Q&A time: What size copper wire is best for a ferrite ring?
Thank you for your inquiry. Choosing an optimal design for a ferrite ring and associated elements, such as wire types and insulation, usually elicits a discussion with many viewpoints, mostly because the various design elements are working against each other: Increasing one element often times reduces another (maybe important) element. It is a complex situation.
Another requirement that makes it hard to choose which elements are going to be prioritised is that amateur radio operators often ask for the ferrite rings that are used in their chokes, UnUns and Baluns to operate satisfactorily over a very wide range of frequencies, maybe 3MHz to 30MHz, putting further pressure on the designer. Ultimately the only way forward is to test the designs in practical, real life situations and optimise from there.
Today’s question and answer is about the best thickness of the wire to use.
The two major design principles when choosing a wire are: Current carrying capacity, and distance from the core. We’ll talk about them in a moment.
Just remember that many other factors come into play to a lesser extent, for example the core cross section, the magnetic path length, the length of the wire (inductance), the ohmic resistance of the wire, insulation material, turns spacing, inter-turn capacitance or even ferrite material. This is a discussion for another time.
Starting with the current carrying capacity (assuming you are going to use copper wire) is to refer to the national safety standards like South Africa’s SANS 10142-1. These standards provide ampacity tables and for our purposes it translates roughly to 14A (e.g. 3.25kW at 240V) for a 1mm2 which is again roughly equivalent to 1.1mm Ø. Safely.
Let me place that on the table: A copper wire of about 1mm can carry about 14 Amps.
Working backwards from a 50Ω antenna accepting a full 100 Watts continuous power, the numbers are about 70.7 Volts and 1.414 Amps. Yip, that is about 10% of the capacity of the wire. And remember we have already allowed the safety factors into the original numbers.
Similarly one can assume for 400 watts an average voltage of 141V and 2.8A, and pushing a Kilowatt down the coax can raise that to 224V and 4.5A … still only a third of the way down the suggested current capacity of a 1mm wire. To summarise: For 100W a wire diameter of 0.4mm is more than sufficient and even less will perform adequately, and going up in range to 1000W a wire diameter of 1mm will perform excellently.
So what happens when you use thicker wire? As my late Father-in-Law used to say: “RF is Art, not Science” and with that he suggested that using thicker wire is not necessarily going to give better results. But why?
Thus the second design element that one should consider when winding ferrite rings has to do with the effect that a thicker wire will have on the overall performance. Two things immediately stand out: The centre of the copper wire in now placed further away from the centre of the ring (and remember that distance is important), plus the thicker wire is more difficult to handle over the number of turns and chafing through the insulation can happen. And don’t even consider using wire with thicker insulation – it exacerbates the original problem, especially at the higher frequencies. Of course you can do it successfully, but be careful of the Gotcha’s. Thinner wire is easier to handle and usually results in better measured performance.
This is one of the reasons why we at Radio-Shop / Radiowinkel always dip all our final toroids in a RF-transparent varnish and then test for insulation up to 1000V.
So watch out for more articles on the topic of winding ferrite rings for your shack!
73 de Leon Uys ZR6N (A21LU when I work in Botswana) (AG5RH when I visit the kids in Texas)