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Overheating balun cores – an explanation discussed a scenario where an operator reports unstable VSWR after 30s of ATU adjustment.

Where the antenna system incorporates ferrite elements, a possible / likely explanation is that loss in a ferrite core has been extreme and raised core temperature to the Curie temperature at which it quickly loses its magnetic properties.

In that scenario, theoretically, the complete temperature curve would look like this.

The initial rate of temperature increase here is 5°/s, and we can safely assume that almost all of the power absorbed by the core is stored as heat energy, little energy is lost the the air when the temperature difference is very small. Continue reading Overheating balun cores – how much power does it take?

Correspondents raise instances of damage to baluns with me from time to time, and there is a steady stream of reports online.

One of the very common reports is of something unexpected happening while adjusting an ATU, after perhaps 30s of power applied, VSWR suddenly becomes unstable, changing for some unknown reason, and attempts to find optimal settings of the ATU fails.

A likely cause of this is non-linear behavior of the ferrite core in a balun in the system.

Let’s talk about that.

A theoretical model of temperature rise

A simple model that gives useful insight is to consider the case of a toroid core in still air, being heated by constant applied RF power giving rise to core loss.

Core temperature rises quickly initially, then more slowly as the core heats up and loses more and more heat to the surrounding air.

We can write and expression for core temperature T: \(T=T_{max}\left(1-\mathrm{e}^{-\frac{t}{\tau}}\right)\) where τ is the thermal time constant and Tmax is the final temperature if things continued without disruption.

Above is an example where τ=20s and temperature rise is 100°.

Note that at the beginning (t=0), the slope of the line is pretty constant, but as temperature increase, slope decreases until eventually it is almost zero. Continue reading Overheating balun cores – an explanation

Correspondents raise instances of damage to baluns from time to time, and there is a steady stream of reports online.

The thinking is often “my balun is rated at 5kW, how can I damage it with a transmitter that is only 100W?”

This article discusses heat dissipation from a small sealed plastic enclosure. This is not an unusual problem, it exists widely in industry and is solved routinely for enclosures ranging from small ones like these to large switchboard enclosures.

An example balun deployment

Above is a balun enclosure located under the soffit of the building, so outside in free air and where the sun will not shine on it. Continue reading Power dissipation of a sealed plastic balun enclosure