Month: May 2024

NE6F’s common mode current tester – Part 2

NE6F’s common mode current tester – Part 1 ended with the following:

Common mode current adjacent to a small choke

Consider a straight section of coaxial feedline not close to other materials, and with a small common mode choke inserted in the feedline. A “small” choke means one that is a very tiny fraction of a wavelength, say λ/100, from connector to connector.

Q1: Ask yourself that if say 1A of common mode current flows into one connector, what is the common mode current at the other connector?

Q2:What is your answer if you were told the balun was specified to have a CMRR of 20dB?

The answer to Q2 is relatively easy, CMRR is not a meaningful statistic for a common mode choke deployed in a typical antenna system, it would not change the answer to Q1.

The answer to Q1 needs a longer explanation… let’s do it!

Common mode current distribution is almost always a standing wave.

Above is a plot of current distribution of an example dipole antenna system with coax feed. The dipole is slightly off centre fed to drive a significant common mode current on the vertical coax feed  which is grounded at the lower end. Continue reading NE6F’s common mode current tester – Part 2

Last update: 1st June, 2024, 2:54 PM

A test of NanoVNA s21 series through impedance measurement

A test of NanoVNA s21 series through impedance measurement.

This technique is very popular, its users believing it gives superior accuracy, especially on higher impedance DUT.

Above is the test setup, a nominally 100Ω resistor is connected from Port 1 inner contact to:

  • Port 2 shield for s11 reflection measurements; and
  • Port 2 inner contact for s21 through measurements.

Note: my NanoVNA-H4 has been modified by addition of a short direct wire between the ground side of Port 1 and Port 2 jacks inside the case (see above).

I have intentionally used an ‘ordinary’ metal film resistor with wire leads to demonstrate with a readily available component… but you could buy some similar value 0.1% tolerance SM resistors (50pcs of 100Ω 0.1% 1210 resistors for less than $10 on Aliexpress) and solder some pigtails to them and depend on their marked value.

The instrument was SOLIT calibrated with the reference plane at the Port 1 connector. A 200mm coax jumper was used for the through calibration, it has a delay of 1.5ns and loss of 0.05dB @ 1MHz which needs to be adjusted out. Continue reading A test of NanoVNA s21 series through impedance measurement

Last update: 3rd June, 2024, 8:51 AM

Improving ‘s21 shunt-through’ and s21 series through’ measurement of impedances

This article describes some operational changes to two calculators on this site:

Calculate Z from series s21 – enhanced

The previous versions of the calculator required input of the impedance of Port 2 (Zl), and it calculated and reported the Thevenin equivalent source impedance of Port 1 (Zs) from the Zc measurement.

The important data is actually Zs+Zl and this can be directly calculated from Zc, so the calculation has been simplified and entry of Zl is no longer needed. See Derivation of the expression for the unknown impedance in an s21 series through measurement.

The results for Zu are unchanged. Continue reading Improving ‘s21 shunt-through’ and s21 series through’ measurement of impedances

Last update: 29th May, 2024, 2:50 AM

NE6F’s common mode current tester – Part 1

A correspondent asked my thoughts on a Youtube video featuring…

NE6F’s common mode current tester

Above is the schematic of NE6F’s common mode current tester.

The concept is that current probes A and B are placed either side of a current mode choke, and by calibrating and switching between them, a relative reading of current on one side compared to the other may be found. Continue reading NE6F’s common mode current tester – Part 1

Last update: 16th August, 2024, 12:51 PM

Baluns: you can learn by doing!

This article presents a simple way to make measurements of a prototype Guanella 1:1 current balun, measurements that can guide refinement of a design.

The usual purpose of these transmitting Guanella 1:1 current baluns is to reduce common mode feed line current. Not surprisingly, the best measure of a device’s effectiveness is direct measurement of common mode current (it is not all that difficult), but surprisingly, it is rarely measured.

Above is the prototype balun being a Fair-rite 5943003801 (FT240-43) wound with 11t of solid core twisted pair stripped from a CAT5 solid core LAN cable and wound in Reisert cross over style. Note that Amidon #43 (National Magnetics Groups H material) is significantly different to Fair-rite #43. Continue reading Baluns: you can learn by doing!

