Category Archives: Antennas, Baluns and EMC

The Private Life of Coaxial Cable

This is a common theme from several of my talks over the years about Ferrite Chokes and Baluns. By expanding the topic into a blog format, I hope to be able to explain it in more detail, and add some FAQs as time goes on.

This well-known image originated with Walt Maxwell W2DU, the author of many articles about transmission lines. It brings out several important points.

  • The inside of a coaxial cable truly is “private” – completely screened from the outside world. This is one of the rare statements that can be handed to absolute beginners as a simple fact, and still remains valid all the way through to graduate-level EM physics.
  • The private interior of coax is due to the skin effect which means that, at RF, current flows only on the surfaces of conductors.

    There are many confusing illustrations in textbooks that make some people imagine the skin effect only applies to electrical conductors of some particular shape and size, or in some particular type of circuit. That isn’t true! The best and most generalized derivation of the skin effect I have been able to find is reproduced here. All the high-level EM physics and maths leads to a simple and very powerful conclusion:
    Whenever and wherever an RF current is flowing (regardless of the circuit configuration, regardless of the reason) the skin effect will always be present.
    RF current will be forced to flow very close to the outer surface of the conductor, and there is no net current flow into or through the thickness of the material. 
    Because we know the skin effect will always be present, we can use that as a starting point when tracing RF current pathways on complex shapes such as coaxial cables and shielded loops. Suddenly, the whole topic of shielding (and what you need to do to make it work correctly) will begin to make sense!
    There is one condition, though: you have to hold onto that idea consistently. Many of the myths in RF engineering arise because people aren’t being logically consistent. They imagine they can switch these basic ideas on-and-off to suit their own preconceived notions… and physics just ain’t like that.  
  • The skin effect in coaxial cables means that the outside surface of the shield is a completely separate conductor from the inside surface.
    The outside surface of the shield is part of the outside world. This is where the so-called “common-mode” currents flow.
  • Let’s look again at that diagram. It shows the open end of a coaxial cable carrying three separate RF currents (I1, I2 and I3). The cable is connected to two wires coming from the antenna and carrying currents I4 and I5.

Private Life of Coax

  • Inside the coax, current I1 flows on the outside surface of the centre conductor, and current I2 flows on the inside surface of the shield. Because the centre conductor is entirely surrounded by the shield, electromagnetic coupling is very tight and this forces I1 and I2 to be exactly equal and opposite. Even a very short length of coax is sufficient to enforce this to a very high degree of accuracy.
    • Due to the way coax is constructed, the inside of the coax will only support energy transfer in the differential, transmission-line mode.
    • Inside the coax, the common-mode current is zero.
  • Now look at the currents I4 and I5 close to the feedpoint of the antenna. In the real world, there is no such thing as a perfectly balanced antenna. Here are a couple of slides to make that point:

Because a perfectly balanced antenna is such an UN-natural phenomenon, I4 and I5 will almost always be UNequal. 

  • When we connect the antenna to the coaxial feedline, the current I4 becomes re-labeled as I1 flowing on the centre conductor, but it’s still the same current.

    Now if I4 and I5 happened to be exactly equal, I5 would flow entirely into the inside surface of the shield to become I2, and everything would be fine. But I4 and I5 never are the same, so what happens to the difference?
  • Point X on the diagram is where I5 from the antenna can divide into two parts, shared between I2 and I3. I2 is the part the flows into the inside surface of the shield, and as we have already seen, this must be numerically equal to I1.

    The difference between I4 and I5 becomes I3, flowing on the outside surface of the shield. Note that this happens only at the open end of the coax, where the inside and outside of the shield meet together. Everywhere else along the coax, I2 and I3 each create their own skin effects on opposite surfaces of the shield, that keep those two currents physically separated.

Those are a lot of observations to keep in mind, but they all lead to one simple conclusion.


  • Because the currents I4 and I5 in the antenna are never quite equal, they can be mathematically resolved into a differential-mode component and a separate common-mode component.

  • However, a coaxial feedline will physically do the same thing! At RF, where the skin effect creates a third conductor on the outside of the shield, the pure differential-mode component of the incoming two-wire connection will automatically be routed into the private, shielded interior of the coax to become I1 and I2. Meanwhile the common-mode component is routed separately onto the third conductor, the outside of the shield, to become I3.

And that’s the private life of coax.



Ooh, we’ve got lots of those… if you follow this blog, I will add to and answer these from time to time.

FAQ1. “I don’t feel comfortable about using the term ‘common-mode’ in relation to coax.”

The classical concepts of common mode and differential mode were based on twin-wire transmission lines, which can support the two different modes on the same two conductors. However, this was long before coax was invented, and coax doesn’t work like that.

The difference is that, at RF frequencies, the skin effect creates a third conductor on the outside of the shield, so the coax becomes a three-conductor transmission line which will carries its common-mode current separately on the outside of the shield. Seen from that viewpoint, it is perfectly justifiable that engineers call I3 “the common-mode current”… because that’s exactly what it is! 



Clean Up Your Shack – 2019

The online follow-up to my talk for the 2019 RSGB Convention.

