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[Measuring] Is there an easy way to measure balun loss?

 

Hello team,

Is there an easy way to measure balun loss, without another one in tandem S11-S21 ?

I only have one and I would like to have its characteristics, including loss.

Cheers and thanks in advance.

Lu¨ªs CT2FZI
 

On Wed, 20 May 2020 10:01:05 -0700, you wrote:

Hello team,

Is there an easy way to measure balun loss, without another one in tandem S11-S21 ?

I only have one and I would like to have its characteristics, including loss.

Cheers and thanks in advance.

Lu¨ªs CT2FZI
What do you mean by "without another one in tandem"?

OE8UWW
 

Probably the best inexpensive technique would be using back-to-back or tandem identical devices.

The back-to-back system allows use of 50-ohm meters at each end, or S21 and S12 measurements with network analyzers, for loss and mismatch errors.

Taking half the measured loss through two identical devices can get us very close to the real loss.

balun test.jpg

 

But I only have one unit, and I would like to know if there is an easy way - ie, without an oscilloscope etc - to measure the transmission loss through a unun.
 

Consider building a balanced R pad. Then measure the loss of the pad with and without the balun. The difference is the loss of the balun. I assume, if a 50 ohm balun, then you require the same Z of the R pad.

Alan
 

Hi Alan

Thanks for the reply.

You mean like a attenuator circuit made with resistors?

I am not sure if I understood.

If the unun ratio is 49:1 means that the R pad should receive on the input 2450R and the output would be 50R.

How can I measure the R pad loss to subtract it from the unun + R pad loss?

Cheers

On Wed, May 20, 2020 at 06:43 PM, alan victor wrote:


Consider building a balanced R pad. Then measure the loss of the pad with and
without the balun. The difference is the loss of the balun. I assume, if a 50
ohm balun, then you require the same Z of the R pad.

Alan
 

Get two of them back to back but at those impedance transformation
ratios you are looking at
~20dB of attenuation per pad so that could be a problem.

On Wed, 20 May 2020 at 21:31, CT2FZI <ct2fzi@...> wrote:

Hi Alan

Thanks for the reply.

You mean like a attenuator circuit made with resistors?

I am not sure if I understood.

If the unun ratio is 49:1 means that the R pad should receive on the input 2450R and the output would be 50R.

How can I measure the R pad loss to subtract it from the unun + R pad loss?
 

Luis,

One method: Measure the return loss of the balun with the balun shorted. The return loss should be about twice the balun loss. This is similar to measuring the loss of a shorted or open piece of cable.

--John Gord

On Wed, May 20, 2020 at 10:01 AM, CT2FZI wrote:


Hello team,

Is there an easy way to measure balun loss, without another one in tandem
S11-S21 ?

I only have one and I would like to have its characteristics, including loss.

Cheers and thanks in advance.

Lu¨ªs CT2FZI
 

Hi Luis. Your query stated BALUN. However, if an UNUN, then a simple R pad, either PI or T with a Z value that matches the UNUN transform ratio is required.

The method in either case requires a measure of only s11 or the return as this is directly tagged back to the insertion loss as loss= 1/(1-s11).

If it is a large transformer ratio, the prior post of measurement of short and open s11 is a method to try.
 

The loss relationship should be written as 1/(1-|s11|^2)...

Need to check, is this a valid form for a passive transformer network?
 

Hello again Alan,

Thanks for the detailed answer.

I had to google for the design, I have found this website:

I have made a small screen shot of the T design. If one terminates this circuit with a 50 ohms resistor it will work, correct? Assuming the R T values will convert the input impedance to 50 ohms...
 

Yes. If we are trying to measure the so called TRUE INSERTION loss... |S21|. And the vna has a reasonable dynamic range, then we could build an appropriate MINIMUM LOSS PAD. It would provide a match between 50 ohms and the UNUN transform ratio. Example, say it was a 9:1. Then I would need a 450 ohm to 50 ohm minimum loss pad. That pad has a minimum loss of ~ 16 dB. Higher loss will result in "nicer R values"... So now measure the pad, then measure the pad + UNUN and find the delta. Yes, something so simple has the devil in the details. And it can be more difficult than first realized!

Alan
 

Hi John,

Do you agree this will also work with an UNUN, shorted to ground?

On Wed, May 20, 2020 at 09:17 PM, John Gord wrote:


Luis,

One method: Measure the return loss of the balun with the balun shorted. The
return loss should be about twice the balun loss. This is similar to
measuring the loss of a shorted or open piece of cable.

--John Gord

On Wed, May 20, 2020 at 10:01 AM, CT2FZI wrote:


Hello team,

Is there an easy way to measure balun loss, without another one in tandem
S11-S21 ?

I only have one and I would like to have its characteristics, including
loss.

Cheers and thanks in advance.

Lu¨ªs CT2FZI
 

I ran a test case with a 4:1 xmfr after adding resistive loss to the xmfr.
A plot of return loss (from s11) and then transmission loss (from s21) is provided.

Looks like 1/(1-|s11|) where S11 is obtained from the return loss value matches up with the calculated transmission loss.
 

Luis,
Yes, it should also work with an UNUN. A caveat: It should give a low VSWR (high return loss) when properly terminated.
--John

On Wed, May 20, 2020 at 03:38 PM, CT2FZI wrote:


Hi John,

Do you agree this will also work with an UNUN, shorted to ground?

On Wed, May 20, 2020 at 09:17 PM, John Gord wrote:


Luis,

One method: Measure the return loss of the balun with the balun shorted.
The
return loss should be about twice the balun loss. This is similar to
measuring the loss of a shorted or open piece of cable.

--John Gord
 

try to see
and
 

Thanks for sharing will look into it !

On Thu, May 21, 2020 at 02:35 PM, <gaionim@...> wrote:


try to see
and
 

I just joined the nanoVNA group a couple of days ago and followed your thread on the balun measurement. If possible, why don¡¯t you measure the scattering params, save the data, build a 3x3 scattering parameter matrix with the data and convert it to mixed signal to get the diff output? See for example . You would need one of the math software tools out there and I understand that practice and theory... have some issues to get along! but eventually they should go hand in hand. Just a suggestion.

Luciano
 

On Thu, May 21, 2020 at 06:35 AM, <gaionim@...> wrote:


try to see
and

Interesting way to measure balun loss with a single balun and a VNA. Thank you for posting this information.

Roger
 

I did this about 2 years ago similar to Roger Need's second link. On the output of the 1:49 balun I connected a 2400 ohm resistor and in series with this resistor on its ground end I connected an HP power meter. The total load is then 2450 ohms. If the 1:49 transformer has a 0 dB loss the the power meter would measure 16.9 dB below the input power. Anything higher than 16.9 dB down due to the loss in the 1:49 transformer (it is not an autotransformer or and unun, BTW.....it is a conventional transformer with a primary and secondary winding). This based on the ideal S21 = 10 x log (50/2450) = -16.902 dB. You can do the same with a 1:9 transformer (10 x log (50/450) etc.. When I received my Nanovna back last August, there was good agreement in the loss measurement against the HP 435B with an HP8482 power sensor. This is how I determined the loss of a 1:49 EFHW transformer to be 1 to 1.5 dB depending on the freq. A resistance of 2450 ohms is far from what an EFHW antenna's feed point Z is but close enough for a ball park check. A back to back test had fairly good agreement. 73

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