Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: How to configure the return path of a dipole antenna?
Answer: Well, when we are making a dipole antenna, we use two conductors the same length and put them in a line. At the point where they touch in the middle we separate them by a little and connect them to the balanced side of a balun. The unbalanced side of the balun is connected to our radio receiver or transmitter. We make the lengths of each conductor equal to a quarter of a wavelength at the frequency we wanted to receive or transmit.
It’s easy to make such antennas, and they work well, but the frequency range over which they are sensitive is not very large, which is why EMC test labs use antennas constructed from large numbers of different-length dipoles, and other weird and wonderful constructions of conductors, to get good-enough sensitivity over a wide frequency range (e.g. the log-periodic, Bilog™, etc.).
The whip antennas fitted to cars and hand-portable radios are dipole-like, but only use one conductor. The metal body of the car and the conductors in the portable radios act as the other half of the dipole antenna, but of course they are not “tuned” to a quarter wavelength and so we call them a counterpoise.
(I know the question probably wasn’t about intentional antennas – but I will get to the point eventually!)
To relate discussions of dipoles to the fact that current always flows in a closed loop, perhaps it helps if we say that electromagnetic (EM) energy always flows in a closed loop.
It must do this, because energy cannot be created or destroyed (although it can be converted to an especially dense form of energy, which we call matter, or mass, but usually our electronic devices are not powerful enough to convert mass to energy or vice-versa so we ignore this possibility). So, when we send some EM energy off to somewhere (e.g. by radiating it from an antenna) then the loop always gets closed, somehow, and it is not possible to prevent this from happening (at least, in this universe).
So what actually happens in the case of a radiating antenna is that one arm of the dipole (or the whip of a whip antenna) emits EM energy in one phase, while – at exactly the same time – the other arm of the dipole (or the counterpoise of a whip antenna) emits EM energy in antiphase.
If we could add all of the energy radiated from both legs of the dipole (or from the whip and its counterpoise) together – at any instant in time – they would cancel out perfectly and we would have no resultant energy, which is just another way of saying that we can’t create or destroy energy.
It is incorrect to think of the current or EM energy flowing in a loop (the way I tend to describe it!) because if we think about it we realize that if it flows around a loop then it takes some time to travel around it and get back to its source. During that time there would be an energy unbalance in the universe, which our universe does not permit, so although we (well, I) often talk about currents or EM energy flowing in a loop what actually happens is that energy sets off in one phase at the same time as the exact same amount of antiphase energy sets off in a different direction – the total amount of energy at any instant as the energy flows always summing to zero.
Eventually, the in-phase and anti-phase EM energies meet up and cancel out (which is why we draw the lines of electric and magnetic field distributions using closed curves, and can talk about current loops).
Now, all conductors, whatever we call them (wires, cables, brackets, front panels, PCB traces, IC lead frames, connector pins, etc., etc.) always act as antennas (although perfectly-matched transmission lines are very poor at it).
And because there must always be a return path for any flow of EM energy, when any conductor acts like an antenna we can generally identify the location of the other half of its dipole that must occur because of the law of conservation of energy.
People who are clever with mathematics (I’m not) model any conductor as a series of microscopic dipoles all strung together in a line along its length. For example, see Chapter D.3.8 on page 461 of Edition 4 of Tim Williams’ excellent book “EMC for Product Designers” (Newnes , 2007, ISBN-10: 0-750 68170-5, ISBN-13: 978-0-75-0681670-4), or else see Chapter 7.1 on pages 421-426 of Edition 2 of Clayton R Paul’s excellent book “Introduction to Electromagnetic Compatibility” (Wiley, 2006, ISBN-10: 0-471-75500-1, ISBN-13: 978-0-471-75500-5). Some types of EM field solvers do this type of analysis automatically.
What was the question again?
Oh yes: “How to configure the return path of a dipole antenna?”.
Well, unless we are actually making an intentional dipole antenna out of two conductors the same length, the return path just happens in accordance with the laws of physics.
As we have seen, it is strictly incorrect to talk about the return path of any antenna, but that is how I think of them because of my low-frequency circuit design background.
In any complex structure, such as a printed circuit board (PCB), if we fit it with a whip antenna (e.g. because we are designing a cellphone) or if we consider any conductor as an “accidental antenna” (e.g. because we are trying to do good EMC design), the antenna’s return path – the other arm of the dipole; the counterpoise, whatever – can be difficult to locate precisely, and there may be a complex arrangement of them. This is because it depends on the field patterns associated with the antenna and the structure of conductors and insulators (dielectrics) it finds itself in proximity to.
However, we can deliberately create a good counterpoise (other arm of dipole) by driving the antenna with respect to large, unbroken PCB 0V plane, or a well-shielded metal enclosure. Field plots of such arrangements show that nearly all of the antenna’s electric field lines connect the antenna conductor to the 0V plane or enclosure, telling us that nearly all of the antenna’s “return current” is flowing in the plane or enclosure.
If we are trying to make a good cellphone antenna, we make the antenna conductor stick out at 90 degrees to the PCB 0V plane or metal box counterpoise – so that the electric field lines spread out as far as possible.
But I we are trying to do good EMC design by making all of our accidental antennas as inefficient as possible, then we arrange for the “return path” for each conductor to be as close as possible so that its electric field pattern is very small and compact. If the conductor is a PCB trace, we arrange for its relevant 0V plane to be on an adjacent PCB layer.
If the conductor is a wire or connector pin, we either arrange it to be surrounded by its return conductor (e.g. a shielded or coaxial wire), or for it to have its return conductor very close by (e.g. twisted-pair wire). Of course, we can do both, for example by using a shielded twisted-pair, in which the return wire carries the differential-mode return current (EM energy) and the shield carries the common-mode return current (EM energy).
When I was struggling with understanding all this some years ago, I asked my former University Electromagnetics Tutor how do the send and return currents (EM energies) “know” how to flow so that they meet up and totally cancel out at the end. Where there was just a single conductor in space I could see how that worked as a dipole (or series of microscopic dipoles) but what about a wire or conductor with a complex ground structure such as we get in our products, systems and installations? He simply said: “Get a copy of ‘QED – The Strange Theory of Light and Matter’ by Richard P Feynman, and read it.”
So I did, and it gave me all the answers. You can do the same. My copy was published by Penguin Books in Great Britain in 1990, ISBN: 0-14-012505-1, but in the USA it was published by Princeton University Press in 1985.
-Keith Armstrong
