NanoVNA – interpolation – part 3

This article continues on from NanoVNA – interpolation – part 1 and NanoVNA – interpolation – part 2 which illustrated jagged scans at up to 900Mhz where the reference plane was displaced by 5m of RG58A/U.

A quite practical example where care must be taken is the following one at HF. Let’s say you wanted to measure the feed point impedance of some HF antenna, and the online gurus explained that one way to do that was to calibrate the NanoVNA and normal antenna coax feedline as a fixture, setting the reference plane to the feed point end of the coax.

A Simsmith model for illustration

A Simsmith model was constructed of a 30m (~100′) length of RG213 with a short circuit termination, and the real and imaginary parts of s11 as would be seen by the NanoVNA were plotted.

Let’s say you wanted to sweep from 1.5-33MHz (to include a little each side of the 160-10m bands… partly for reasons to be explained later.)

30m of RG213 @ 33MHz, step size 0.3MHz

Lets focus on the high frequency end where the jagged response is worse.

Again we see the periodic variation of s11 real and imaginary components as shown in the earlier articles. In the plot above, Simsmith as done a linear interpolation of the sweep points, and at 0.3MHz per step, the curves a jaggy. The actual minimum of the blue curve is at 33.88MHz, and the value is about 5% higher than the linear interpolation… which will introduce measurement noise to any VNA sweeps with such a configuration. Sweeps such as this are inputs to the calibration process. Continue reading NanoVNA – interpolation – part 3

NanoVNA – interpolation – part 2

NanoVNA – interpolation – part 1 introduced the principle on which VNA measurements are made and corrected based on a set of error terms derived from measurement of some known loads at the reference plane.

The technique of interpolation as a convenient means of increasing the utility and flexibility of a calibration data set was also introduced, and example raw (uncorrected) sweeps of an OC at the end of about 5m of RG58A/U were given to illustrate the challenge in interpolation with insufficient samples or control points.

A more common data flow is that shown above, where the correction terms are calculated for each of the frequencies in the calibration sweeps, and then those terms are interpolated to the frequencies actually used for a DUT measurement sweep. Continue reading NanoVNA – interpolation – part 2

NanoVNA – interpolation – part 1

A simple two port VNA allows measurement of  S parameters s11 and s21 of a DUT. Port 1 contains a directional coupler to transmit a signal into the DUT, and to capture and measure the amplitude and phase of the reflected wave relative to the forward wave at Port 1 (s11). Port 2 simply measures the amplitude and phase of the signal at its input, the forward wave after it has passed through the DUT relative to the forward wave at Port 1 (s21).

This is typically done by stepping (sweeping) the source on Port 1 through a range of frequencies, specified for example by start and end frequency and the number of discrete steps.

There are several source of error in such a measurement, but by making a series of measurements of some known configurations (Short, Open and Load on Port 1, Isolation and Through to Port 2), those errors can be determined and corrected out of subsequent measurements. So, there is a calibration process to measure and save measurements on these known loads, and then a correction process to apply the calculated corrections to raw measurements.

Early VNAs invalidated the calibration data if sweep parameters were changed, and so corrections were applied to raw measurements at corrections measured and calculated at exactly the same frequency.

This was really inconvenient, especially where no facility was provided to permanently save and restore a set of calibrations.

Later VNAs included the facility to interpolate (but not extrapolate) the calibration / correction data to a new set of sweep parameters. This was really convenient, but introduced a new source of error, the interpolation error.

When all this is done under the covers, users with little understanding of what is going on under the covers can easily obtain invalid / worthless results.

Let’s focus on s11 measurement, though the same issues exist for s21 measurement.

Above is a plot of uncorrected or raw s11 sweeping a NanoVNA-H4 101 points from 1 to 900MHz with nothing on Port 1 (approximately an open circuit OC). Ideally s11 should be 1+j0, but the directional coupler circuitry and small distance to the connector means the amplitude and phase vary as shown in the plot. Continue reading NanoVNA – interpolation – part 1

nanoVNA-H – can firmware be updated if it has a broken USB socket?

The usual method used for firmware upgrade is DFU (Direct Flash Update) using the USB interface and one of many PC clients to load the firmware.

Before attempting a firmware upgrade, be certain of the hardware you have, and the appropriate / compatible firmware file and format. Look for a label on the back, or on the silkscreen of the PCB (though sometimes hidden under the battery… doh!)… know what hardware you have to ensure you load compatible firmware.

