The toroidal inductor is an indispensable component in amateur shortwave radio equipment. It is commonly found in RF circuits for filtering/attenuation, differential/common-mode suppression, and choke (e.g., balun) lines, as well as traditional power conversion, EMC solutions, and PFC (power factor correction). I have a basic understanding, and I'm attempting to write a brief introduction on selecting toroidal inductors. There may be inaccuracies or omissions, but I hope it will serve as a starting point.
The materials used for magnetic rings are mainly divided into:Ferrite, magnetic powder, silicon steel, and amorphous metalRegarding the four main categories (although this categorization may not be entirely accurate), I have a better understanding of the first two. Their shapes are also very diverse, including those resembling small beads, cable-clip types, and common ring, two-hole, E-shaped, and rod-like (for medium wave antennas). Sometimes, due to their shape, they are also called magnetic core, magnetic bead, or magnetic bar.
Iron oxide magnetic ring
The specific materials for ferrite rings (note: do not confuse with "fair-rite"), can be further divided into nickel-zinc ferrite and manganese-zinc ferrite. The initial magnetic permeability of these types of ferrite rings is around 100-1000. They have low core loss, especially nickel-zinc ferrite rings, which are preferred by Baolin. Their appearance also resembles the traditional gray-black magnets that people use.
Core loss refers to the energy lost due to hysteresis, eddy currents, and residual effects in the core material.
Eddies are currents that are induced within the magnetic material, which then convert directly into heat. The hysteresis phenomenon refers to the heat generated during magnetization to overcome friction. Remaining losses occur due to the magnetic induction strength B lagging behind the change in magnetic field strength, which results in energy loss. [I may have explained this incorrectly; please correct me.]
Ferrite magnetic rings, due to their material composition, are sometimes referred to as Ni-Mn ferrite. These ferrites typically have a protective coating applied, and the uncoated surface is prone to powdering and degradation. The primary use of these magnetic rings is for cable filtering, such as the plastic housing found on common data cables (e.g., those used with printers).
Manganese-zinc iron oxide magnetic rings are easy to magnetize and demagnetize. The initial magnetic permeability (Ui) is higher than that of iron oxide, approximately 1000+? Visually, they appear smooth and less prone to powdering, with a dark color. If painted, it's typically green paint. Material designations usually start with K, such as K5, K5B, or K7.
Metal magnetic powder magnetic ring
Metal magnetic powder materials are also known as iron powder core materials, containing resin, carboxyl-based adhesives (?), and iron powder. They are further divided into various material formulations with suffixes such as -2, -8, -10, -14, -18, -26, -28, -33, -38, -40, -45, -42, etc.
Its initial permeability is around 10-100. These types of toroidal inductors are typically small and used in amateur radio equipment. They can be used for various purposes, such as:
* Providing good DC saturation characteristics
* Being suitable for high frequencies
* Using power conversion and line filtering
The -8, -2, and -33 materials all have relatively good linearity, but the -33 material has a higher magnetic core loss. The -8 material is generally preferred for use in anti-noise coils.
With so many materials available, how do we differentiate them? Fortunately, there's a standard coating system (see table below) to use.
| Material suffix | Initial magnetic permeability | Painting |
|---|
| -2 | 10 | Red/Transparent |
| -8 | 35 | Yellow/Red |
| -10 | 6 | Black/Transparent |
| -14 | 14 | Black/Red |
| -18 | 55 | Green/Red |
| -26 | 75 | Yellow/White |
| -28 | 22 | Gray/Green |
| -33 | 33 | Gray/Yellow |
| -38 | 85 | Gray/Black |
| -40 | 60 | Green/Yellow |
| -45 | 100 | Completely black |
| -52 | 75 | Green/Blue |
Silicon steel magnetic ring and amorphous magnetic ring
I have rarely encountered these two types. Silicon steel magnetic rings are typically made by stacking or winding multiple thin sheets. Depending on the specific alloy material, silicon steel magnetic rings can be further divided into iron-silicon-aluminum, boron-iron-alloy, and iron-silicon-molybdenum varieties. Some articles also mention a high-permeability magnetic ring coated with blue paint, but I'm unsure which category it belongs to. Specific materials have suffixes such as -026, 060, 075, 090, 125, 147, and 160. They are commonly used in output inductors for switching power supplies, PFC (power factor correction) inductors, and resonant inductors, as well as for noise filtering in electrical circuits.
The initial magnetic permeability of the amorphous magnetic ring is 10 k.~Hundreds of k, typically made of plastic casings. When opened, they contain coils. This is because the material is relatively brittle and prone to breaking. They are commonly used for making common-mode inductors to suppress low-frequency conduction interference. I haven't seen any applications in wireless radio yet.
