Transmission Lines and Transformers - Part 1

Off Topic
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Over the last 2-3 weeks SOTA Activators in GM and elsewhere have been out making the most of the propitious UK weather. Their endeavours will surely be keeping them physically fit.

In turn, keeping mentally fit is of equal importance, especially for those with more than a few ‘turns on their PA coil.’

When it comes to RF transmission lines, most hams use coaxial cable, fewer 2-wire line, and fewer still wave-guide.

1 Coax

As some may know, coax and 2-wire line typically operate in what is known as TEM mode - Transverse Electric and Magnetic fields. For coax, something like this:

coax_TEM
coax_TEM409×358 17.2 KB

In TEM mode, we typically have a 2-wire circuit. For coax, a net RF current travels along the centre conductor, and has a return path along the INSIDE of the braid. Practically, we may use a ‘choke balun’ to deter current flow along the OUTSIDE of the braid.

Nominally, the Electric and Magnetic fields seen internally within the coax do not escape to the outside. A cable such as LDF4-50, with its solid-copper outer, may well do a better containment job than, say, RG-58 with a simple braid.

Note that the centre conductor, the electric field, and the magnetic field are ORTHOGONAL - all at right-angles to each other. What is more, neither the Electric field, nor the Magnetic field, ‘flow’ along the cable - hence the Transverse part of TEM.

2 Wave-guide

By comparison, wave-guide operates in various TE or TM modes - either Transverse Electric or Transverse Magnetic fields. It cannot run in TEM mode.

With a simple TE-mode wave-guide, the Magnetic field effectively ‘flows’ along the wave-guide transferring RF energy without the need for a 2-wire circuit. On-line, there are useful pictures of this behaviour, better than I can draw.

Some of this material can be quite difficult to grasp, and you are never far from Maxwell’s equations and their daunting maths. I try to keep my distance from such things :slight_smile:

In part 2, we’ll look at the difference between conventional transformers and so-called ‘transmission line’ transformers; the behaviour of the ubiquitous EFHW 49:1 UNUN will be reviewed.

The English language has an expression ‘Coals to Newcastle’. It arises where you might be explaining something that people already understand.

For some, the above may well be ‘Coals to Newcastle’, my apologies.

73 Dave

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I agree with your statements about coax feeders, but I feel a bit of clarification might be needed for some readers.

It’s due to the ‘skin effect’ at RF frequencies that the forward current flows on the outer surface of the inner conductor, while [as you say] the return current flows on the inner surface of the outer shield/braid

In a perfect setup, the return current travels only on the inner surface of the braid, creating a sealed electromagnetic field between the centre conductor and the shield (i.e. inside the coax, so no ‘stray’ RF).

When the antenna is unbalanced and connected to an unbalanced feeder (coax), the current from the inner conductor cannot return solely on the inside of the shield. This creates a ‘stray’ or common-mode current that travels down the outer surface of the braid. Hence the need for the choke.

NB: Some have incorrectly written that the CM current flows outside (above) the braid. That is physically impossible. What they really mean is, the CM mode RF energy is outside, hence problems in your shack.

On a frequently-misunderstood point (for those who were mistaught or not taught this properly at school): In all DC and AC (including RF) circuits, the energy transferred from the source (e.g. DC battery, RF transmitter) to the load (e.g. light bulb, antenna) is by electric and magnetic fields outside of the connecting wires / transmission line and not by the electrons in the conductors.

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What is current? Flow of electrons

One in, one out.

Imagine a tube full with balls inside. If you put one more ball at the begginig one at the end will fall out.

By the way, what is the definition of 1 ampere

Electron rules

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Hi Damir, not sure if you are just adding to the conversation or challenging my statement about the role of electrons in the transfer of energy. Are you conflating ‘current’ with ‘energy’?

Anyway, w.r.t. current …

More strictly, it’s the net rate of flow of charge (free electrons or ions)

This is a commonly-used analogy but despite its simplicity I don’t care for it as it’s misleading. The analogy suggests the balls are pushing each other all the way through, suggesting high-speed transport. In reality, the electric field (voltage) forces electrons to drift very, very slowly - typically only a few mm per second (in a DC circuit) - while the ‘push’ (the electric field) moves at near the speed of light.

In an AC circuit like the 50Hz mains electrons drift about 3 micrometres before the 50Hz cycle swaps direction. In RF circuits the electrons oscillate about 10^-8 to 10^-10m in each direction.

