An online expert held forth on the design of ferrite chokes and transformers, and to quote one paragraph:
Equally selfevidently we don’t want ANY real part of the reactance in a transformer and, for a practical transformer, we want the self inductance on each side (primary and secondary) to be at least j10*R(Load or Source) and the coupling to be as close to 100% from primary to secondary. It is the real part that heats up transformers a LOT and, since ALL of the current is seen by the ferrite in a transformer, not just the part that got reflected back on the outside of the coax in a choke, losses are abos-posilutely-undubiously NOT desired and the u”R needs to be as close to zero as we can get at the designed frequency for minimum loss and minimum power dissipation.
Setting aside the hyperbole and the wooly thinking, let’s drill down on
u”R needs to be as close to zero as we can get at the designed frequency for minimum loss and minimum power dissipation.
It is a pretty general statement without really specific quantities,
needs to be as close to zero as we can get and
minimum loss and minimum power dissipation does not give useful guidance of acceptable values of µ”, and may even impart the impression that the following chart is for material that is not suitable above perhaps 200kHz, if that.
Above, µ” is greater than 10 above about 200kHz, greater than 100 from about 2 to 100MHz. Is this what the quote condemns?
Let’s pull up a previous design.
A prototype small 4:1 broadband RF transformer using medium µ ferrite core for receiving use gave details of the design of a small transformer which used the above #43 material.
Above is the prototype transformer in a test jig which has been SOL calibrated. One end of the winding has a 150Ω resistor (measured as 149Ω) in series to ground, the other connects to the output connector centre conductor. This arrangement puts a 200 load on the transformer, and there will be 6dB of loss from input to output due to the divider action of the 150Ω resistor and 50Ω instrument input.
Above is a scan of S11 and derived input impedance and VSWR. As expected from the earlier work, Insertion VSWR is below about 1.2 from 1.8MHz up, and looking at the Smith chart, you can see that the departure from an ideal transformation is a little +ve reactance and slightly lower resistance which is the expected effect of the shunt magnetising impedance.
The S21dB (black) plot is |S21| in dB, the gain through the device, and bear in mind that due to the resistor load division, -6dB represents zero loss. So, the InsertionLoss is tenths of a dB, demonstrating that efficient transformers can be made from lossy ferrites.
We can extract from the s11 and s12 figures and the 149Ω series resistor, the InsertionLoss and its components TransmissionLoss (or simply Loss) and MismatchLoss.
It is the TransmissionLoss that heats the core and wire, MismatchLoss does not cause heat in the transformer (and does not necessarily cause heat anywhere else).
In this case, the TransmissionLoss varies from 0.1dB to 0.25dB, by most measures a reasonably efficient transformer.
Let’s look at a Simsmith model of the transformer based on published material characteristics and calibrated to the measured response.
Referring to the manufacturer’s chart given earlier, µ” is greater than 200 from 2 to 30MHz, yet the TransmissionLoss is quite low.
The Rm value plotted in blue against the right hand Y axis is the series equivalent resistance component of the of the magnetising impedance due to µ”, it is what the quote refers to with
we don’t want ANY real part of the reactance in a transformer. It is near zero at the left but goes up to quite large values on the right, yet the TransmissionLoss is quite low.
It is easy to wave hands around and say core materials must be nearly lossless and to talk about achieving minimum loss, but the challenge for designers of practical devices is to find a compromise set of many competing effects / parameters that provides a design that is acceptable on many criteria for a particular application.
Read widely, and think… there is some pretty wooly stuff elaborated on social media.