Amidon encoding and Fair-rite part number
In the introduction we just had, we all noticed that a magnetic ring has one. Sometimes we hear HAMs say "FT240-43", and sometimes we see a string of 10 digits (which may also be labeled as "fairrite"). What is the relationship between these two?
Yes, but not entirely. Let's start with the first type of encoding: systems starting with "FT" are based on the Amidon naming convention used in the American amateur radio community. "FT" stands for ferrite trodoid, and the numbers that follow represent the outer diameter (in inches), such as FT114 = 1.14 inches, which means an outer diameter of 29mm. The number after the hyphen is the material formula code.
But which specific FairRite magnetic ring is it?
Fair-rite is a well-established manufacturer of ferrite magnetic rings based in the United States, with a history dating back approximately seventy years. They have an internal part number system, such as 5943001001. The first two digits, "59," indicate the series (you might occasionally see those starting with "26"), followed by the material formula (which is the "43" from FT240-43). The remaining six digits are an internal serial number and do not follow a pattern. (By the way, the second to last digit represents the coating material: 1 is for Poly toluene, 2 is for thermosetting plastic, and 0 indicates no coating.)
If you want to determine the corresponding relationship, you can only check the inner and outer diameter height on its product page, and then see if it can be classified into a specific Amidon code on toroids.info.
Of course, magnetic loops are not produced by Fair-rite alone. Early amateur radio articles mentioned NXO-100, NXO-80, and others with domestic designations, as well as NGO, GTO, Rx**C, and R*H*For example, the series. Personally, I think that products from domestic manufacturers are mainly used in industrial applications and are not specifically produced for the HAM (amateur radio) market. Furthermore, due to path dependence, people have become accustomed to using Amidon/Fair-rite systems.
Please note that for some domestically produced models, the specifications only define properties such as magnetic permeability, resistivity, magnetic flux density, and operating frequency. However, even for the same model number, there can be multiple size specifications. For example, NXO-100 has at least 8 different size specifications, including 4 ring-shaped and 4 double-hole versions, which can create some inconvenience for buyers.
Then, let's look for one.
First, open a distributor's sales page for a magnetic disk and check what parameters are listed. In this case, I found the part number to be 5943001101.
This magnetic loop antenna comes from unibalun – a Balun/UnUn design with customizable transformation ratios, which requires a larger FT82-43.（5943000601)Please use it.
The Mouser warehouse shows:
[image-20251110091330527](C:\Users\Administrator\Desktop\认识和挑选磁环\image-20251110091330527.png)
It can be seen that it is a ferrite magnetic ring, with an outer diameter of 13, an inner diameter of 7.9, and a height of 6.4. The material composition is "43", sold in bulk.
Open its manufacturer's page again. Circular （5943001101） - Fair-Rite There are four options: basic information, electrical information, mechanical information, and material information.
The basic information describes its basic materials, dimensions, and applications (specifically, "suppressing conduction EMI from 20 MHz to 250 MHz. This material is also used in inductor applications, such as high-frequency common mode chokes."). It also specifically lists the properties of -43 material, providing several parameters: initial permeability (μi), residual magnetic flux density Br, coercivity Hc, loss factor tanδ/μi, and initial permeability temperature coefficient αμi.
In simpler terms, a higher initial permeability results in a higher inductance for the same current. However, excessively high inductance can lead to saturation. The magnetic permeability of material -43 is at an intermediate level, and it's also mentioned that it's commonly used in EMI suppressors and variable transformers.
If you're not interested in electrical constants, you don't need to read this section specifically.
Σl/A (Core Constant) Permeability constant: Used to estimate permeability (reluctance). Simply put, it represents the resistance of a magnetic material to magnetic flux. The higher the value, the smaller the inductance.
(Effective Path Length) Effective magnetic path length – I'm not sure what this refers to.
Ae (Effective Cross-Sectional Area) Effective cross-sectional area: This is used to calculate magnetic flux density B, where B = Φ / Ae. A larger effective cross-sectional area indicates a higher magnetic saturation capability.
Effective Core Volume (ECV) Effective core volume – The larger it is, the greater the power it can handle, but there are also side effects, and losses may be higher (especially for applications such as baluns/unnus with variable impedance/voltage transformation).
AL (Inductance Factor) Inductance coefficient: The inductance generated by each coil on the core, measured in nF/turn². Calculate the inductance value using.
Alternatively, you can refer to the user manual for the magnetic ring, which contains similar information.
Under the Fair-Rite product page, there are several graphs showing how permeability decreases as frequency increases (note the x-axis), and the curve of permeability changing with temperature (which shows a rapid decrease after 160°C). The third graph shows the hysteresis loop at the reference frequency (10 kHz) (i.e., the change in magnetic flux density B with changes in magnetic field strength H, which clearly shows saturation), as well as curves showing how permeability decreases with temperature or DC bias at different frequencies.
