanating from these devices may very well cure the problem.

I learned from these measurements that the ferrite material itself can be responsible for an inductor having a high resistive component . The permeability of the material , which is a function of frequency, accounts for the interesting behavior the inductance over the frequency range. Also, there is resistive component that varies with frequency.

The maximum attenuation of a wire passing through or wound on a ferrite core does not necessarily occur at the frequency where the inductive reactance is maximum, but instead where the magnitude of the impedance is maximum. Useful attenuation may even occur at a frequency that is higher than the self resonant frequency where the reactance is capacitive.

The type of material one uses for a common mode choke whether it be 33, 43, 61, or 77 material is a function of the band of frequencies that need to be attenuated. A core material should be chosen such that the frequencies to be attenuated are near the frequency where theta crosses zero. However, good attenuation may be had at frequencies removed from this resonant point because of the low Q of the coil.

One must consider resistivity and permeability when slipping ferrite beads over a conductor to gain some common mode attenuation, because the attenuation and loss is effected by all of these parameters.

Also, a good understanding of the loss factor of a ferrite might save you from a very hot core.

Remember that there is big difference between a ferrite core and a powdered iron core. A coil wound on a powdered iron core behaves much like an air core as long as the core does not saturate, but a coil on a ferrite core may be very lossy. Also, since permeability is temperature dependent a ferrite core should not be used in a resonant circuit.

If nothing else this strange phenomenon of a coil of wire looking resistive and even capacitive has been interesting to me. I hope you have found it interesting, too!

Member Comments:
This article has expired. No more comments may be added.
 
All Toroids Are Not Created Equal  
by N4DSP on April 14, 2007 Mail this to a friend!
Nice real nice. This is what eHam should post more often.
Thank You guys.

73 All
john-n4dsp
 
All Toroids Are Not Created Equal  
by KI6CDF on April 14, 2007 Mail this to a friend!
Thank you for your thought provoking article. The impedance/reactance measuring instrument provided you with some great information.

I'm puzzled by one issue though. How much of the toroid heating is a result of hysteresis, particularly at the higher frequencies? You did not mention that in your article.
 
RE: All Toroids Are Not Created Equal  
by W0IVJ on April 14, 2007 Mail this to a friend!
KI6CDF asks:

I'm puzzled by one issue though. How much of the toroid heating is a result of hysteresis, particularly at the higher frequencies? You did not mention that in your article.
--------------------------------------------------------
That is a good question! Like I said, I am no expert on magnetics, but as I understand it, hysteresis loss happens when the B-H curve does not track back on itself. This is enhanced when the core is driven into saturation. The link below explains hysteresis loss far better than I can.

http://www.ep2000.com/Templates/white%20papers/MagneticDipolesEP.pdf

73, Tom
 
RE: All Toroids Are Not Created Equal  
by K3AN on April 14, 2007 Mail this to a friend!
Consider what happens with a coax-fed dipole antenna. The current from the center conductor of the coax goes out onto one leg of the dipole. The current from the inside of the coax shield goes out onto the other leg of the dipole as well as down the outside of the shield. The division of current follows Ohm's law, and depends on the relative impedance of both conductors at the frequency of interest.

Installing ferrite beads over the coax at the feedpoint raises the impedance in the outside shield path, forcing more of the current onto the dipole leg and less current down the shield. So cores in series not only provide more dissipation (the heat is distributed among several cores) but the added impedance also reduces the current and therefore reduces the amount of power the cores have to dissipate.

It seems logical, but to be honest I'm not sure whether it's a proper analysis. Any comments?

 
RE: All Toroids Are Not Created Equal  
by W6TH on April 14, 2007 Mail this to a friend!
.
I'm puzzled by one issue though. How much of the toroid heating is a result of hysteresis, particularly at the higher frequencies? You did not mention that in your
article.
--------------------------------------------------------
That is a good question! Like I said, I am no expert on magnetics, but as I understand it, hysteresis loss happens when the B-H curve does not track back on itself. This is enhanced when the core is driven into saturation. The link below explains hysteresis loss far better than I can.

.....................magnetic saturation...............
........hysteresis can be called;inefficiency........

The relationship between magnetic field strength (H) and magnetic flux density (B) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation.

