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Years ago I found graph of Input Z of a transformer vs the multiple of the primary inductance to the drive impedance.

Most amateurs us a 4 time multiple, as in 50Ω drive, the transformer primary would require 200Ω at the lowest frequency of operation.

I'll post the graph at the end.

My transformer is 1 to 1 ratio, with a 2 turn primary and secondary on a BN72-202 binocular core.

I'm using a basic voltage divider circuit to measure the input impedance of the transformer. I'm on my second iteration of my test jig but have not found a great change in my numbers. My primary has a 59.3?H primary. This is 50Ω XL at 134,300 Hz or 1X. 4X is 200Ω at 537,000 Hz. You will need to recalculate these frequencies for you primary inductance.

?I have a jig as shown. A drawing describing the method and the graph showing the 4 X rule.

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I'm a little disappointed I can not get my transformer to look like 50Ω with a 50Ω load.

At multiple of

10X I get 48.68Ω

4X I get 46.77Ω

3X I get 46.77Ω ? ?? Notice 3X and 4X are the same, I have found this consistent over about 15 tests.

2X I get 46.15Ω

1X I get 39.29Ω

At 0.5X I get 26.14Ω

Can I get anyone to verify my figures?

Use any method you want, a VNA, an NanoVNA, etc.

You can also use any core you want, would be interesting to see how a toroid does.

??????????????????????????????????????????????????????????? Mikek

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Hi Mike,

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I think what you are seeing is not unusual. No transformer is perfect. The parallel magnetizing and loss current across the 50 ohms is bringing it down to 46 ohms. At the lowest frequency transformer action is failing as the core can't sustain the higher flux required so it begins to look like a short circuit. It would be interesting to plot the transformer input impedance across the frequency range with both an open and short circuited secondary. That's how we were taught to measure transformer characteristics (admittedly at power frequencies but there's no reason why it can't be done at RF).?

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A VNA, nano or otherwise, would be very helpful in analysing this. Ditto a vector impedance meter. I'd love to duplicate your experiment but can't do it soon as my bench is totally occupied and will be for while with another project.

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Cheers,

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Morris

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Morris said, "It would be interesting to plot the transformer input impedance across the frequency range with both an open and short circuited secondary."

?My interested started, because I wanted to know what Z normalized really meant, I kind of assumed the Z would drop, but I wanted to verify.

Here is the paper where I found the Z normalized graph.

A lot of math and the graph at the end.

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?I added my results to the graph, (in Blue) to show how poorly it matches the original line.

OK, it's really not that bad, but it is consistent over multiple tests, I'm 3.5% low compared to the

only source I know that discusses the 4 percent rule.

???????????????????????????????????????? Mikek

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I borrowed a friends NanoVNA and ran a scan.

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Hi John KN5L,

?I see I forgot the link in my previous post.

?The paper has the derivation of the graph at the end.

The numbers don't quite match, and I wonder why?

Is it the Leakage inductance?

?I next plan to try a toroid and see if it gets closer to the derived curve on the graph.

???????????????????????? Mikek

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Thanks much for that input.

????????? Mikek