There are two elements that are critical to efficient near ideal impedance transformation over a wide frequency range, low flux leakage and sufficiently high magnetising impedance. While low magnetising loss is essential for efficiency, it does not guarantee sufficiently high magnetising impedance for near ideal impedance transformation.
Magnetising impedance can be estimated using one of the following calculators, but keep in mind that there are quite wide tolerances on ferrite cores.
- Inductance of RF cored inductors and transformers
- Calculate ferrite cored inductor – rectangular cross section
- Calculate ferrite cored inductor – circular cross section
- Calculate ferrite cored inductor (from Al)
- Calculate ferrite cored inductor – ΣA/l or Σl/A
- Ferrite permeability interpolations
Magnetising impedance can be measured (eg with an analyser), but it should be measured with only the measured winding on the core. Did I mention the wide tolerance of ferrites?
Example – FT240-43 3t @ 3.6MHz
You might ask the question is 3t sufficient for the primary of an EFHW transformer that delivers a 50+j0Ω load to a transmitter.
Estimation from published datasheets
Estimating with a calculator, we get the following.
Plugging the real part of Y into Estimate core loss for ferrite cored RF transformer we obtain the following.
Estimation from measurement
Measuring a core with a 3t winding using very short wires to the AA-600 coax socket, the following results were obtained.
Plugging the R,X pair into Estimate core loss for ferrite cored RF transformer we obtain the following. (You could also just enter just the R|| from this analyser value for Rpm.)
Above, the results from measurement are a little better than expected from the datasheets, I did mention that ferrites have quite wide tolerance.
Depending on your loss criteria, the 3t winding might be adequate from a loss perspective on 3.6MHz. On the other hand you might be tempted to test 4t, but there is a limit as more turns tends to compromise the higher frequency performance, especially on a large core.
A follow up article will consider the effect of magnetising impedance on impedance transformation.