Featured: T68-2 / T50-2 Iron Powder Toroid Cores at Amazon.
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Look inside almost any solid-state HF amplifier, low-pass filter or matching network and you will find toroids. A toroid core concentrates magnetic field inside the ring, which keeps inductors compact and reduces coupling to nearby parts. For HF inductors, the red "-2" iron powder cores, such as the T68-2 and T50-2, are among the most commonly used.
What the "-2" means
The number after the dash is the core material. Amidon lists its Material 2 iron powder cores with Permeability = 10 and a frequency range of 2 Mhz - 30 Mhz, color-coded red. Iron powder cores like these are valued for stable inductance and low losses in tuned circuits, which is why they turn up in filters and matching networks.
The number before the dash is the size. Amidon lists the dimensions in inches:
- T50-2: inner diameter 0.300, outer diameter 0.500, height 0.190, AL value 4.9.
- T68-2: inner diameter 0.370, outer diameter 0.690, height 0.190, AL value 5.7.
Amidon gives AL values in nH/Turn2, meaning inductance per turn squared.
Using the AL value
Inductance on a toroid rises with the square of the number of turns. To estimate inductance, multiply the AL value by the square of the turn count, and the result is in nanohenries. Working backward, divide the inductance you need by AL and take the square root to estimate the turns. Winding style and spacing change the result a little, so builders often wind slightly more than needed and remove turns while measuring.
Where these cores fit in amplifier work
A common use is the low-pass filter on an amplifier's output, which reduces harmonics before the signal reaches the antenna. Low-power filters, driver stages and receive circuits are natural places for small cores like these. Core size matters as power rises: a core has to handle the magnetic flux and heat in the circuit, and higher-power output filters use larger cores. Follow the design you are building, and do not substitute a smaller core in a high-power position.
Use magnet wire of a suitable gauge for the current involved, keep the enamel intact where the wire passes over the core edges, and secure the winding so it cannot shift once the inductance is right.
A note on buying cores
The figures above are Amidon's published specifications for its Material 2 cores. Cores sold under other names on marketplaces may be made differently, so check the seller's data before relying on an AL value. Measuring a wound inductor with an LC meter or antenna analyzer is the surest way to confirm what you have built.
And a reminder for any amplifier project: external linear amplifiers are not legal for use on CB radio. Clean output filtering is part of operating any legal amateur station responsibly.
Where to get it
Check current price and availability: T68-2 / T50-2 Iron Powder Toroid Cores at Amazon.
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