Even though the distance moved is tiny and the contribution to the current by individual electrons is very tiny, there are typically 10^20 free electrons in 1mm of typical wire, so the net current is a sensible macroscopic measurable value.

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Yes, I am challenging your statement that energy is transfered by electric and magnetic fields.

These fields are side effects of current. Without flow of current there are no electric and magnetic

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You don’t need to believe me but you should challenge your belief, e.g. read an undergraduate level physics text on the topic, or go online and ask if my statement is true or not.

The concept is well known and uncontroversial in Physics, i.e. energy flows via the Poynting vector, which represents the direction and magnitude of electromagnetic energy flow, perpendicular to both the electric and magnetic fields. Even many professional electrical / electronic engineers (of which I was one for 35 years) were taught only simple analogies which by virtual of being simple are usually flawed and don’t match the physical reality.

NB: we are talking about a classical physics explanation here. I didn’t want to get into a quantum explanation which involves weird concepts like virtual photons and photons that travel in all possible paths in the universe.

That’s partial correct: the magnetic field is a ‘side effect’ of the current. But the electric field that the electrons ‘feel’ is created by the source (e.g. battery, transmitter) and by opposite charges (in this case ions) on the surface of the conductors.

The drifting electrons (current) create a magnetic field, and it is the combined electric E and magnetic B fields that transport the energy, not the slow-moving electrons themselves.

When a voltage is applied (e.g. from the DC battery, RF transmitter), the free electrons accelerate but drift very slowly, while the surrounding E & M fields propels energy along the conductor at nearly light speed.

This YouTube video has some excellent animations that describe the concepts …

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Yes, I know you would point me to this channel. He is talking nonsence. After I watch his video that in one direction light travel with half of speed and in oposite direction in the moment I blocked him

There are many “information” spreading nonsence and stupidity on internet

Electron are moving very slowly but effect, as you said before, spreads with speed of light

All effects are because electrons. Wires are hot, transistors are amplifaing signal by controling flow of electrones.

And, you told me to serch online for informations. I am online :slight_smile: give me and the rest of us explanation

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And, I would not continue this discusion. Three posts are enough.

I thank you for exchanging our thoughts and thank readers for their patience

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Addressing the wider readership (if any have stayed with it this long!), a point of clarification:

Veritasium’s original “The Big Misconception About Electricity” YouTube video sparked some controversy. As a result, he made this follow-up video (released April 2022) [the one I linked above] with the help of some top engineers to 1) address some of the points or assumptions critics said that he didn’t make clear in the original, and 2) address the specific objections from critics (including clips from their objection videos).

I hope open-minded readers won’t be put off if you would like a quick (23 minute) description of the concepts. Or check other sources, read some Physics or ask Google AI.

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I do remember in around 1988/89 ish measuring the drift velocoty of electrons in a conductor. It was either a practical part of A level physics at the time or first year Electronic enginering at Leeds Uni. I cant remember which or even how we did it, but what I do remember that was counter-intuitive to my previous previous 12 years at school where we had been taught that the electrons zoomed around a circuit at the speed of light. I also know it took me a while to grasp the concept to overcome the years of indoctrination.

I sometimes wonder that if we were taught the more complex topics from the start whether our understanding overall would be further along due to not having to “unlearn” something first. Its almost like something is taught because it has a simple equation that “sort of works for the real world” and that can be used in an examination. Rather than saying , look, its complicated, but remember that “This” does “That” to “Something”. If your interested and want to learn how and why then do your O’Levels, A’Levels and Degree and it will all follow on rather than the learn this for O’level, now for A’level forget that and learn this because it better and then for Degree, forget the A’level, this is better.

And don’t even get me started on the guff that was being taught around models of the atrom in chemistry back then.

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You probably used a Hall probe to measure the free electron density (indirectly), and measured the current & conductor diameter to calculate the drift velocity.

Another Leeds Uni graduate. I was there about 15 years before you but you might have predated another Andy, @GM4LLD.

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Me? 80-83 B.Sc Electrical & Electronic Engineering. It seems it’s now called the School of Electronic & Electrical Engineering but I’m sure it was just the Dept. Electrical & Electronic Engineering back in my day. One of the PhD students who helped out with practicals and final year undergrad projects I remember dealing with was Chris Snowdon who became Sir Chris Snowdon FRS, FIET. He has quite an impressive C.V. :wink:

Most of the time the simpler explanations work as long as you know when they don’t work. e.g. thick cable vs thin cable carrying current is like a thicker pipe vs thin pipe carrying water. This breaks down with RF and skin effect.

