Mick and Smithy Talk Antennas
Chapter 13
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 says far too much about ‘near’ and ‘far’
Smithy was late for the next club meeting, only arriving just before the advertised talk got underway. So it was a couple of weeks before the pair could get together to continue their discussion on the mysteries of antenna operation.
Smithy grabbed a cup of tea and started off at once “Mick, you remember I introduced the idea of there being induction fields around the antenna, these being quite different to the radiation field which moves away from the antenna.” The radiation field is self-supporting, energy is interchanged between the two fields and averaged out, half the energy is stored in each of the fields. This process doesn’t happen with the induction fields, they may store different amounts of energy over the transmitter cycle. In fact magnetic induction fields are more efficient for energy storage than electric induction fields. Which is why most electrical machines use magnetic fields, such as in transformers and motors. Mick had been cornered, he’d not even had chance to get himself any tea “Yep”. “Well” continued Smithy “ can you imagine in the immediate vicinity of the antenna, these induction fields changing in sympathy with the RF currents in the antenna. Because the effect of these fields decay faster with distance than that of the radiation field does, there exists around the antenna what are called zones, where the different fields predominate. Now a practical antenna will have all these fields associated with it and conditions around the antenna can then be described in terms of three zones:
The near zone - close to the antenna where the electric and magnetic induction fields predominate because they are so strong relative to the radiation field.
The intermediate zone - where the induction fields and radiation fields are both strong enough relative to each other to be important.
The far zone - where only the radiation field can be detected. This is the zone in which true radio communication takes place.
Now, I have stuck to the terminology I first saw in an American engineering book when I was briefly involved in some radio interference work many years ago. Sometimes though these zones are referred to as near and far field. I’ve always thought this to be confusing as I think ‘zone’ underlines the fact that we’re talking about the region in which the types of field predominate.” Smithy stopped here, for Mick had just waved his hand over to the tea area, and so off they wandered for tea and those unwanted chocolate biscuits that always find their way down to the club after Christmas.
But Mick was given hardly any time in the tea area, even though he wanted to show off to the others about his EMS-14, the PS 8250 and the CSW201G he’d got at Christmas. Smithy had pulled some papers out of his sports bag and was beckoning Mick over. “Mick, there are not precisely defined boundaries to the three zones, although some practical guidelines are commonly used. The size of the near zone depends on the wavelength L (and thus the frequency) of the RF from the transmitter and is usually taken to be within a L/2Pi radial envelope around the antenna. The intermediate zone is not so easily specified and sometimes account is taken of the maximum linear dimension of the antenna. The far zone can be considered to start at several wavelengths away from the antenna, so it is convenient to regard the intermediate zone as occupying the region between L/2Pi and 3L radii of the spherical envelopes. Some years ago I plotted out a graph on this basis which shows the zonal boundaries at different frequencies. Next time I’ll use the graph to explain some little-known properties of antennas.”
“I’ll look forward to that” replied Mick, managing to suppress a yawn with immense difficulty. (Mick’s world of EMS-14s, PS 8250s and CSW201Gs is too far away from the dry old stuff that Smithy is interested in; it’s beginning to seems possible that Mick might lose interest in delving any further into the subject of antennas. That would be a shame - but there you are that’s the way people are. We shall just have to see how much more of this dry old stuff Mick can take.)
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.
next episode (Chapter 14).