nanoVNA-H – measure ferrite transformer

This article demonstrates the use of the (modified) nanoVNA-H to measure Loss (Transmission Loss) and Insertion Loss of a small ferrite 64:1 RF transformer, and the Insertion VSWR and Return Loss. The transformer was designed for a receive application at 9MHz.

Firstly let's define the meaning of the terms:

  • Loss: PowerIn/PowerOut, may be expressed in dB as 10log(PowerIn/PowerOut), referred to in this article as Transmission Loss for clarity;
  • Insertion Loss: (Power in a matched load)/(Power in the mismatched load), may be expressed in dB as 10log((Power in a matched load)/(Power in the mismatched load));
  • Insertion VSWR: the input VSWR when the device is terminated in its nominal load;
  • Insertion Return Loss: the input Return Loss when the device is terminated in its nominal load.

Since the transformer has a turns ratio of 1:8 or nominal impedance ratio of 1:64, we will insert a large resistor to raise the 50Ω Port 2 impedance to close to 50*64=3200Ω.

Further, since Zin of Port 2 is critical to measurement accuracy, we will calibrate the VNA with a good 10dB attenuator on Port 2.

Insertion Loss

Insertion Loss is easily found with a VNA, InsertionLoss in dB is simply -|s21|dB.

Yes, the sign is right. It is a hammy thing to insist on prefixing loss dB figures with a – sign, and betrays a lack of understanding.

Insertion Loss says nothing about heating of the transformer.

Transmission Loss

Determining Transmission Loss for this scenario is a little more complicated. We start with -|s21|dB but must adjust for two things:

  • the effect of input mismatch causing reduction in InputPower; and
  • Port 2 captures only a portion of the power delivered to the combined load.

The difference between InputPower and OutputPower is converted to heat in the transformer core and windings.

Insertion Loss is of relevance if the source is well represented by a Thevenin equivalent circuit with Zs=50+j0Ω. Ham transmitters are not usually well represented as such.

Transmission Loss gives a means of calculating the RF power that will be converted to heat.

Input VSWR / Return Loss

The other perspective that is often of interest is the input ‘match', the VSWR and Return Loss.

This transformer gives a reasonably good impedance conversion from a 3200+j0Ω load to 50+j0Ω input from about 3.5-10MHz.