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Help on understanding the NanoVNA v2 wheatstone bridge design

spam 2021/10/03 09:31

I have some difficulties on understanding the design of the coupler in the NanoVNA v2. From what I understand is that this is a wheatstone bridge but I have some problems understanding the initial design process for that and where all the additional resistors are coming from. What was the initial design process to come up with 10dB transmission.

OK1VAW 2021/10/04 07:36


It is not a pure Wheaston Bridge, but loaded resistive bridge. The main
difference is, that the diagonal (where you put the Galvanometer with
high impedance) is loaded with a 50 ohm too. In most cases of
reflectometric bridges there is a balun to get the diagonal output
asymetrical with one pin grounded. If you do the math (and do not forget
to use the 50 ohm generator  - voltage source in series with 50 ohm
resistor), you should get the power getting from diagonal in proportion
to reflection coefficient.

Here is detailed description from the Rohde&Schwarz ZRB2 bridge manual.

Dne 03.10.2021 v 18:31 napsal(a):

Roger Henderson 2021/10/04 19:07

The last couple of messages in the below thread are also worth a read.
There is an article from the HP VNA guru Joel Dunsmore, and a newer article
from Brian (virtualparticles) who is an RF guru from Copper Mountain.

On Mon, 4 Oct 2021 at 18:36, OK1VAW <> wrote:

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