Antennas – disturbing the thing being measured – open wire lines #3

The article Antennas – disturbing the thing being measured – open wire lines #2 did a simple analysis of current flows in the model scenario using ideal voltage balun drive.

That begs the question, what difference would a good current balun make?

We can get a good approximation of what happens by inserting the current balun’s Zcm in series with Z3. Let’s take Zcm to be 1130+j1657Ω (11t on a FT240-43). Continue reading Antennas – disturbing the thing being measured – open wire lines #3

Antennas – disturbing the thing being measured – open wire lines #2

The article Antennas – disturbing the thing being measured – open wire lines illustrated the pitfalls of a simplistic model of an antenna presented on two terminals of an open wire line.

A more complete representation of the antenna can be formed by making three impedance measurements (Schmidt nd). Continue reading Antennas – disturbing the thing being measured – open wire lines #2

Antennas – disturbing the thing being measured – open wire lines

A common question in online forums relates to inability to reconcile analyser measurements of an antenna system with the transmitter system antenna facing VSWR meter.

The cause is often that the antenna system was changed significantly to connect the analyser.

Seeing recent discussion by the online experts of how the measure the impedance of an antenna system looking into a so-called balanced feed line gives advice that is likely to cause reconciliation failure.

I will make the point firstly that the line is not intrinsically balanced, it is the way the it is used that may or may not achieve balance of some type. I will refer to that type of line as open wire line.

Let’s explore the subject using some NEC models.

I have constructed an NEC-4.2 model of an approximately half wave dipole at 7MHz, it is 20m above the ground, and fed slightly off centre with open wire line constructed using GW elements. At the bottom, I have connected a 2 segment wire between the feed line ends, and two sources in series. Continue reading Antennas – disturbing the thing being measured – open wire lines

nanovna – a surfit of choices

An oft cited advantage of the nanovna are choices:

  • hardware (several clones of the basic thing, the ‘improved’ -H series, the coming -H with bigger screen, the -F with bigger screen… and the future v2);
  • firmware (lots and lots of forks, some hardware targeted);
  • external clients (PC clients, web interfaces, Python / Octave / Matlab code etc).

There is not necessarily interoperatibilty between all instances of each level of this tree. For example, nanovna-F may not share firmware images with the original nanovna and its clones, and vice versa due to a different display protocol.

Some PC clients support features not implemented in all current firmware versions, eg screen capture. Continue reading nanovna – a surfit of choices

nanovna-H – coax connectors

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

The NanoVNA uses PCB end launch SMA connectors, and if one is tightened to anywhere near the SMA specification torque of 1Nm, the assembly is ‘soft’ as the board flexes… a warning that this may cause damage (track cracking or outright separation of the SMA connector).

If you have a bare board, you can counter this torque with a wrench applied across the flats of the female connector, but in my -H, it is fitted in a plastic case and the flats are not accessible. Continue reading nanovna-H – coax connectors

nanovna-H – supplied cables

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

I reported issues with the cables supplied with my nanovna-H at nanovna-H – Chinese junk?

Kurt Poulsen reported some cable measurements, including measurement of a cable supplied with a nanovna. In this case, the cable is a little longer than mine, and although his report does not identify the cable type, it seems that RG174 type cable is reported by most users.

Above is Poulsen’s measurement of s11 of an open circuit 33cm cable of presumably RG174 type. Continue reading nanovna-H – supplied cables

nanovna-H – a summary of the experience so far

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

I purchased what appears to be a ‘genuine’ nanovna-H and it has firmware NanoVNA-H_20191018.dfu installed. During checkout of the delivered device, an issue became evident, an issue worth describing in its own article.

Nevertheless, one online expert assured me it is a fake because genuine ones use green solder mask. Continue reading nanovna-H – a summary of the experience so far

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

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.