There was no electron drift measurement either at A-level physics or Uni for me. We did repeat JJ Thomson’s e/m experiments at A-level. That might have actually been part of the A-level practical exam which was done on a continuous testing basis, i.e. we did one practical exam per term as there was only a limited amount of apparatus. Someone would do e/m and another something to do with different radioactive sources or pendulums etc.

Electron flow in a conductor was explained at the time as “you push an electron in a cable at one end and it pushes one out of the other end but the drift velocity is quite slow”. It met the needs of the time. I should really do a little study of these fundamentals once I have infinite free time in 7 weeks time :slight_smile: I looked at some of the maths behind pendulums just now on Wiki and none of it caused me to remember formulas, as it was 46-47 years and I’ve not needed the maths since!

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Exactly this, but examinations don’t test that progression, they only test the latest taught elements. So you don’t get to show that you know which model to use when and why.

I was 1988 intake. I enjoyed myself quite a bit at Leeds (socially at least), it was a great place and an easy bus ride from Headingly to Otley (Caley Crags) and to Ilkley for the cow and Calf. Later years I had a car and thus very popular which brought Almscliffe and Brimham Rocks into reach.

My course choices were somewhat affected by things I had been advised before O’Levels. Being part of the early 1980’s home computer boom (Spectrums, BBCS C64, I had Orics) I was advised not to do O’level computing as I’d find it boring. This stopped me from going on to A’Level and closed off further avenues (possibly for the best looking back, more later on this) .So being newly licensed I plumped for Electronic Engineering of which the only enjoyable parts for me was learning C and spending a few days in the rather intimidating high voltage lab.

After scraping through year 1 I decided to switch course which led to one of my only regrets in life. A choice of 2. Materials Science which was the big thing at the time and Leeds was quite high up the list of best places to study and Genetic Engineering for which a move to Cambridge would have been the best option but I didn’t check to see if this was a viable option.

My A’level Biology teahcer had practically begged me to go down the genetics route as apparently he thought I had a flair for it to the extent he asked my grandfather to put pressure on me (he was also a teacher). But my young and practical mind was thinking “yes I like it and enjoy it, but what use is it ever going to be” - yes well., hindsight is a fantastic thing isn’t it.

The only enjoyable thing about material science was learning Fortran 77 (I’m seeing a pattern here) which almost got me a job maintaining the software to control a glass furnace for making fibreglass ( a ridiculously complex and fascinating subject) .

When I say I have regrets, I actually don’t mean it either. All decisions and chocies have led to where I am today , so to have a regret would mean I am not happy now which is most certainly not the case.

As for the computing route, I am not sad I didnt go down that route, could you imagine a graduate from 40 years ago stumbling around in todays world (yes, one would hope one wouldn’t stick at the 1980’s level and keep up with the times).

So full circle, almost 40 years later I’m into the very final stages (1 assignment and 1 project remains) of a Computing Degree (with the OU). Now that has been very enjoyable and a totally different learning experience from the brick uni days.

As for famous people, Richard Ashcroft (The Verve) was the year below me at 6th form college who my only memory of was an arguement when he said to my friend that Ronnie James Dio (based on a T-shirt) was rubbish (not exact words) and nowhere near as good as his band. He seemed a miserable bugger back then too.

I think Bill Bruford (of Yes fame) would have been there about the same time as yourself Andy (CPZ) and you may have been a bit later than Mark Knopfler.

And here endeth the lesson in both taking a subject way off topic :slight_smile: and procrastinating when I should be wrting an assignment on transfer learning (in AI).

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My first programming language was Fortran on the Maths computer in the Maths-Physics block (on or below Red Route). I recall some fellow students getting addicted to programming (albeit having to use the now-antiquated punchcard entry method) and neglecting their final-year studies. I realised the effect on me too, stopped going there, and managed to scrape through my finals.

Of course, in them days it was all long hair, beards and flared jeans …

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I have read some of these ‘revealing” statements but my question arising, Mr Speaker, is in that case how do valves work? Electrons passing charged grids and getting to the anodes seem to be the way they work. Basic Semiconductors like diodes appear to require the current to stop flowing when the diode is reverse biased.. more complex semiconductors like LSI ICs and CPUs, memory chips, etc would all fail if the fields interlaced and interacted. Wouldn’t they?

Hmmm

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