.:
 
RE: All Toroids Are Not Created Equal  
by W6EM on April 14, 2007 Mail this to a friend!
K3AN said:"Consider what happens with a coax-fed dipole antenna. The current from the center conductor of the coax goes out onto one leg of the dipole. The current from the inside of the coax shield goes out onto the other leg of the dipole as well as down the outside of the shield. The division of current follows Ohm's law, and depends on the relative impedance of both conductors at the frequency of interest.

Installing ferrite beads over the coax at the feedpoint raises the impedance in the outside shield path, forcing more of the current onto the dipole leg and less current down the shield. So cores in series not only provide more dissipation (the heat is distributed among several cores) but the added impedance also reduces the current and therefore reduces the amount of power the cores have to dissipate.

It seems logical, but to be honest I'm not sure whether it's a proper analysis. Any comments?"

Just a short one. RF is alternating current. As such, as you describe current from from the inner conductor "out" on one leg of the dipole, current comes back "in" on the shield from the other leg. In other words, the direction of current flow from the coax center conductor is out, the shield, back into the shack.

Keeping that thought in mind, whether one core, or, as you point out, many, when slipped over coax, form what is considered essentially a "single" turn transformer.
In fact, it is what is referred to classically as a current transformer. What is supposed to happen, is that the magnetic field in the toroidial core(s) will cancel. That happens since the "out" direction creates a magnetic field, and forces equal and opposite current flow in the shield in the opposite direction back "in". Now, you want that to occur, and if everything's perfectly balanced, it should happen anyway. But, the core(s) help that to happen via the magnetic field forcing equal and opposite current in the other conductor---the shield. That's where the balun effect comes in. The core forces differential action to happen to cancel the fluxes in the core.

The other advantage is creating an overall common-mode impedance to same-direction currents in both shield and center conductor. Things like transient pulses of the same magnitude on both dipole legs would be helped by having common-mode series inductance. Yes, theoretically, even lightning currents can be attenuated in coax via such additional inductance if a very good ground path exists to a good ground.

73,

Lee
W6EM



 
RE: All Toroids Are Not Created Equal  
by W0IVJ on April 14, 2007 Mail this to a friend!
K3AN said:"Consider what happens with a coax-fed dipole antenna. The current from the center conductor of the coax goes out onto one leg of the dipole. The current from the inside of the coax shield goes out onto the other leg of the dipole as well as down the outside of the shield. The division of current follows Ohm's law, and depends on the relative impedance of both conductors at the frequency of interest.

Installing ferrite beads over the coax at the feedpoint raises the impedance in the outside shield path, forcing more of the current onto the dipole leg and less current down the shield. So cores in series not only provide more dissipation (the heat is distributed among several cores) but the added impedance also reduces the current and therefore reduces the amount of power the cores have to dissipate.

It seems logical, but to be honest I'm not sure whether it's a proper analysis. Any comments?"
--------------------------------------------------------------

I believe your analysis is correct. W9CF points to the models of a choke balun derived by W7EL and W2DU in the link below:

http://fermi.la.asu.edu/w9cf/articles/balun/balun.html#SECTION00030000000000000000

73, Tom
 
All Toroids Are Not Created Equal  
by N0EW on April 14, 2007 Mail this to a friend!
Nice article, thank you for sharing this with us. As one of the above said, "more!" hehe.

One comment / question regarding "Figure 11: Attenuation of Seven SB-1020-43 Ferrite Beads" has to do with what I recall of Walter Maxwell W2DU's comments about this style of balun...

I thought he recommended upwards of 25 or more cores to be placed on the coax, and had commented a number of times that using less isn't a fair comparison. I thought he pointed out too few cores would be a poor performer.

Question 1 - When you added the types of mix in series, did each have 7-cores, for a total core count of 14?

Question 2 - Have you considered a comparison of the 7-cores as you presented above to a much greater number of cores, as a minimum 25-cores? I think that'd be in interesting comparison.

Regarding the question about the dipole, I have two comments...

First, perhaps I misread it, but I had the impression the cores were being placed at the station end of the coax, as opposed to near the antenna feedpoint. I have always presumed such baluns were at the antenna feedpoint to be effective. I question how useful they'd be at the greatest distance from the antenna feedpoint, given common antenna installations.

