This article continues on from several articles that discussed the ARRL EFHW kit transformer, apparently made by hfkits.com, and the revised design at ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 1.

This article presents a saturation calculation.

You will not often see saturation calcs (for reasons that will become apparent), though you will hear uninformed discussion promoting FUD (fear, uncertainty and doubt).

Lets assume that the core is capable of maximum continuous power dissipation of 10W (limited by factors like safe enclosure temperature, human safety, Curie point etc).

## Now let’s estimate the magnetising current for 10W of core dissipation with 3t primary

Starting with the expected permeability above…

Gm is the magnetising conductance, the real part of Y above, 0.00231.

We can approximate the primary voltage for 10W core dissipation as \(V=(\frac{P}{G_m})^{0.5}=(\frac{10}{0.00231})^{0.5}=65.8V\) which implies 87W in a 50Ω load.

Magnetising current can be calculated at 10W core dissipation as \(I_m=\frac{V}{|Z|}=\frac{65.8}{222}=0.296A\)

In fact, under load, the net magnetising force may be just a little below 0.296A due to the effects of leakage inductance.

## Let’s estimate saturation current for a 3t primary

Lets assume that under load, magnetising force due to current in the secondary offsets most of the magnetising force due to current in the primary and that the net magnetising force is due to magnetising current.

So, let’s solve.

Above is Fair-rite’s published data for their #43 mix (do not assume it applies to pretenders). Let’s take saturation flux density to be 1500 gauss, 0.15T.

Above is a calc of the saturation current for the 3t primary (peak), 4.01 Arms.

## Conclusions

The saturation current is 14 times the magnetising current at 10W core dissipation, and is unlikely to be a significant limitation for low duty cycle modes.

This EFHW transformer is loss limited rather than saturation limited for most practical applications.

If you had in mind that this transformer was suited to peak power \(14^2 \cdot 87=17000 \text{ W}\) or more, then it may be driven into saturation.