Before discussing how to upgrade firmware if the USB interface is not functional, be sure that this problem is not driver related, that there is a real hardware problem. Continue reading nanoVNA-H – can firmware be updated if it has a broken USB socket?

Balanced ATUs – the Holy Grail?

It seems that the Holy Grail of many ham HF enthusiasts is a “true balanced ATU.”

The word “true” in there bodes poorly!

It seems that while there are plenty of online experts who have very strong opinions on common mode current, baluns and ATUs, it is very rare that we see quantitative evidence of their assertions, measurements even.

Less commonly does a “true balanced ATU” description include valid measurement of common mode current as evidence of operation.

A “true balanced ATU” project by LY1O in unusual in that it contains a probe purported to measure and display current balance.

Above is a schematic of LY1O’s measurement system, it has a pair of current transformers each with half wave diode detectors in each leg of ATU output. It is important to note that the detectors convert the RF AC wave into a DC value close to the peak value of the AC wave… so they respond to the magnitude only of the current in each leg. Continue reading Balanced ATUs – the Holy Grail?

An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03)

The article A simple Simsmith model for exploration of a common EFHW transformer design – 2t:14t proposed a simple model.

The previous proposal

Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side.

The model works quite well for low leakage inductance / low ratio transformers but falls down for the higher leakage inductance / higher ratio transformers.

An improved model

The improved model is similar, but Cse in the model above is distributed to the outer sides of the lumped constant model.

Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side. Continue reading An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03)

Modelling an antenna as a simple two terminal resistance is often naive

in the article A simple transformer model of the Guanella 1:4 balun – some further observations I stated:

Note that a two terminal impedance is a naive representation of many if not most antennas, popular, but a naive over simplification that does not facilitate evaluation of current balance.

An example was a recent posting above that used the model to make assertions about the behaviour of a Guanella 1:4 balun.

This article reports results of two experiments with NEC to model an ‘imperfect’ half wave dipole. It is not exactly resonant, but the main issue is that it is tilted from one end to the other, it is not parallel to the ground surface. Continue reading Modelling an antenna as a simple two terminal resistance is often naive

A simple transformer model of the Guanella 1:4 balun – some further observations

A simple transformer model of the Guanella 1:4 balun discussed a simple model for the operation of the device, but a model that is too simple for most RF baluns. Notwithstanding that, it does expose some interesting issues that are not only valid at lower frequencies, but will also be manifest in an RF balun.

Isolated load

Consider the effect of breaking the connection at the red X, so that we now have  what is often referred to as an “isolated load”. Continue reading A simple transformer model of the Guanella 1:4 balun – some further observations

A simple transformer model of the Guanella 1:4 balun

(Guanella 1944) described a 1:4 balun, of a type often known as a current balun.

From Definition: Current Balun, Voltage Balun:

An ideal current balun delivers currents that are equal in magnitude and opposite in phase.

A good current balun will approach the ideal condition. It will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance (including symmetry).

Common mode current will be small.

If the load impedance is not symmetric, then the voltages at each output terminal will not be equal in magnitude and opposite in phase. (Note that for a truly ‘isolated’ load, one well represented as a two terminal load, the currents MUST be equal in magnitude and opposite in phase, but the voltages may not be equal in magnitude and opposite in phase.)

A simplified model

 

Above is a schematic of the Guanella 1:4 balun as often presented, this is an edited graphic from the ARRL manual, so may be familiar to readers. Continue reading A simple transformer model of the Guanella 1:4 balun

Some wooly thinking on Antenna Factor online

Antenna Factor is often given / used as a parameter for an antenna (system).

An antenna with (nearly) constant AF can be quite convenient to simple field strength measurement where the AF value establishes a simple relationship between antenna terminal voltage and the external electric field strength.

Antenna Factor (AF) is the ratio of field strength to antenna terminal voltage for an antenna, dimensionally \({AF}=\frac{E}{V}=\frac{V/m}{V}=1/m\), AF units are 1/m or can be expressed in dB as \(AF_{dB}=20 \log_{10} AF \text{ dB/m}\).

It is lazy practice (though not uncommon) to simply express AF in dB, but wrong.  Continue reading Some wooly thinking on Antenna Factor online