Mick and Smithy Talk Antennas
Chapter 14
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
.....in which Smithy attempts to keep Mick interested.
They meet up at their local club again, gathering around the teapot and the last of the post-Christmas chocolate biscuits. Before the talk could turn to antennas, Mick could see that Smithy was annoyed about something. He was right. Smithy pulled a recent edition of the local newspaper out of his bag, turned the pages until he found what he wanted and handed it to Mick, saying “just look at that, Mick.” Mick read out “LOCAL MAN REFUSED REFUND FOR SLIPPERS IN CITY STORE” (which was in one inch high headline print with the story occupying a whole page!) “No - not that - across on the other page” said Smithy “ look at that chap who’s just ‘invented’ a mobile phone app for kids - you’d think he’d done something tremendous - I hate that sort of thing.” (We’ll mention in passing here that these days Smithy also hates white van men who block pavements etc. etc. Nowadays, of course, parking on the pavement is regarded as being considerate (to other motorists) and so Smithy increasingly found himself out of touch with much of everyday life. Apart from the Maradona/Madonna confusion, he thought that Britney Spears was a vegetable and Kelsey Grammer a school. In his mind ‘Workshop’ was a place equipped with benches, machines and/or test equipment, and ‘tools’ were things like screwdrivers and soldering irons - but of course these days people using tools at a workshop were most likely to be sat on office chairs in front of computers.) Mick, however, just nodded and said nothing, except that a vision flitted across his mind of Smithy joining Waldorf and Stadler up in the balcony on the Muppet show, shouting “rubbish” at everyone and everything.
With that off his chest, Smithy started off “you know Mick if you’re having trouble visualising fields, can you imagine the trouble several generations of engineers have had in accepting mathematical expressions as a description for RF radiation? No wonder RF became something of a black art, little understood by engineers who were not actually involved with it. You can see this if you were to go back and analyse the situations that developed when EMC legislation was introduced. All of a sudden many, otherwise competent, electrical and electronic design engineers were finding that to produce a satisfactory product, they would have to learn something of RF. For myself” continued Smithy” I always had a difficulty accepting a mathematical explanation without some mental picture of what it’s supposed to represent. People who were good at maths always seemed to be able to accept what they were taught without worrying about this aspect.”
“Now” said Smithy, with chocolate smeared on his lips “ Let me go over again that way of visualising the radiation field. After all, it’s only familiarity with the idea that is needed, you can accept contour lines without even thinking about it. Imagine a piece of paper with a horizontal line going across it from left to right. This is the propagation axis. Actually Mick, this axis can represent time or distance. The E field is represented by sets of vertical field lines occupying the vertical plane, that is the plane of the paper and it is called the plane of polarisation because of engineering convention. The E field lines must be imagined as moving along the horizontal axis at the speed of light, whilst maintaining their exact line density pattern. This density pattern represents the cyclic changes in field strength caused by the corresponding changes in the cyclic RF transmitter currents in the antenna. Ok still?”
“Yep I can picture that” nodded Mick. “OK” said Smithy “ Now from a three-dimensional point of view, the third axis is at right angles to the vertical plane and to the propagation axis, so that from a two-dimensional diagram on a vertical page the magnetic field would extend out on a horizontal page at right angles to the vertical page. This horizontal plane is occupied by the magnetic H field lines, changing in complete step with the E field lines. The whole thing is called a plane wave that exists in the far zone of a transmitting antenna. But of course we’re not really talking about two pages at right angles, the two fields exist in three linear dimensions as well as in time, it is a sample section of each of the expanding fields that we consider to be on each page. I won’t confuse you by trying to describe other radiation waves, such as spherical waves. If you’ve got this, you can go on to understand antennas in greater depth. Next time I’ll explain just how those antenna zones I was telling you about affect the performance of antennas.”
A much happier Mick left the club that night for he had indeed formed a sort of mental picture of fields moving away from an antenna; something that Smithy had tried so hard to instill into his gadget-obsessed mind. Even Smithy was happy - someone had bought him his tea!
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.
next episode (Chapter 15).