Mick and Smithy Talk Antennas
Chapter 11
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
.....Oh dear, it’s getting complicated.....
Mick and Smithy meet up back at their local radio club a few weeks after the Christmas break. Mick is telling Smithy what he had received in the way of presents - well of course he had ordered them himself “Yep, I had an EMS-14, a PS 8250 - I had £20 knocked off that, and a CSW201G for only £20.” Smithy said nothing, firstly he had no idea what any of that stuff was but more to the point he was reluctant to divulge the exact nature of what he himself had received - socks and handkerchiefs. What Smithy would have liked though was a Collins Radio Diary, but of course such diaries had vanished from the shelves over forty years ago. In fact Smithy still treasured his last Collins Radio Diary for 1966, for it contained a useful table of decibels, a section on attenuator design and much other useful stuff. But despite this, Smithy had enjoyed the holiday for, whilst browsing in a secondhand bookshop, he had acquired a classic American Radio book from the 1940s: Transmission Lines, Antennas and Wave Guides by King, Mimno and Wing. This had made his Christmas, he was almost too excited to start the book! An American classic! (It’s all a bit sad really, isn’t it?) He would save it for later, giving himself the pleasure of anticipation even though, like all such books, the treatment was largely mathematical and not one of Smithy’s strong points.
They were first at the tea area, so sipping his tea, as they moved away to a corner spot, Smithy started on the latest of his antenna explanations. “Mick, last time I briefly mentioned photons as the agent responsible for RF radiation. I said I would tell you about ‘Fields’. If I remember rightly you had some problem with ‘Fields’ when I mentioned them in connection with ‘Feeders’. Mick, if you want to progress beyond a limited understanding of antennas, somehow you have to grasp the concept of fields. But before I tell you about a dodge on how to do this, I’ll just underline how important they are. You see, Mick, antennas are in fact transducers: devices that change one form of energy into another.” Mick nodded, but in truth he wondered to himself what use all this was - with stuff like EMS-14s, PS 8250s and CSW201Gs, did you really need to know any of this now? But it was too late to stop Smithy, he had started so he would continue!
“The transmitting antenna” continued Smithy ”converts RF electrical power into RF radiated field energy, whilst the receiving antenna does the reverse. The mechanism for doing this is, of course, the photon as I briefly explained before. Mick, you remember I told you that there has been a century of DESCRIBING RF radiation in terms of fields. Well, you may still come across books etc that attempt to explain the CAUSE in terms of ‘Fields’, for instance by saying the radiation field ‘breaks away’ from the antenna because of effects called ‘retarded potential’ - meaning interaction of the field immediately around the antenna. There have also been misguided attempts to design antennas as though the fields were the CAUSE and not the EFFECT.”
Mick nodded vaguely as Smithy continued” We must use a sort of ‘contrivance’ to allow us to visualise the fields. We’ll start with something that I remember from my school Physics days: you have to imagine an isolated point electrical charge with a potential of V+ on it. Around this charge we can imagine concentric lines, called equipotential lines, which represent energy levels around the charge. If you were to move another positive charge, we call it a probing charge, towards the V+ charge, work has to be done in moving from one equipotential to a higher one closer to the V+ charge. Now if we were to draw lines at right angles to the equipotential lines, these will represent the force which acts on such a probing charge. These lines are called the electric field lines and a pattern of such lines can be considered to represent the electric field, called the E field. Perhaps it might help if you remember how iron filings sprinkled over a card with a bar magnet underneath reveal the magnetic field.”
Mick was still awake, but he felt his concentration beginning to go. But there was no stopping Smithy now ”The E field is measured in volts per metre or a sub-division more appropriate such as microvolts per metre. The field lines are lines of constant force; they do not exist in reality, in exactly the same way that contour lines and isobars do not exist, yet they are a convenient way of visualising electromagnetic fields. In fact, Mick, contour lines were once defined as gravitational equipotentials which interface with the earth’s surface. (We can say here that this particular bit of information did not help Mick at all.) Imaginary though these electric field lines are, we can make good use of them in picturing what happens around a transmitting antenna.” At this point Mick interrupted to say ”Smithy, wouldn’t it be easier if you explained receiving antennas first?” “Actually no” replied Smithy “believe me Mick, the receiving antenna is rather more difficult to explain from a field viewpoint. But I’ll do it in due course.” Smithy took from his bag the old Services Text Book of Radio - volume 5 - Transmission and Propagation that he had previously recommended Mick to acquire. Thumbing through it he showed Mick the diagram on Page 260. It showed the radiation fields of a small dipole represented by field lines. “Using this approach” Smithy said “we can imagine a three-dimensional picture as:
The electric field, imagined as a series of expanding toroidal rings, each containing the field lines in spaced patterns, according to the originating sinusoidal current in the short dipole.
The magnetic (H) field, exactly in phase with the electric field, but as a set of expanding rings in a plane at right angles to the plane of the E field, again with a pattern of field lines.
The old Admiralty Handbook of Wireless Telegraphy also included a diagram of field lines emanating from an antenna. But whilst this is useful in visualising the radiation field, be sure to disregard any accompanying description that supposedly describes the radiation process itself.
All field lines, as they move away from the antenna, maintain their exact relationship in time and space to each other. Once again it must be stressed that the lines have no physical reality, serving only to illustrate the behaviour of the field. If we imagine ourselves far enough away from the antenna, we can visualise electromagnetic radiation sweeping past us as a changing E field and a changing H (magnetic) field, in planes at right angles to each other and to the direction of propagation, the field lines now being straight as they extend out to infinity. These fields are self-supporting, one generates the other in their onward propagation through space. In radio propagation we normally think only about the E field, but in fact it doesn’t matter which field you consider, as long as you keep to one field only. At any point in time or space only one of the fields can be considered to act.”
Looking at his friend, Smithy realised Mick could take no more so he finished by saying ”Mick, the E and H fields at right angles to each other, and in phase, represent the radiation at a distance from the antenna, but close to the antenna the situation is much more complicated. If you want to persevere with this I tell you about it next time. I’ll also describe what happens at the feeder/antenna interface.”
Mick agreed, he didn’t want to upset Smithy by admitting it was too complex, but this session had tested his mental powers to the full. He was a gadget man, quite happy to discuss the merits of an EMS-14, a PS8250 or a CSW201G.
Restorative tea was needed urgently.
If you have missed our other episodes:
Episode 1.
Episode 2.
Episode 3.
Episode 4.
Episode 5.
Episode 6.
Episode 7.
Episode 8.
Episode 9.
Episode 10.
next episode (Chapter 12).