ECLECTIC AETHER - Adventures with Amateur Radio

Balun Loss

This was an experiment in order to determine Broadband RF transformer loss could be determined by measuring the input SWR when the secondary is either Open or Short Circuit.
I'd been trying various simple methods of determining 'ball park' loss figures for broadband matching transformers for folks who don't have access to much test equipment.

The basic idea was to measure the input SWR of a transformer with the secondary Open or Short circuit, and I have been able to check this technique against a few different transformers in my collection.

The technique can also be used to estimate the loss that is likely to occur when feeding a High (almost O/C) or Low impedance (almost S/C) load, and is a useful way of assessing broadband transformers that are likely to be presented with large variations in the value of terminating load.

The basic idea is not new, and a similar technique for measuring coax cable loss was described by Frank Witt, AI1H, in QEX back in 2005 (although that is based on earlier work too)

RF engineers tend to use 'Return Loss' as an alternative to SWR. The basic idea is that you measure the level of reflected signal in dB relative to the incident signal and this gives you a value of return loss, which tends to provide greater resolution than a basic SWR ratio. 

The return loss is the two way loss though a device, as the signal travels out and then back again to the source. If you halve the return loss value it indicates the loss in one direction.

SWR can easily be converted to Return Loss and this in turn can be charted to show attenuation (1/2 RL value).

Below is a simple chart showing this relationship, which I hope will help with this measurement method.

This method works well when the losses are relatively high, but it becomes much less accurate as the losses decreases, and the overall accuracy is likely to be made even worse by the use of a cheap inaccurate SWR meter as the measuring device.

Note that in the following examples I have shown the amount of loss as a negative value. This is in order to facilitate a comparison of the two methods in a format that is similar to the display on a typical VNA.

The charts show a comparison between the Insertion Loss of two identical transformers connected back to back and the calculated value of Open and Short Circuit Insertion Loss which was calculated by halving the Return Loss figures.

The charts are shown in a rough order, with transformers demonstrating the greatest loss towards the top of the page.

The orange lines show the equivalent SWR under O/C & S/C conditions.

The dotted line shows the average of the Open and Short Circuit values derived from the Return Loss, which, if the method works, should be close to the Back to Back insertion gain curve shown in Black.

Some examples are plotted on two scales in order to show more detail when the loss value is low.

Bottom line:-

Overall it's perhaps a bit better than not making any measurements at all, but not by much.

However it's fairly safe to say that if the SWR under S/C or O/C conditions is less than 10:1 it is probable that you have some losses (>1dB), if it's less than 3:1 you are likely to have appreciable losses (>3dB), and if it's less than 2:1 you are more than likely to be using a dummy load.

Comments welcome