Calculation of impedance of a ferrite toroidal inductor – from first principles

A toroidal inductor is a resonator, though it can be approximated as a simple inductor at frequences well below its self resonant frequency (SRF). Lets take a simple example, a ferrite toroid of rectangular cross section.

From the basic definition \(\mu=B/H\) we can derive the relationship that the flux density in the core with current I flowing through N turns is given by \(B=\frac{\mu_0 \mu_r N I}{2 \pi r}\). Continue reading Calculation of impedance of a ferrite toroidal inductor – from first principles

nanoVNA – measuring cable velocity factor – demonstration

The article nanoVNA – measuring cable velocity factor discussed ways of measuring the velocity factor of common coax cable. This article is a demonstration of one of the methods, 2: measure velocity factor with your nanoVNA then cut the cable.

Two lengths of the same cable were selected to measure with the nanoVNA and calculate using Velocity factor solver. The cables are actually patch cables of nominally 1m and 2.5m length. Importantly they are identical in EVERY respect except the length, same cable off the same roll, same connectors, same temperature etc.

Above is the test setup. The nanoVNA is OSL calibrated at the external side of the SMA saver (the gold coloured thing on the SMA port), then an SMA(M)-N(F) adapter and the test cable. The other end of the test cable is left open (which is fine for N type male connectors). Continue reading nanoVNA – measuring cable velocity factor – demonstration

nanoVNA – measuring cable velocity factor

With the popularity of the nanoVNA, one of the applications that is coming up regularly in online discussion is the use to measure velocity factor of cable and / or tuning of phasing sections in antenna feeds.

‘Tuning’ electrical lengths of transmission line sections

Online experts offer a range of advice including:

  1. use the datasheet velocity factor;
  2. measure velocity factor with your nanoVNA then cut the cable;
  3. measure the ‘tuned’ length observing input impedance of the section with the nanoVNA; and
  4. measure the ‘tuned’ length using the nanoVNA TDR facility.

All of these have advantages and pitfalls in some ways, some are better suited to some applications, others may be quite unsuitable.

Let’s make the point that these sections are often not highly critical in length, especially considering that in actual use, the loads are not perfect. One application where they are quite critical is the tuned interconnections in a typical repeater duplexer where the best response depends on quite exact tuning of lengths. Continue reading nanoVNA – measuring cable velocity factor

Rigexpert Antscope2 – v1.1.1

A new version of Antscope2 has been released.

Online posters are excited that it supports some versions of nanoVNA, and one thread attempts to answer the questions:

The SWR image shows that the SWR minimum is at the center phase angle as you would expect. My question is:

  1. what are the other points that look like resonance,

  2. and should I trim my antenna based on phase?

  3. If so which one?

They are interesting questions which hint the ham obsession with resonance as an optimisation tartget.

Properly interpreting VNA or analyser measurements starts with truly understanding the statistic being interpreted.

In this case, the statistics being discussed are Phase and VSWR, and their relationship.

What is the Phase being discussed?

Above is an Antscope2 phase plot for an archived antenna measurement. The measurements are of a 146MHz quarter wave mobile antenna looking into about 4m of RG58C/U cable. We will come back to this. Continue reading Rigexpert Antscope2 – v1.1.1

Rigexpert AA-600 N connector dimensions

A recent post by David Knight described dimensional issues with the N connector on his AA-600 and problems with the seller in having it resolved.

Warned of a potential quality issue, I measured my own AA-600.

Above, the test of the inner pin forward surface distance from the reference plane on the N jack on the AA-600. The acceptable range on this gauge for the female connector is the red area, and it is comfortably within the red range.

Above is a table of critical dimensions for ‘ordinary’ (ie not precision) N type connectors from Amphenol.

This dimension is important, as if the centre pin protrudes too much, it may damage the mating connector.

Pleased to say mine is ok, FP at 0.192″.

I used a purpose made gauge to check this, but it can be done with care with a digital caliper (or dial caliper or vernier caliper), that is what I did for decades before acquiring the dial gauge above.

nanoVNA-H – measure ferrite transformer – Noelec balun

At nanoVNA-H – measure ferrite transformer I gave an example of using a nanovna to measure loss of a ferrite cored transformer.

Noelec makes a small transformer, the Balun One Nine, pictured above and they offer a set of |s11| and |s12| curves. Continue reading nanoVNA-H – measure ferrite transformer – Noelec balun

VSWR ripple

Having seen some recent discussion where the online experts opined that an example given of a VSWR plot that contained a fairly consistent ripple was quite normal, this article suggests there is an obvious possible explanation and that to treat it as quite normal may be to ignore the information presented.

Above is a partial simulation of a scenario using Rigexpert’s Antscope. It starts with an actual measurement of a Diamond X-50N around 146MHz with the actual feed line de-embedded. Then a 100m lossless feed line of VF=0.66 is simulated to produce the plot that contains a ripple apparently superimposed on an expected V shaped VSWR curve.

This is the type of ripple that the expert’s opine is quite normal. Continue reading VSWR ripple

Transformer loss from half return loss

The popularity of the nanovna with new users has led to a lot of ‘beginners’ seeking advice of the online experts.

A recent question and the responses exposes some common misunderstanding / woolly thinking.

A beginner in search of wisdom asked if there an easy way to measure balun loss?

One of the responses was…

Measure the return loss of the balun with the balun shorted. The return loss should be about twice the balun loss. This is similar to measuring the loss of a shorted or open piece of cable.

This was expanded by others, basically supporting the concept. Continue reading Transformer loss from half return loss