Measure transmission line Zo – nanoVNA – CCS RG6

There are many ways to get a good estimate of the characteristic impedance Zo of a transmission line.

One method is to measure the input impedances of a section of line with both a short circuit and open circuit termination. From Zsc and Zoc we can calculate the Zo, and the complex propagation constant \(\gamma=\alpha + \jmath \beta\), and from that, MLL.

Calculation of Zo is quite straightforward.

The solution for γ involves the log of a complex number \(r \angle \theta\) which is one of the many possible values \(ln(r) + j \left(\theta + 2 \pi k \right)\) for +ve integer k. Conveniently, the real part α is simply \(ln(r) \). The real part of γ is the attenuation in Np/m which can be scaled to dB/m, and the imaginary part is the phase velocity in c/m. The challenge is finding k.

Let’s take an example from recent measurements of 35m of CCS RG6 coax, and extract the s11 values recorded in saved .s1p files @ 1.87MHz. The saved data in MA format, magnitude and angle (in degrees).

Calculate Zo and gamma is flexible and can accept the MA format data directly.

Above, the results. Zo is 74.73-j1.156Ω, and matched line loss MLL is 0.03281dB/m. This MLL is quite a deal higher than you might find in many line loss calculators, they often fail on CCS cables. Continue reading Measure transmission line Zo – nanoVNA – CCS RG6

Nichols: The Two Bird Experiment

With the following introduction, (Nichols nd) tries to demonstrate some important principles. He says…

This really tests your understanding of transmission line theory.

Above is Nichols’ test setup, simple enough.

With the transmitter keyed, the transmatch is adjusted to show zero reflected power on Bird Wattmeter #1. Transmitter is then adjusted to generate exactly 100 watts of forward power indicated on Bird Wattmeter #1. Bird directional Wattmeter #2 indicates about 36 watts of REFLECTED power. (Charts are readily available to show that a 4:1 mismatch gives about 36% reflected power).

Continue reading Nichols: The Two Bird Experiment

Conductors for a Guanella 1:1 balun – discussion

This article discusses some design factors that should be considered when designing / implementing a Guanella 1:1 balun (often known as a common mode choke).

The behavior of a Guanella 1:1 balun can conveniently be separated into its concurrent common and differential modes.

It is the differential mode that is of most interest when it comes to conductors. Continue reading Conductors for a Guanella 1:1 balun – discussion

The requisite knowledge does not come in the box

Every so often, I see a post in an online forum setting out a problem along the lines of:

I just put my brand new dool-band vertical up with 100′ of fitted Super400, and connected my brand new analyser and selected measure coax loss and it says my coax loss it 9.2dB. I gotta pull this crap Super400 down and return it to the shop, who can recommend good coax.

Continue reading The requisite knowledge does not come in the box

nanoVNA – measure Transmission Loss – example 5

This article is demonstration of measurement of Transmission Loss in a section of two wire transmission line embedded in a common mode choke. The scenario is based on an online article  MEASURING DM ATTENUATION of YOUR CMC USING THE NANOVNA AND NANOVNA SAVER.

The reference article publishes measured attenuation or loss being -1.45dB @ 28.4MHz. Of course, the -ve value hints that the author is lost in hamdom where all losses MUST be -ve dB..

The meaning of loss in a generic sense (ie without further qualification) is \(loss=\frac{Power_{in}}{Power_{out}}\) and can be expressed in dB as \(loss_{dB}=10 log_{10}(loss)\).

Some might interpret the result to imply that \((1-10^{\frac{-loss}{10}})*100=28 \%\) of input power is converted to heat in the choke.

The result given (and corrected) as 1.45dB was taken simply from the nanoVNA \(|s21|\) result, and so it is actually InsertionLoss, not simply Loss.

What is the difference? Continue reading nanoVNA – measure Transmission Loss – example 5

High voltage test of a couple of PTFE insulated silver plated copper wires

This article documents a high voltage test of a couple of PTFE insulated silver plated copper wires.

In each case, a single wire is tested, one electrode to the wire and another being an alligator clip clipped onto the wire about 30mm from the end. This approximates a knife edge test which subjects the insulation to the highest electric field strength.

At the time of the test, temperature was 21° and relative humidity 65%. Whilst not extreme humidity, it is sufficient to degrade breakdown often giving rise of an arc over the surface of the wire to the cut end. For that reason, about 30mm of insulation is left clear at each end. Continue reading High voltage test of a couple of PTFE insulated silver plated copper wires

An example and explanation of unexpected common mode choke flashover

An online discussion is developing the design of an ultimate common mode choke, at it reached a stage considered final when a transmit test revealed it could not withstand the unstated transmitter power.

The designer did report measurement at the choke looking into the feed line giving Z=493-j740Ω @ 3.8MHz. There are questions about the validity / uncertainty of the measurement, but let’s take is as correct for the purpose of this discussion.

We can calculate the expected differential peak voltage at a given power level at the point where Z=493-j740Ω. Continue reading An example and explanation of unexpected common mode choke flashover

Mornhinweg ferrite core measurements – #61

Further to Amidon’s method of rating ferrite inductors and transformers, this article discusses some interesting measurements of ferrite toroids by Manfred Mornhinweg (Mornhinweg 2019).

Mornhinweg ferrite core measurements – #31 discussed his measurements of a #31 suppression sleeve.

Above are his measurements of a FB-61-6873 sleeve. Essentially there are two measurements at each frequency, and the expected flux density B is in the ratio of approximately 2:1. He has fitted a straight line on a log/log graph to the measurements at each frequency. The similarity of the slopes is not unexpected, and is a tribute to his experiment design, execution and calculations. Continue reading Mornhinweg ferrite core measurements – #61

Using complex permeability to design with Fair-rite suppression products

Fair-rite allocates some of its closed loop ferrite products to two different categories:

  • inductive; and
  • suppression.

Sometimes the same dimensioned cores are available in both categories with different part numbers and possibly different prices, implying some real difference in behavior, eg 5943003801 and 2643803802 are both FT240-43 sized cores.

Material datasheets often contain a note like this from the #43 datasheet:

Characteristic curves are measured on standard Toroids (18/10/6 mm) at 25°C and 10 kHz unless otherwise indicated. Impedance characteristics are measured on standard shield beads (3.5/1.3/6.0 mm) unless otherwise indicated.

I sought to clarify my interpretation of this clause by asking Fair-rite …whether the published material permeability curves / tables apply to suppression product. Can I use the published permeability curves / tables to predict inductor impedance reliably for suppression products?  Fair-rite’s Michael Arasim advised… Continue reading Using complex permeability to design with Fair-rite suppression products

Black body emissivity of ferrite core material

Some of my articles have contained thermal pictures of ferrite cored inductors and transformers.

I have been asked several times recently about the assumed emissivity and the accuracy questioned, I assume this has been discussed online somewhere.

When first measuring ferrites with non-contact thermometers, I performed some experiments to discover whether the default emissivity ε=0.95 applied. It would be convenient if it did, and permit use of some instruments that do not allow adjustment of ε.

In the past, I have compared the reading of non-contact thermometers with several K thermocouple meters and a Thermomelt indicator, and observed insignificant difference (ie less than the variance of repeated measurements).

The following experiment is a thermal pic of a FT240-43 core on the black plastic case of the instrument. The setup has had hours to stabilise thermally.

Above is a combined thermal image and faint visual image. This instrument has only one readout point, and by moving it around, only 0.1° variation was observed between the background and the core. Continue reading Black body emissivity of ferrite core material