There are several key indicators to be aware of, such as Curie temperature (Tc), which should ideally be around 130°C. Once this temperature is exceeded, the material loses its ferromagnetic properties. In practice, the usable operating temperature limit needs to be reduced by another 20–30 °C, so it's important to monitor the load on the magnetic ring.
Additionally, there is the resistivity ρ (measured in Ω·cm). The lower the resistivity, the lower the high-frequency eddy current losses. Manganese-zinc materials typically have a resistivity of around 10²–10⁴ Ω·cm, while nickel-zinc materials can be three orders of magnitude higher.
If you were a mischievous child and disassembled the transformer's primary coil, you should have seen the thin sheets of silicon steel stacked together. Each sheet was made so thin to reduce eddy current losses.
So, how should I choose?
Based on a practical approach, I first recommend that novice HAM radio enthusiasts start with simple assignments to avoid unnecessary detours. For example,
If you want to choose your own equipment, the first thing to consider is the maximum (short-term) power tolerance. Given applications like FT8 that use a high percentage of bandwidth (i.e., transmitting very frequently and densely), it's advisable to leave a 2x margin to avoid overheating. For example, if I'm using a QRP transmitter with a maximum power of 5W, I could consider choosing one with a maximum tolerance of 25W or 20W. According to the experience of others, the FT240-43 can tolerate 500W, the FT114-43 approximately 150W, and the FT82-43 about 50W.
Regarding insulated wire for winding, it is recommended to use polyurethane-coated wire (QA). For larger magnets, choose a wire diameter of approximately 1mm or 1.2mm; for smaller ones, choose 0.65mm. Some vendors use Chinese wire gauges (CWG) and American wire gauges (AWG) for marking.
Secondly, pay attention to the operating frequency. The magnetic flux/permeability of the toroidal inductor is related to the frequency, and this is also shown in the charts on the official website. The operating frequency depends on the application; for example, some HAMs mention that the maximum usable operating frequency of the NXO-100 resonant inductor is 15 MHz, so it is not suitable for HF (High Frequency). I think if you use it to build a Balun, you don't need to worry about the 15 MHz limitation (it has inductance properties, and the 15 MHz limitation is not a problem). Of course, if you are concerned, you can buy the NXO-80. The maximum usable operating frequency of the NXO-80 is either 50 MHz or 30 MHz.
So, where should I go to buy it?
Personally, I recommend buying magnetic rings through the overseas purchasing page on Licheng Mall. You don't need to pay international shipping fees; you only need to pay a small amount of customs duties, and domestic shipping coupons are often available, making it very affordable. Plus, it can guarantee that they are genuine products. The downside is that you need to be patient and wait for about two weeks. Companies like Mouser and Digikey are large electronic component storage companies, but you need to confirm the part number before placing an order. Here's a list of common Amidon models and Fair-rite part numbers (I haven't received any advertising fees from Fair-rite XD).
| Amidon number | Fair-Rite number | Materials | Outer diameter (mm) | Inner diameter (mm) | Height in mm | Reference price per unit / Yuan |
|---|
| FT140-43 | 5943002701 | Nickel-zinc | 35.5 | 23 | 12.7 | 30 |
| FT114-43 | 5943001001 | Nickel-zinc | 29 | 19 | 7.5 | 14 |
| FT114-43 | 2643801002 | Nickel-zinc | 29 | 19 | 7.5 | 12 |
| FT240-43 | 5943003801 | Nickel-zinc | 61 | 35.5 | 12.7 | 81 |
| FT240-43 | 2643803802 | Nickel-zinc | 61 | 35.5 | 12.7 | 56 |
| FT82-43 | 5943000601 | Nickel-zinc | 21 | 13 | 6.35 | 11 |
| FT82-43 | 5943001801 | Nickel-zinc | 22.1 | 13.7 | 6.35 | 6 |
| / | 5943001101 | Nickel-zinc | 13 | 7.9 | 6.35 | 5 |
| FT50-43 | 5943000301 | Nickel-zinc | 12.7 | 7.15 | 4.9 | 4 |
| / | 2643625002 | Nickel-zinc | 15.9 | 7.9 | 14.28 | 8 |
| FT37-43 | 5943000201 | Nickel-zinc | 9.5 | 4.7 | 3.3 | 12 |
Additional Content: How to Test the Magnetic Permeability of a Magnetic Ring
There is a simple algorithm: first, measure the outer diameter D, inner diameter d, and height h (all in mm), then wind approximately ten turns (number of turns n) using enameled wire, and finally measure the inductance L (in uH). The magnetic permeability of the magnetic ring itself is u = 2500.*L*(d + D) / [(D - d)]*h*L²]
I was also planning to discuss the different winding methods for solenoids (such as the difference between current and voltage solenoids), but I thought it would take too long, so I'll start a new post later if I have time.
闽ICP备2021006864号-7
闽公网安备35020602002794号