Second, the answer --as I understand it-- is all about the impedance at the antenna feedpoint. The current will follow the path of least resistance, or split along impedances sufficiently close to one another (the 3-leg theory). If the impedance is 10 times greater on the outside of the coax shielding as compared to the inside of the shielding, all is well.

If the impedance on the outside of the coax shielding is much less than on the inside of the shielding (or they are similar in value) trouble is brewing.

So there are four impedances interplaying at the antenna feedpoint when using coax, the: (1) antenna; (2) center conductor; (3) inside of coax shield; and (4) outside of coax shield.

To fully answer what takes place, all four impedances must be taken into account. If this is wrong, I stand ready for edification! ( But talk slowly and be painfully clear; I may be an idiot! Hehehe... )

--End Aside--


I really enjoyed reading your article. Thanks again. I'll have to think about eating rice and beans for a while a get one of those AIM 1470's. Looks like a lot of fun!

Thanks again,
Erik n0ew
 
RE: All Toroids Are Not Created Equal  
by N0EW on April 14, 2007 Mail this to a friend!
Clarification.... Regarding placement of the balun in the dipole question....

It depends upon where you are trying to suppress potential RF radiation. If you wish to suppress feedline radiation the balun is properly placed at the antenna feedpoint.

If that isn't a problem / concern, and one wishes to only suppress RF radiation in the shack, that W2DU balun is placed near the shack entry point. This would attempt to keep RF out of the shack, but not attempt to suppress RF along the feedline.

Such is my understanding. It remains possible I am a terribly disturbed individual ;) Hehehe
Erik n0ew
 
RE: All Toroids Are Not Created Equal  
by W6EM on April 14, 2007 Mail this to a friend!
W0IVJ, Tom said:"K3AN said:"Consider what happens with a coax-fed dipole antenna. The current from the center conductor of the coax goes out onto one leg of the dipole. The current from the inside of the coax shield goes out onto the other leg of the dipole as well as down the outside of the shield. The division of current follows Ohm's law, and depends on the relative impedance of both conductors at the frequency of interest.

Installing ferrite beads over the coax at the feedpoint raises the impedance in the outside shield path, forcing more of the current onto the dipole leg and less current down the shield. So cores in series not only provide more dissipation (the heat is distributed among several cores) but the added impedance also reduces the current and therefore reduces the amount of power the cores have to dissipate.

It seems logical, but to be honest I'm not sure whether it's a proper analysis. Any comments?"
--------------------------------------------------------------

I believe your analysis is correct. W9CF points to the models of a choke balun derived by W7EL and W2DU in the link below:

http://fermi.la.asu.edu/w9cf/articles/balun/balun.html#SECTION00030000000000000000

73, Tom"

Tom: If you look carefully at the simplest example, W7EL's ideal transformer, unbalanced coax, Figure 7, notice that the ZBalun is across the shield path. But, it is a correct model to show impedance across an ideal current source, the current transformer. It only has a voltage induced across the impedance, when resultant common-mode current flows. And, that's what you want in an ideal balun, as W7EL later summarizes. Under true, balanced, differential current flow (equal center conductor and equal and opposite shield current, no net current should be seen through the impedance ZBalun.

No, its not simple current division at all. He is confused, I think, by the twin-lead model of W2DU's which has little relevance to unbalanced coax reality.

Keeping differential mode impedance very small should be the goal, while increasing common-mode impedance is desirable.

73,

Lee
W6EM
 
All Toroids Are Not Created Equal  
by K6GC on April 14, 2007 Mail this to a friend!
A very interesting and informative article, KUDOS!

It has been my experience using a Choke Balun of the type described herein that the balun not only stops common mode currents, it also attenuates them on many frequencies.

To Wit: I once had an Unadilla multi-band trap dipole. As the sun destroyed the plastic upon which the windings were made, deep "Vees" developed as the plastic shrunk and could no longer cover the circumference. Rain water ran into these cracks and shorted the capacitor in the traps. The first to go were the 80 meter traps, followed in more or less chronological order right up to the 10 meter traps. After initially replacing the first few traps, I realized the futility of replacement and I put jumpers around the damaged traps.

