Transmission lines – Page 6

nanoVNA-H – sweep of a coax line section with OC termination

This article discusses the use of the (modified) nanoVNA-H raw accuracy and the implications for calibrated measurements.

Introduction

VNAs achieve much of their accuracy by applying a set of error corrections to a measurement data set.

The error corrections are obtained by making ‘raw’ measurements of a set of known parts, most commonly a short circuit, open circuit and load resistor (the OSL parts). The correction data may assume each of these parts is ideal, or it may provide for inclusion of a more sophisticated model of their imperfection. This process is known as calibration of the instrument and test fixture. nanovna-Q appears to have some fixed departure compensation to suit the SMA cal parts, less suited to other test fixtures.

So, when you make a measurement at some frequency, the correction data for THAT frequency is retrieved and used to correct the measurement.

What if there is not correction data for THAT frequency? There are two approaches:

a calibration run is required for exactly the same frequency range and steps (linear, logarithmic, size) as the intended measurement; and
existing calibration data is interpolated to the frequency of interest.

The interpolation method is convenient, but adds uncertainty (error) to the measurement. Some commercial VNAs will NOT interpolate.

The nanoVNA will interpolate, and with interpolation comes increased uncertainty.

An uncorrected sweep of a reasonably known DUT is revealing of the instrument inherent error.

The DUT is a 12m length of LMR400.

Expected behavior

Let’s first estimate how it should behave.

The VNA contains a directional coupler nominally designed / calibrated for Zo=50+j0Ω, and in use, VNAs are invariably used to display measurements in terms of some purely real impedance, commonly 50Ω.

Though the DUT characteristic impedance (Zo) is nominally 50Ω, it is not EXACTLY 50+j0Ω and so there are departures in the displayed values wrt 50Ω from what might happen in terms of the actual Zo.

We can calculate the magnitude of Gamma for our 12m OC section of LMR400 over a range of frequencies.

|Gamma| vs frequency is a smooth curve as a result of line attenuation increasing with frequency. As a result in the small departure in Zo, |Gamma| wrt 50Ω has a superimposed small decaying oscillation. Continue reading nanoVNA-H – sweep of a coax line section with OC termination

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – T-Check test

Rhode & Schwarz describe a test for accuracy of a VNA at T-Check Accuracy Test for Vector Network Analyzers utilizing a Tee-junction.

A nanoVNA-H PCB v3.3 (modified to fix decoupling problem on mixers) was calibrated from 0.1-900MHz using the supplied parts.

A T piece with extra 50Ω termination was inserted between the supplied original cables and s11 and s21 captured. The assembly was turned around and measured again to capture s22 and s12 (though recorded as s11 and s21). The two files were merged to obtain a full two port bothways .s2p file.

The T-Check value was calculated and is plotted here in VNWA.

Above, the T-Check results are not stunning at all, the ideal result is 1.0 at all frequencies. Rhode and Schwarz recommend that more than 15% error is unacceptable… of course that is in a commercial grade VNA.

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – measure ferrite transformer

This article demonstrates the use of the (modified) nanoVNA-H to measure Loss (Transmission Loss) and Insertion Loss of a small ferrite 64:1 RF transformer, and the Insertion VSWR and Return Loss. The transformer was designed for a receive application at 9MHz.

Firstly let’s define the meaning of the terms: Continue reading nanoVNA-H – measure ferrite transformer

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – measure ferrite core permeability

This article demonstrates the use of the (modified) nanoVNA-H to capture data from which the complex relative permeability of an unknown ferrite core is calculated and plotted

Above, a single turn of wire through the sleeve allows measurement by the nanovna. The nanoVNA fundamentally captures s11 parameters which we need to convert to relative permeability. Continue reading nanoVNA-H – measure ferrite core permeability

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – measure equivalent core loss resistance

A very common design of a n:1 transformer for EFHW antennas uses a 2t primary on and FT240-43 (or even smaller) ferrite core.

In a process of designing a transformer, we often start with an approximate low frequency equivalent circuit. “Low frequency” is a relative term, it means at frequencies where each winding current phase is uniform, and the effects of distributed capacitance are insignificant.

Above is a commonly used low frequency equivalent of a transformer. Z1 and Z2 represent leakage impedances (ie the effect of magnetic flux leakage) and winding conductor loss. Zm is the magnetising impedance and represents the self inductance of the primary winding and core losses (hysteresis and eddy current losses). Continue reading nanoVNA-H – measure equivalent core loss resistance

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – a ferrite cored test inductor impedance measurement – s21 series vs s11 reflection

One often sees experts online insisting that s11 reflection impedance measurements of common mode chokes is inaccurate, and the ONLY way is s21 series measurement.

