Dave Arrich AD6AE
Transmission Line Transformers (TLT’s) – Part 1
FERRITE TOROID CORES AND CHARACTERISTICS
INTRO
This series will only address sintered, ferrite material. Powdered iron cores are neither designed nor recommended for RF applications and therefore, will not be covered. Note that ALL HF antennas require a matching transformer or CM choke.
Ferrite toroid cores are doughnut-shaped rings, that are dark grey or black in color, and composed of either of two ceramic mixtures – nickel-zinc (NiZn), or magnesium-zinc (MnZn) to which various forms of iron oxide (NOT powdered iron) has been added to create functional characteristics such as desired permeability, operating frequency range, and temperature tolerance. The materials are combined by sintering. A link explaining core manufacture: https://search.app/4WMGAYmbxWZMMrfZ8
Additionally, many other shapes such as rods, split beads, sleeves, binoculars, etc., are made for specific RF uses. This series will focus on the doughnut-shaped and bead, snap-bead (barrel-shaped) cores for use in antenna matching transformers and common mode line chokes.
Ferrites are classified by a specific “MIX” or ratio of the above compounds to tailor its magnetic properties. Those are listed in Table 1. Naming conventions vary among manufacturers, but most are designated by first a letter, followed by a number and generally have a permeability tolerance of +/- 20%. Cores are relatively inexpensive, but, being made of a brittle, ceramic material, are prone to breaking if dropped on a hard surface (this has already been tested).
Cores are identified by type (ferrite), outer ring diameter (inch), and mix. For example: a Ferrite Toroid having an outer diameter of 1.40-inches and composed of a #61 mix would be identified as, FT140-61. Fair Rite Corp. also lists many of their cores by part numbers such as: 4061378111. The second pair of numbers indicates the mix. The rest of the specs are listed on the data sheet for that core. See Fair-Rite Corp Materials Index – Fair Rite (fair-rite.com) and click on ‘Tech Resources.’ It’s an informative site but takes some poking around for the info.
Easily available cores come in various sizes from very small (⅛”dia. that slips over a resistor or capacitor lead to toroids having an outside diameter of 4 inches or more for custom cores.
So, What’s The Big Deal About Ferrites And TLT’s Anyway?
Simply put, it is about achieving maximum power transfer from source to different loads. In Direct Current circuits, in order to achieve maximum power transfer from source to load, both must present equal internal impedances (or resistance) to the flow of current (Ohm’s Law P=I2R). In RF TLT’s, there’s a little more to it than simply winding as much wire as will fit on a core. But for this series it’s correct to say that TLT’s “translate” the unequal impedances between source and load while minimizing ‘translation’ (or insertion) losses. Many factors cause insertion loss and is beyond the scope of this series but can be minimized with proper core and wire selection, and careful winding geometry.
THE UNUN
UNbalanced to UNbalanced. Also called an autotransformer. This transformers job is to match the UNbalanced impedance of the transmitter and feedline to a myriad of UNbalanced antennas that may be referenced to ground. Ground can be anything from a deeply driven copper rod, multi-wire radial system (ground plane) or an end-fed antenna using a single wire counterpoise. Verticals and end-fed antennas can present a wide range of impedances ranging anywhere between 12 for vertical and 4,500 ohms for end-fed wire antennas. Antennas
THE BALUN
BALanced to UNbalanced. The BALanced terminals connect the output side of the transformer to an antenna , load or instrument. The UNbalanced terminals connect the input side of the transformer to a transmitter, feedline or signal source. This transformers job is to match the unbalanced (or referenced to ground) impedance of the transmitter and feedline to that of an ungrounded, ‘balanced’ or floating antenna not connected to ground such as a dipole. These types of antennas present impedance ranges of 12 – 150 ohms.
THE COMMON MODE (CM) CHOKE
Common mode current is present and problematic in all coaxial feedline systems regardless of where it is terminated. CM chokes, are also referred to as 1:1 baluns, beads, ugly baluns, or line chokes. Common mode current will be discussed in later articles.
PERMEABILITY (μ)
Permeability (μ) is the degree to which a magnetic substance changes or influences the magnetic flux in a magnetic field, and how well the material allows or restricts magnetic lines of force pass thru it. The higher the permeability, the higher the flux flow.
High permeability provides high flow of flux at low frequencies. By the same token, it also restricts the flow of flux at higher frequencies. Specific mixes are generally used for specific frequency ranges. Table 1. shows commonly used cores that are best suited for HF with #31, #43, #52 and #61 Mixes being suitable for TTL applications in this series.
VOLUME RESISTIVITY
A fancy term for the surface resistance of a toroid core. Unmarked cores can ofttimes be identified by measuring the surface resistance with an ohmmeter. A low ohmic reading between 200-2000 ohms indicates an MnZn material. A high or infinite ohmic reading indicates a NiZn material. This method may work to ID unmarked cores at ham-fests but won’t indicate the mix. Further research will give the method used to measure the unknown ‘per turn’ inductance or AL that’s expressed in nano-Henries per turn. Even then, it’s dubious and should be avoided for anything beyond experimentation.
AN EXCELLENT PAPER FROM PALOMAR ENGINEERS
This is one of the best written articles I’ve seen on the subject. So, I’ve pasted it here and tweaked it a bit for clarity (italics are mine). One thing I added to Table 1., was data for a #77 mix that I copied from Fair Rite Corp. It was mentioned in Palomar’s narrative, but not entered in the table.
From Palomar Engineers
Home – Palomar Engineers® (palomar-engineers.com)
Follow the above link for more information.
FERRITE MATERIAL TYPES
Ferrite Mix Selection Guidelines. Quite often we are asked which mix is best for a certain application or frequency range and there is a lot of misinformation on the internet and elsewhere regarding the selection of proper mix for a given application.