Mick and Smithy Talk Antennas
Chapter 18
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
Mick and Smithy meet up again at their local radio club. Smithy is part way through explaining how a receiving antenna extracts energy from a RF radiation field. On Smithy’s advice they take their tea early - and strong - for he is sure that Mick’s attention span is going to be marginal for the task ahead!
“One thing from last time” started Mick “you talked about the effective area of an antenna, now I’ve been puzzling over this ever since and for the life of me I cannot see what would cause this.”
“OK” rejoined Smithy “I don’t want to deal with this until I’ve told you how the field is modified by the presence of the antenna - but I’ll give you a clue - think back to our discussions on the transmitting antenna. I told you something then about the fields around the immediate vicinity of the antenna. Well the properties of receiving antennas are inexorably linked to those of transmitting antennas. Now, let’s get back to the vertical antenna we have arbitrarily positioned in a vertically polarised RF radiation field.” Smithy took some scrap paper out of his bag and drew an oblique view of a short vertical wire surrounded by a vertical section of a plane which he labelled THE WAVE FRONT. At right angles to this he drew an arrow pointing to the centre of the antenna which he labelled DIRECTION OF PROPAGATION.
“Now” said Smithy “We’ll start with an isolated antenna, one which has no receiver connected to it. On the wire the E field falls to zero due to it’s conductivity being so high. Or in terms of basic physics, photons are captured by the wire and cause a current to flow. This current, not unexpectedly, behaves just as the current in a transmitting antenna does. In other words the antenna creates its own induction and radiation fields and these distort the incoming field. Some RF energy forms a distinct (reflected ) field, whilst in the vicinity of the antenna the spherical fields combine with the incoming plane wave to form standing wave field patterns around the antenna over a distance of a wavelength or so. We say the antenna has ‘scattered’ the incoming field. Mick - this is of course how radar works.”
“This standing wave pattern of E field around the antenna varies with time but is stationary in position. The exact form that the pattern takes is determined by the antenna length expressed as a proportion of signal wavelength - which incidentally Mick, is how the director and reflector elements act in the well-known Yagi antenna. Just as in standing waves in antenna wires and feeders, this standing wave field pattern is only a concept - only the separate fields have a real identity.”
Smithy looked at Mick. Mick could only say “Smithy - you’ve done my head in!” “Well” said Smithy “I think we should continue so that I can wrap up this description of ‘scattering’ In the immediate vicinity of our receiving antenna, the reflected field is not far enough away to act as a plane wave, and the standing wave pattern is caused by the interference between the incoming plane wave and the reflected spherical wave. Let us take the simplest case of a resonant half-wavelength isolated vertical antenna and see how this scatters the E field.” Smithy drew out on his sketch a section of a vertical plane along the propagation axis normal to the wave front. He then drew a series of gradually diminishing standing waves from the antenna back against the direction of propagation. Smithy explained “directly in front of the wire, along the axis of propagation, the standing wave pattern decreases, effectively disappearing within a few wavelengths. Directly behind the wire, along the propagation axis the E field increases from zero (at the wire) to be re-established as the passing field at two or three wavelengths distance (known as ‘shadowing’). In the wave front through the wire, the standing wave pattern is symmetrical about the wire. Again decreasing in a few wavelengths distance. This whole pattern of standing waves around the wire exists in three-dimensions, of course, as an envelope that vanishes to nothing behind the wire. Of course, if the wire is a different length to our half-wave, then the pattern will be different. Next time I’ll tell you what happens when we connect a receiver to the wire.”
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.
Episode 11.
Episode 12.
Episode 13.
Episode 14.
Episode 15.
Episode 16.
Episode 17.
next episode (Chapter 19).