This one is built upside down so the connections are visible.

In this experiment using the SDR-KITS VNWA testboard (above), a series of measurements are made of an inductor, and the test setup.

The nanoVNA-H (modified) was calibrated using the test board and its associated OSL components. The test board is used without any additional attenuators, it is directly connected to the nanoVNA-H using 200mm RG174 fly leads.

s11 reflection measurement

Above is the s11 reflection measurement. It is quick, and most tools directly display R and X vs frequency (though some of the new age tools display X as pF or µH equivalents). This is nanoVNA MOD v3 (a derivative of nanovnasharp).

Ok, so we have 4856-j852Ω. Continue reading nanoVNA-H – a ferrite cored test inductor impedance measurement – s21 series vs s11 reflection

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – Chinese junk?

The NanoVNA is a new low cost community developed VNA with assembled units coming out of China for <$50.

It is not really a project as such, but a loose collection of hardware versions manufactured in variously compliant forms, and several PC and web clients competing for space. Information is partly in the public domain, partly in restricted access forums where the controller’s identity is hidden… the modern way of ham radio.

After some research, and with some residual uncertainty, I decided upon the so-called nanoVNA-H design by Hugen79.

Hugen79 nominates a seller on Alibaba, but I was unable to purchase there because it would not accept my suburb in an address, and unwilling to put at risk the minimal buyer protection by purchasing outside of the selling platform, I went to Aliexpress for sellers. (No listings on eBay or Amazon for AU account holders, but many listings have appears on eBay since purchase).

Above is the pic from the seller’s listing. Continue reading nanoVNA-H – Chinese junk?

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – a ferrite cored test inductor – drilling down on a glitch – the fix

The NanoVNA is a new low cost community developed VNA with assembled units coming out of China for <$50.

This article documents some tests using a small ferrite cored test inductor that provides a similar impedance to that of a common mode choke suited to HF antenna feed lines.

The nanoVNA-H has firmware NanoVNA-Q-0.4.3-20f33ba.dfu installed. The nanoVNA appears to be a nanoVNA-H v3.3 but it is hard to be sure, it is from China, the land of copyists and fraud. Indeed the nanovna-Q author asserts that genuine nanoVNA-H have a green overlay whereas mine is blue. Uncertainty is part of the territory!

A small fixture was OSL calibrated and impedance measured by reflection (ie s11).

The results broadly reconcile with previous measurements of the inductor, but the jitter is worth of examination.

Above is a chart showing the measured impedance components, we might expect a reasonably smooth curve for each. It has a lot of jitter on it, and you might excuse that as a result of applying the instrument and method to such a high impedance… but for two things: Continue reading nanoVNA-H – a ferrite cored test inductor – drilling down on a glitch – the fix

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – a ferrite cored test inductor – drilling down on a glitch

The NanoVNA is a new low cost community developed VNA with assembled units coming out of China for <$50.

I have been evaluating one for common tasks around a ham station and noticed a persistent glitch around 7.5MHz in scenarios where |s21| is high.

Above is an example where a ferrite cored inductor similar to an antenna common mode choke is measured, and the underlying curves should be smooth with some random measurement noise. The glitch around 7.5MHz is persistent and much larger than the random noise, so I have investigated further. Continue reading nanoVNA-H – a ferrite cored test inductor – drilling down on a glitch

Last update: 2nd February, 2020, 7:11 AM

nanoVNA-H – standalone VHF antenna VSWR sweep

The NanoVNA is a new low cost community developed VNA with assembled units coming out of China for <$50.

This article documents a task to perform a VSWR sweep of a VHF antenna stand alone, and to capture the data and screen image for download to a computer.

The nanoVNA-H has firmware NanoVNA-Q-0.4.3-20f33ba.dfu installed.

The instrument was OSL calibrated for reflection at Port 1 (Ch0 is nanoVNA speak, convention means little) for 100-500MHz.

A sweep was then performed from 143-149MHz, forcing some interpolation of the calibration data.

The nanoVNA was configured for VSWR and Smith chart displays, much as you would to use it stand alone.

The sweep was then “Paused”, and the nanoVNA carried to the computer and plugged into the USB. The power must not be disturbed, the controls must not be altered, the captured data is really just what is frozen on the paused display.

Note that if the thing is unplugged from the USB the nanoVNA resets and the data is lost, so plug it in properly just once.

I then connected to it using using nanoVNA MOD v3 and captured the screen, clicked get data once, and saved the .s1p file.

Above is the cropped screen capture. Continue reading nanoVNA-H – standalone VHF antenna VSWR sweep

Last update: 2nd February, 2020, 7:11 AM