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As measured impedance get significantly higher or lower then 50 ohms the variance in bridge resistors create greater errors.

I guess a revisit to using an impedance matching network to solve this issue is required.

Once a upon a time we spent significant effort building RX bridges, noise bridges, etc... and spent much time on finding ideal variable R's and C's as well L's and involved transformers. Application of some of that effort may be re used and supplemented now that a low cost vna is available.

Consider building a frequency independent L matching network or even a PI although it adds the challenge of adding a 3rd element to the challenge. That challenge is to incorporate L and C's whose self resonate frequency is far removed from the measurement frequency. Example, a 20 meter bobtail antenna has a driving point Z of nearly 5k with -j290 series ohm. Not easy to measure. However, readily matched to 50 ohms, the Qm is only 9.8 and the element values are 5.5 uH and shunt 23 pF at 14.1 MHz.

Both of these components are constructed with SRF values well above 20 meters, with care. And will provide as measured by the vna a return loss in excess of 30 dB. Reading off the value of the L and C at the measured frequency will permit you to find the antenna port Z. So in advance of the measurement, if you have some idea of what Z is expected, it should be straight forward to construct a match system that facilitates that measurement. I have done this simple exercise with some unknown R-X values with high gamma and it works out fine.

Alan

Hi Alan,

I did some modelling and there are quite a few places where errors can creep in. I was a little surprised on the significance of Tx driver source impedance and balanced Rx output loading impedance have. It will take more analysis to understand what can be calibrated out.

The excitation driving source impedance has a resistor attenuator pad to improve the source impedance control but a compromise must be made on the tolerable drive signal loss. The capacitance of the synthesizer driver output and input of the SA612 effect high frequency end.

The sophistication of the calibration correction algorithm may be an area to examine as the correction algorithm for low Z measurements can be different from high Z measurement. How you might interpolate calibration points may depend on what is creating the errors, resistor matching, absolute resistor values, and/or stray reactance. At low freq end stray reactance effects are minimal but may have to begin to worry about spurious mixing products not being attenuated enough by LPF effecting ADC readings.

But this is a low cost 'hobbyist' unit. I would like to see a better synthesizer IC and mixer IC but that would run up cost. I have a NanoVNA, NavoVNA-F, and N1201SA unit. I like the N1201SA with its better AD4351 synthesizer chip and high frequency capability but it is limited on low freq end and doesn't seem to have open source code available. My N1201SA was built with AD4351 chip which could go down to 35 MHz but firmware stops it at 137.5 MHz as if it has the AD4350 chip. I would definitely go for the extra bucks of the NanoVNA-F over the original NanoVNA. The -F has 5v boosted supply for mixers where the original NanoVNA runs directly from 5v USB that drops to LiIon battery voltage when USB supply ls not connected. The -F also has the more powerful M3 processor and extra flash memory along with several other improvements including a real housing.

On Mon, Dec 16, 2019 at 03:25 PM, <roncraig1@...> wrote:

where the original NanoVNA runs directly from 5v USB that drops to LiIon
battery voltage when USB supply ls not connected.

That's wrong assumption. The Injoinic IP53xx chips are working as step-up converters
when powered from LiPo, so it's always 5V on mixers.

Old thread here (it was about the clones of transverters-store 3GHz RF bridges) .. I eventually got around to looking at these bridges.

I ended up with both a blue board and a green board, the latter with red sma caps. The transverters-store apparently ships red caps too, describes a green board but pictures a blue board.

The ones I got both performed badly and were identical. They appear to both have nearly correct coupler wiring : they do connect the core of the second coax(which is shown unconnected in the original site) to the outers. at both ends. Not sure if that matters. They also both had the 100R resistors incorrectly linked but a solder blob sorts that out, after which they behave roughly as I expect.

These were bought in 2019 at the time of that discussion so the situation may be different now. Given that the board only costs $18 from transverters-store.com I'd suggest getting it from there if you want one. I shall probably do that, though I've got some more traditional N-type metal bodied directional couplers on the way.

There's a seller with a large number of the 100kHz - 3GHz Hameg bridges that were built for my HMS-X SA at the moment. Unfortunately he wants ?600 each for them. Which I think is cheaper than Hameg sold them but still out of budget by at least 10dB.



A green (not blue) ~US$10 reflection bridge
e.g. from eBay
with matched SMA references (want a matched pair to calibrate nanoVNA CH1)
Why is a green reflection bridge better than a blue one ?
(Presumably it's not just a matter of preferred colour ..)
Blue are bad clones of transverters-store design and are wired wrong.