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.
The core will never reach Tmax, temperature increase is terminated when the Curie temperature is reached and the core will be at 130° or more… likely to be sufficient to cause damage to wire insulation and enclosure.
There is something seriously wrong!
So this raises the question of just how much power does it take to increase the temperature of a FT240-43 core by 5°/s.
The Heat Capacity of ZnMn ferrite is around 1050J/kg/K, and the mass of the FT240-43 is around 0.12kg… so the energy to raise it 5° is 1050*0.12*5=630J. The average power \(P=\frac{E}{t}=\frac{600}{1}=630 \text{ W}\)
Is it feasible to supply 630W of RF to an antenna balun or transformer from a transmitter capable of 1500W (or more). Certainly, and especially if there is a fault in the antenna system.
In this case, the heat input is extreme and the core reaches Curie temperature in just 30s. More subtle failure may occur where the average core heating power us just 10 or 20W and the temperature rises slowly past the safe temperature for the enclosure and wire insulation.