To operate the antenna on bands where the traps were damaged, I attempted to load the coax against ground as a "T" antenna. What I found was that my signals were greatly attenuated and I received poor signal reports. I "fixed" this by running more power and found evidence of heating on the balun at the feed point.

Since this experience, I have formed a new plan of action:

Now days I connect my coax directly to the dipole with no balun of any sort. I treat the coax as though it was open line. That is, if the bottom of the coax is grounded, as normally is the case, it should form an even number of quarter waves from the ground to the feedpoint. This will assure a high impediance to common mode currents. In the old ARRL handbooks you can find a discussion of feedline lengths, both grounded and ungrounded at the transmitter end, and charts showing "best" lengths for the various bands that will present high impedances for common mode currents.

I would like to suggest this as a possible alternative to the use of baluns.

TR, WB6TMY
 
RE: All Toroids Are Not Created Equal  
by WE9L on April 14, 2007 Mail this to a friend!
Use mix 31 beads instead of 43 or 77. Mix 31 is the better attenuator for ham radio frequencies between 1 and 60 MHz.

Harvey WE9L
 
RE: All Toroids Are Not Created Equal  
by AE6RO on April 14, 2007 Mail this to a friend!
I guess everyone else is either on a hot date, yakking on 75 meters, or just passed out. 73, AE6RO
 
RE: All Toroids Are Not Created Equal  
by KC6TOA on April 14, 2007 Mail this to a friend!
Reminds me of reading the "measuring beads" section of K0BG website. For measuring surplus beads, i would venture a guess the AM-4170 would be superior to the MFJ-259B.
 
RE: All Toroids Are Not Created Equal  
by KI4NX on April 15, 2007 Mail this to a friend!
Great article and good comments. This is what ham radio experimentation is all about. And, all this is timely for me. I've been building some baluns for the past several weeks and not getting the results I was expecting. With this article I have more insight about what's happening. Now, I have to get that piece of equipment for better design and construction. I just can't afford it just yet - maybe I could after the IRS finishes all their mangling of my '06 taxes.
 
RE: All Toroids Are Not Created Equal  
by N3OX on April 15, 2007 Mail this to a friend!
"To operate the antenna on bands where the traps were damaged, I attempted to load the coax against ground as a "T" antenna. What I found was that my signals were greatly attenuated and I received poor signal reports. I "fixed" this by running more power and found evidence of heating on the balun at the feed point. "

You were trying to drive current on the outside of the coax shield ON PURPOSE! Definately have to take a choke balun out in that case.

Dan
 
RE: All Toroids Are Not Created Equal  
by ARRLBOOSTER on April 15, 2007 Mail this to a friend!
Sorry too tell you this, but the differential currents cause a more than zero net flux at the core. This amount is logarithmic and although a small degree..it renders your calculations invalid. Go back to school...
 
RE: All Toroids Are Not Created Equal  
by AE6RO on April 15, 2007 Mail this to a friend!
Toroids are too newfangled for me. Although the low Q makes them well suited for chokes. Just give me an old broomstick or toilet paper tube. 73, AE6RO
 
RE: All Toroids Are Not Created Equal  
by WA1WIG on April 15, 2007 Mail this to a friend!
>>I'm puzzled by one issue though. How much of the toroid heating is a result of hysteresis, particularly at the higher frequencies? You did not mention that in your article.

Hysteresis itself causes no REAL power loss. All energy stored is returned to the system, if at a different phase. Thus hysteresis can appear as a reactive component, not a resistive component needed for real power loss.

Cores of any type may have real loss, that is a different topic than hysteresis. A classic example in iron core transformers is Eddy currents and the resistivity of the core material and it dissipating any currents in the core material.

73 gerry WA1WIG
 
RE: All Toroids Are Not Created Equal  
by W0IVJ on April 15, 2007 Mail this to a friend!
Cebik gives us a good look at the common mode situation we encounter when feeding a dipole with coax. His conclusions are that we should use a choke balun at the feed point of the dipole and possibly at the point where the coax enters the shack. One thing that is interesting about Cebik's choke balun is that it is only inductive, so there is attenuation but no power heating of the core. If you are using a ferrite where there is resistive part of the impedance, one has to be careful of the placement of the balun and of the power that will be dissipated in the beads themselves.

