At what frequency is a transformers rated impedance measured

Question

Where transformers have a specific impedance rating for the the primary/secondary, what frequency is this measured at?

For example, this transformer is rated at 40MHz - 860MHz with an impedance of 50/50ohm.

https://www.digikey.co.uk/product-detail/en/tdk-corporation/ATB2012-50011-T000/445-6138-1-ND/2465465?utm_adgroup=General&mkwid=sJoLbxdi4&pcrid=153600060853&pkw=&pmt=b&pdv=c&productid=&slid=&gclid=EAIaIQobChMIgaPRubGs4gIVL7ftCh2hCAlBEAAYASAAEgKcD_D_BwE

I assume the impedance cannot be 50ohm at all frequencys within 40MHz - 860MHz?

Answer 1

If you look at the data sheet it does indeed say that the impedance is 50 ohms across that range of frequencies however, on closer inspection, it gives a return loss figure that does vary a bit and is at its lowest at 900 MHz with a value of 18 dB.

Having said that, 18 dB is a pretty reasonable return loss and implies that the 50 ohms (perfect value) might be about +/- 27% different at 900 MHz. At 100 MHz the return loss is about 29 dB implying an impedance accuracy of +/- 7.4%.

Return loss = 20\$log_{10}\bigg[\dfrac{Z-50}{Z+50}\bigg]\$