Category: Transmission lines

A prototype 50:75 RF transformer

The design is of a 50:75Ω autotransformer using a single Fair-rite 5943003801 (FT240-43) core. Note that this data is NOT for an Amidon #43 core.

The winding is a total of 6t tapped at 5t, wound as two close wound layers. The prototype used a single conductor stripped from LAN cable. The winding configuration is an attempt to minimise leakage inductance for improved broadband performance.

This is unconventional, but then the conventional designs on Youtube etc do not usually have credible evidence of low InsertionVSWR or other performance. Continue reading A prototype 50:75 RF transformer

Last update: 12th January, 2025, 7:16 PM

NanoVNA-H4 – Surely Smith is better?

A reader of NanoVNA-H4 – VSWR only contended that “surely a Smith chart is superior for antenna system tuning, a universal display.”

Let’s explore that using both VSWR and Smith displays to make tuning adjustments to a real antenna system.

Baseline

Above is a sweep of the antenna system showing both VSWR and Smith chart traces. Continue reading NanoVNA-H4 – Surely Smith is better?

Last update: 5th January, 2025, 8:29 AM

NanoVNA-H4 – VSWR only

Hams wanting to resolve an antenna system problem often seek the advice of the assembled experts on social media of one form or another.

They are often advised by many that “you need an NanoVNA”… no matter what they have described, no matter their own capabilities.

Now if they follow that path, it might be the beginning of wonderful things, or it might be a huge frustration.

The problem is that although the NanoVNA is relatively low cost for its capability, affordable to lots of hams, and arguably it can replace whatever they were using previously, but… the requisite knowledge does not come in the box.

There was a generation of “antenna analysers” that were effectively one port VNAs with fixed calibration that provided capability to sweep an antenna system over a frequency range of interest and give a graphical presentation of VSWR, and not a lot of learning required to exploit the thing. (Yes, some of them also provided for a custom SOL calibration).

These type of instruments are still of great utility, though considerably more expensive that the NanoVNA. I have used a Rigexpert AA-600 extensively, but cannot recommend any Rigexpert product, or more generally any current product in this category.

A possible solution

Is there a way to leverage to capability of the NanoVNA to replace the scanning antenna analyser?

This article describes a solution for very common NanoVNA-H4 hardware with SD card, and NanoVNA-D v1.2.40 firmware. The solution may not work for other hardware and firmware.

So, the objective is to simplify the interface to present just a VSWR curve as shown above. Continue reading NanoVNA-H4 – VSWR only

Last update: 3rd January, 2025, 1:56 AM

NanoVNA-H4 – inductor challenge – part 9

This article continues on from NanoVNA-H4 – inductor challenge – part 8, this time with an inductor that might be a candidate for a plate RF choke for a HF valve PA.

Above is the prototype DUT, it is 142t of 0.25mm enamelled copper wire close wound on an 18mm PMMA (acrylic) former, it measures 130µH well below first self resonance.

Let’s sweep it with a NanoVNA. Continue reading NanoVNA-H4 – inductor challenge – part 9

Last update: 30th December, 2024, 5:53 AM

Perplexity AI calculating Zo of a two wire transmission line – comparison with measurement – updated

During the process of writing the article Analysis of output matching of a certain 25W 144MHz PA an estimate was made of the characteristic impedance Zo of a section of twisted enamelled 0.71mm copper wires.

Perplexity AI

Let’s ask Perplexity AI for an answer.

Perplexity AI gives approximately 108Ω. Continue reading Perplexity AI calculating Zo of a two wire transmission line – comparison with measurement – updated

Last update: 1st January, 2025, 7:15 PM

Is it possible to predict the terminal voltage of an installed antenna with feed line – #2?

Is it possible to predict the terminal voltage of an installed antenna with feed line? discussed prediction of an intended antenna using an NEC model.

ATU terminal voltage

This article shows how to predict ATU terminal voltage based on VNA or analyser measurement of the antenna system looking into the feed line. We will also look at an example of the voltages within a T ATU.

Above is a calculation of peak voltage at the ATU antenna terminals at 100W calculated from a saved .s1p file made looking into the feed line from the ATU position.

Above 3MHz, these voltages are modest at 100W. These voltages are impressed upon the balun that sits between feed line and ATU via 300mm of RG-400. Realise that at 1600W, the voltages would be four times that in the graph, ie up to about 3000V at some frequencies. Continue reading Is it possible to predict the terminal voltage of an installed antenna with feed line – #2?

Last update: 14th December, 2024, 11:58 AM

Common mode current and RF feed lines – an interesting thought exercise from WA7ARK

Mike (WA7ARK) recently posted details of an interesting experiment: Measuring currents in an unbalanced dipole.

Above is a diagram of his experiment, and several points at which he measured the magnitude of current with his RF current probe. Continue reading Common mode current and RF feed lines – an interesting thought exercise from WA7ARK

Last update: 28th November, 2024, 4:53 AM

Common mode current and RF feed lines

At any one point along an RF feed line, there are currents in the conductors of sinusoidal waveform with magnitude and phase. These currents can be decomposed into differential and  common mode components to assist analysis of behavior which is often mode specific.

This article explains the composition of feed line current, and its decomposition into differential and  common mode components.

Over the length of the feed line, the differential and common mode components of current are typically standing waves each with different phase velocity, so the magnitude and phase of differential and common mode components may vary along the feed line, as will the phase difference between the components.

We will discuss the principles in two contexts:

  • two wire transmission lines; and
  • coaxial transmission lines.

The first is simpler and lays down important concepts for the second case. Continue reading Common mode current and RF feed lines

Last update: 24th November, 2024, 3:01 AM

Measuring RF feed line common mode current – relative or absolute measurement?

Common mode current is one of the very popular topics on social media, but one does not see much quantitative discussion, mostly just a lot of hand waving without supporting measurement data.

There are a a number of commercial instruments instruments purporting to measure RF feed line common mode current (Icm), but many are specious frauds.

There are quite a few articles on the ‘net describing DIY  ‘balance measurement’ techniques / instruments and again, many are specious frauds.

There are a very small number of commercial instruments and DIY designs that would appear to be soundly based, many of them are ‘uncalibrated’ in that they do not give absolute current readings, and even relative readings may be impacted by non-linear response.

So, what are we trying to measure?

Bear in mind that Icm is usually a standing wave, and ‘measurement’ means a survey at several points along the feed line.

In my experience, for HF, Icm with a 100W transmitter feeding a reasonably efficient antenna system is likely to result in Icm greater than about 10mA, and more that 100mA is poor… so the measurement range of interest is really between 10mA and 100mA. Continue reading Measuring RF feed line common mode current – relative or absolute measurement?

Last update: 13th November, 2024, 3:47 AM