Interpreting temperature rise in ferrite cored RF transformers and inductors



We often see statements by hams where they draw inference from observed temperature rise of a ferrite core at RF. Lets consider the following statement.

The FT-240-43 balun MUST be quite efficient as it barely increased in temperature over a 5 minute over at 100W on SSB.

For the purpose of this explanation, lets assume barely increased in temperature means 5° increase in temperature from cold. Under these conditions, we can reasonably assume that almost all of the heat input to the core is consumed in raising the core temperature.

The Heat Capacity of ZnMn ferrite is around 1000J/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.

We can calculate the average power over 5min, 630J over 300s gives P=630/300=2.1W.

Now lets compare that to the average transmitter output power.

The average power of SSB telephony is less than 3% of PEP, a little higher with compression… so the average transmitter output power in this case might be 3W without compression, and losing 2W in the balun core is quite inefficient.

In this case, a temperature rise of just 5° over 5min of uncompressed SSB telephony hints seriously low balun efficiency.

Quite modest increase in temperature might indicate severe loss.