3 chokes

What’s new since 2015


  • Constant changes in the prices of ferrite cores (but still the same Best Buy supplier)
  • Updated Whole-Shack Mains Filter



  • Extended slide pack (PowerPoint) includes more slides showing how to build the updated Whole-Shack Mains Filter.
  • View ‘Clean Up Your Shack 2015’ (YouTube) for more detailed background.
  • Slides from my 2010 talk about chokes and baluns  to drill deeper into that subject.
  • All mains earth (green/yellow wire) connections are to the earth tag on the top end of the filter. 

THE PROBLEM SOLVER: Fair-Rite 0431177081 snap-on core

Not cheap, but well worth it. If RF noise threatens to take your hobby away, then surely it’s worth something to get it back?

  • Manufacturer’s data  Ignore the photograph – this biggest bead doesn’t look like that.
  • Current Best Buy supplier – and always near the top of the list –  is
    Again, ignore the photograph, trust the part number.
    Notice the big price break at quantity 10. This makes an ideal club purchase.
    UK prices are in GB£ but do not include VAT. However, that is all you have to pay. Three-day FedEx shipping from the USA is streamlined and free for orders over £50+VAT.

Parts list for the Whole-Shack Mains Filter

  • Packaged 15A or 16A mains filter, single phase, 250VAC rated.
    Within 2 days of publication, the Roxburgh RES5-F15 filter from Farnell was sold out! The specific type of mains filter is not critical, so instead let’s try the Schaffner FN2030-16-06 (also from Farnell, but many more in stock).
  • Large Fair-Rite core 0431177081: Mouser (see above) or Farnell
  • Qty 2, Fair-Rite oval core 2643167851: Mouser or Farnell
  • Plastic box CE-TEK GR17012 to fit the above parts: CPC  EN84544
  • Qty 2, plastic cable glands, 5-10mm size: CPC CBBR7352
  • At least 3m of 3-core 2.5mm² mains flex, 90ºC rated: eBay, eg here
  • 13A socket strips to meet your requirements
  • 13A plugThe maximum total current supplied to all sockets is 13A, limited by this plug.

Steve Hunt, G3TXQ – In Memoriam

New Year’s Morning, 2019

G3TXQ was well known and respected throughout the technical world of Amateur Radio, through his website and countless postings to a wide range of groups and mailing lists. Always meticulous and deep-thinking, Steve was a fount of good information – and also a very nice guy.

Steve passed away peacefully on 30th December 2018 after a long battle with cancer, which he also recorded in that same meticulous manner.

G3TXQ is known particularly for his optimized versions of the Cobweb and Hexbeam antennas, and for his work on common-mode chokes. His website is already archived at
and if you haven’t already visited, I strongly recommend it. Take the tour, and you will meet the man he was.

Common Mode RF Chokes


In addition to the material on his website about common-mode chokes, in 2015 Steve wrote a stand-alone article for RSGB’s online technical magazine RadCom Plus.  That article is in danger of falling into obscurity, taking with it some new information that has not been published anywhere else, so I am archiving it here in memory of Steve:

Common Mode Chokes: G3TXQ 2015

The article included a new version of Steve’s famous graphic to help in selecting broadband ferrite chokes to cover various HF amateur bands. The best chokes are the ones that cover the required band(s) with both the dark green shading and the black underline. Orange and red shading are to be avoided – the choke may do some good, but may also have problems. (For clarity, I have added a large ‘X‘ across the three chokes at the bottom. As Steve explains, those were included as particularly BAD examples, NOT to be copied.)

Writing this is not the way I had hoped to begin a New Year.
RIP, Steve.


VHF-UHF Baluns

This is the talk that I presented at the RSGB Convention in October 2018.

Update: RSGB have now released the video at (member login required; it will probably be several more months before the video is transferred to YouTube).

I always cringe to hear myself talking, but the sound track does add some context to the previously released slide pack ( still available here ).

Even without the voiceover, the important points are clear:

  • Today, “baluns” are about controlling interference, reducing noise
  • Choke baluns are best for doing this.

So we should aim to:

  • Minimize common-mode RF currents on the outside of the coax
    and along the boom.
  • Don’t upset the good feedpoint balance that VHF-UHF Yagis already have.

Comments and discussion, please!  This is new information for VHF-UHF, and certainly not the last words to be said about it. It’s still very much ‘work in progress’.
Leave a reply


Technical slide content © 2018 Ian White, GM3SEK
Slide template © 2018 RSGB
Images as attributed (except where that would be embarrassing).

Interdigital Filter Design

Here is a scanned pdf of my 1984 Radcom article called A Simple Way to Design Narrowband Interdigital Filters .

Doug McArthur VK3UM (SK) wrote a program to implement this design method. Since Doug’s legacy website has now closed, the program is now hosted here.  Click to download, open the zip file and run the install program.

Many people have used this method – design your filter, build it well, and it will work.

A key requirement for good performance is an excellent electrical connection all the way around the base of each resonator element. Use a lathe to countersink the base and make a sharp, level edge all around (there is really no way to do this freehand). Drill and tap a deep hole into the base of each element, and assemble the filter using strong steel screws to pull each element tightly into the supporting bar (you’re actually hoping to make a cold weld).