Last update: 13th August, 2024, 10:28 AM

Holzforma / Farmertec G372XT chainsaw – early evaluation

I purchased a Holzforma G372XT chainsaw, it is a Chinese clone of the now discontinued Husqvarna 372XP X-TORQ. It is a relatively new technology carburetted engine without introducing electronic auto tune, a 20 year old design.

There are plenty of online discussions about the 372XP X-TORQ overheating, hot seizures, and pics of blued big ends, scored pistons and bores etc, so whilst the design achieves quite high power output for 71cc, one is warned to pay attention to mixture, fuel quality, and lubrication. This might be an engine that should not be used with other than full synthetic two stroke oil. Continue reading Holzforma / Farmertec G372XT chainsaw – early evaluation

Last update: 21st June, 2024, 12:24 PM

Velocity factor measurement

A frequently asked question is how to measure transmission line velocity factor. The wide adoption of the NanoVNA has spurred these questions.

So, it is good that ownership of a NanoVNA stimulates thinking and search for applications of the instrument.

I note that when these questions are asked online, early responses include recommendation of using the VNA to perform a TDR transform to measure the electrical length of the cable and calculate the velocity factor as \(vf=\frac{\text{PhysicalLength}}{\text{ElectricalLength}}\). The resolution of the NanoVNA swept from 1 to 1500MHz with 401 steps is around 60mm, so you can only measure to about 0.25% resolution if you have a test cable 20m long. So this method might not be very practical in a lot of situations for that reason alone. More later…

It is practical to measure the quarter wave resonance of a shorter section of test cable to better than 0.1% resolution.

A significant problem measuring short cables is the contribution of the test fixture, the reference plane is usually not at the very beginning of the uniform cable, if you know where that is anyway (it may be inside a connector).

Let’s look at an example, a measurement of some ordinary nominally 300Ω TV windowed ribbon line.

Above is the test fixture, the VNA and 1:1 transformer board have been OSL calibrated to a point very close to the pins of the grey terminal block. The transformer module is described at Conversion of NOELEC style balun board to 1:1. Continue reading Velocity factor measurement

Last update: 3rd July, 2024, 9:26 AM

Measurement / evaluation of an RF filter response with NanoVNA

One sees questions online about measuring a filter response using the NanoVNA.

Let’s discuss filters, the DUT, for a moment.

Filters are usually designed to operate with a given source impedance and given load impedance. It affects the response, and measuring a filter with different source and load impedance that it was designed for, or specified in a test procedure gives invalid results.

This article gives an example measurement of a low pass filter designed for a 50Ω system. The filter is designed for harmonic suppression from a transmitter on the 40m ham band (7-7.2+MHz).

We are interested in measurement of the filter as a two port device.

The VNA as a two port test instrument and suited to measurement of this filter because the impedances match (well, approximately in the pass band).

Above is a block diagram of the signal flow in a two port test with a VNA. An incident wave (Vi) from Port 1 is partially reflected at the DUT input (Vr), and partially transmitted by the DUT to Port 2 (Vt). We make the assumption that there is zero reflection at Port 2 (ie that Port 2 input impedance is 50+j0Ω… though that might not be a very good assumption with low end VNAs… measure it). Continue reading Measurement / evaluation of an RF filter response with NanoVNA

Last update: 14th June, 2024, 8:21 AM

The obsession with measuring extreme spot frequency impedances of broadband common mode chokes

The fashion for measuring HF broadband common mode chokes for antenna systems is to use the s21 series through measurement technique, the basis for which is specious as discussed elsewhere on this blog.

Let’s look at an example common mode choke, this time for suppression of ham transmitter ingress to a VDSL2 line.

The spectrum of interest is 1.8 to 10.2MHz, this is the overlap between VDSL2 spectrum and ham bands above 1MHz.

Somewhat arbitrarily, a design specification was drawn up for a prototype choke which would be tested for effectiveness. The draft specification was: Continue reading The obsession with measuring extreme spot frequency impedances of broadband common mode chokes

Last update: 29th May, 2024, 5:39 PM