Month: May 2024

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: 20th May, 2024, 12:00 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.

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: 18th May, 2024, 8:15 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: 12th May, 2024, 5:55 PM

Study of suitability of UHF bulkhead adapter to a Diamond x50-A antenna system

There must a a thousand articles on the ‘net on why UHF series connectors are good or bad, this is another.

The example

The example for discussion is a Diamond X-50A 2m/70cm vertical antenna on about 11m of LDF4-50A feed line, N type connectors are used throughout.

At commissioning, a sweep looking into the feed line was made using an Rigexpert AA600 analyser and the results saved. The file used for this study is a sweep from 143-151MHz.

Above is the UHF series bulkhead adapter studied in the simulation. It is 50mm end to end, the simulation uses 60mm to account for the impedance discontinuity in the mating plugs. The adapter is modelled as 60mm of lossless 35Ω line with VF=0.7 (typical of UHF series adapters). Continue reading Study of suitability of UHF bulkhead adapter to a Diamond x50-A antenna system

Last update: 8th May, 2024, 9:12 AM

Conversion of NOELEC style balun board to 1:1 – through measurement

Conversion of NOELEC style balun board to 1:1 left readers with a challenge to measure the through performance of the modified balun board.

The link grounding the centre tap was cut for this test to float the secondary so that one side could be grounded. This will give almost identical response to the case where the centre was grounded.

Above is the test configuration, the yellow thing is a top view of a modified plastic clothes peg which is used to clamp one of the wires from the transformer to the SMA threads. Continue reading Conversion of NOELEC style balun board to 1:1 – through measurement

Last update: 6th May, 2024, 6:42 AM

Conversion of NOELEC style balun board to 1:1

This article describes a small 1:1 balun for use in measuring field strength using a TinySA Ultra and a small loop antenna.

The balun is also useful for measurements using the NanoVNA (or any VNA), eg for measuring two wire transmission line parameters.

Materials

Some “TC1-1T RF Balun Transformer 0.4-500MHz 1:1CT” transformers were purchased on Aliexpress, 5 for $7. The part number is a Mini-circuits part, but these are likely to be clones. The balun boards also came from Aliexpress, about $4 each. Also needed are compensation caps of 10pF (0805).

Conversion

The boards come with a nominal 1:9 transformer and in my experience a capacitor (though I think the NOELEC board may use a TVS). In any event, it should be removed and the transformer removed. Fit the new transformer and solder a short circuit across the cap pads.

High end compensation

See High end VSWR compensation in a ferrite cored RF transformer for an explanation of compensation.

Connect the board to VNA Port 1 and sweep it to 200MHz, adjusting s11 e-delay so that the trace is a dot at the left of the Smith chart X=0 line. This adjusts the reference plane approximately to the position of the capacitor. Write down the e-delay value for later.

Above is the modified transformer and some calibration parts loads. There are two loads, 200Ω 1% and the other has 2×100Ω 1% in parallel to give 50Ω 1%. The calibration parts are made on ordinary long header pin strips, break three off and pull the middle one out with pliers. Slide the pins to a suitable projection and cut off the other end to uniform length then solder the resistors or shorts to them. This calibration effectively sets the Port 1 coupler Directivity to 46dB (which is quite good). Continue reading Conversion of NOELEC style balun board to 1:1

Last update: 11th May, 2024, 4:29 AM