Category: Measurement

Finding the electrical length of the branches of an N type T – #2

Finding the electrical length of the branches of an N type T – #1 posed a problem, this article looks at one simple solution.

For convenience, here is the problem.

An interesting problem arises in some applications in trying to measure the electrical length of each branch of a N type T piece.

Let’s make some assumptions that the device is of quality, that the connection from each connector to the internal junction is a uniform almost lossless transmission line of Zo=50Ω. Don’t assume that the left and right branches above are of the same length (though they often are) and we should not assume that the nearest branch is of the same length as the others (and they are often not).

So, the problem is all the uncertain things that connect to the internal T junction. Lets connect a calibration quality 50Ω termination to the left hand port. We now know that the path from the male port to the remaining female port comprises lengths l2 and l1 of low loss 50Ω transmission line with a 50Ω resistor shunting at the junction of l1 and l2. Continue reading Finding the electrical length of the branches of an N type T – #2

Last update: 10th August, 2020, 2:52 PM

Finding the electrical length of the branches of an N type T – #1

An interesting problem arises in some applications in trying to measure the electrical length of each branch of a N type T piece.

Let’s make some assumptions that the device is of quality, that the connection from each connector to the internal junction is a uniform almost lossless transmission line of Zo=50Ω. Don’t that the left and right branches above are of the same length (though they often are) and we should not assume that the nearest branch is of the same length as the others (and they are often not).

Something to keep in mind is that the reference plane for the female connectors is about 9mm inside the T, and you can see the reference plane on the male connector, the nearest end of the shield connection.

With a ruler, the physical length of the left and right female branches looks to be about 13mm, and around 28mm for the male branch… but electrical length will be longer due to an unknown (as yet) deployment of dielectric of unknown type inside the T.

So, put your thinking caps on.

A solution to follow…

Last update: 9th August, 2020, 7:20 PM

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 frequencies 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

Last update: 19th January, 2021, 10:38 AM

nanoVNA – measuring cable velocity factor – demonstration – coax

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 – coax

Last update: 28th October, 2020, 12:36 PM

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

Last update: 27th July, 2020, 6:48 PM

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

Last update: 14th June, 2020, 5:53 PM

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.

Last update: 12th June, 2020, 7:59 AM

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 in a back to back test. (Note: back to back tests are not a very reliable test.) Continue reading nanoVNA-H – measure ferrite transformer – Noelec balun

Last update: 11th February, 2021, 9:52 AM