For example: I constructed an EZNEC model similar to Cebik's of the common mode situation. I modeled a coax of 1/2 wavelength connected to the ground on one end and to a 40 meter dipole on the other end. This dipole was 0.6 wavelength over average ground. I do not have the expensive EZNEC, so I cannot tie a wire to the ground. To compensate for this, I constructed 30 radials, 1/2 wavelength in length. These radials were 0.1 wavelength above ground. I then set the power to 100 watts and looked at the current in the simulated coax shield right beneath the dipole where one would place a choke balun. The common mode current was at a maximum at this point and the current was 1.3 amps. This resuslts in an unbalance of current in the two legs of the dipole of 1.2 amps. I then determined the choke balun impedance at 40 meters from figure 11 to be 260+J500. I constructed an EZNEC load of this value and placed it in the simulated coax shield right at the feed point of the dipole. This took the dipole leg imbalance down to 0.04 amps. However, due to the resistive part of the impedance, the beads dissipated 3.5 watts. If a balun could be constructed such that there was no resistive part and the impedance was all inductive, then no power would be dissipated in the balun. However, if one looks at figure 6, one can see that the reactive impedance varies considerably over the frequency range so care must be taken to see that the reactive impedance is high enough in the frequency range of interest to choke out the common mode current. If one was using a ferrite close to its resonance point where the resistance was high and the reactance was close to zero, as shown in figure 5, the balun could get very hot. For example if the reactance went to zero in our 7-bead choke the impedance would be 260+J0 resulting in 12 watts being dissipated in the beads.

One of the reasons why I wrote this article, was to distinguish between the use of toroids that have a resistive part and those that don't. You can see from the modeling above the perils of not noting the differences.

The URL below points to Cebik's in depth study of the common mode situation when feeding a dipole with coax.

http://www.cebik.com/trans/cmp.htmCebi

73, Tom W0IVJ
 
RE: All Toroids Are Not Created Equal  
by W0IVJ on April 15, 2007 Mail this to a friend!
I'm sorry. Try this URL for Cebik's article on common mode currents in a coax feeding a dipole.

http://www.cebik.com/trans/cmp.htm

73, Tom W0IVJ
 
RE: All Toroids Are Not Created Equal  
by WA1WIG on April 15, 2007 Mail this to a friend!
I should have phrased hysteresis "loss" better.

Hysteresis does not cause loss, it is the effect of core losses! Some real energy is dissipated when a magnetic material is magnetized. Supplying an equal but opposite polarity magnetizing force will also dissipate energy thus the material will not return to the same state before it was magnetized and then demagnetized with equal but opposite energy. Non-linearity is a factor, just gets complicated ;)

It's a fine detail, but hysteresis is an effect of losses, not a cause.

73 gerry WA1WIG
 
All Toroids Are Not Created Equal  
by NORTHCOUNTRY on April 16, 2007 Mail this to a friend!
Great article and good demonstration of the instrument from Array Solutions
 
RE: All Toroids Are Not Created Equal  
by NO9E on April 17, 2007 Mail this to a friend!
W8JI has measurements of several types of baluns on his site.

Heating of a balun depends on frequency, impedance and common currents. A balun may be cold with 100W with one antenna and warm at 10W with another antenna. The worst case is off-balance and high impedance.

Once I tried to constrct a balun for my 160m vertical that consited of 2 x 70 ft elevated radials and a 100ft vertical radiator. Every balun that I tried overheated at 500W when running a frequency. Changing a balun to a transformer eliminated any sign of heating even at 1.5 KW.

73,
Ignacy, NO9E
 
RE: All Toroids Are Not Created Equal  
by W0IVJ on April 18, 2007 Mail this to a friend!
Danny, a friend of mine, gave me a URL to an excellent article on ferrites and baluns. I hope some of you find it useful.

http://audiosystemsgroup.com/RFI-Ham.pdf

73 Tom W0IVJ
 
All Toroids Are Not Created Equal  
by K5YEF on April 25, 2007 Mail this to a friend!
Fantastic article! Hope to see more from you in the future.
 
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