Mick and Smithy Talk Antennas
Chapter 20
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. Mick is anxious to find out about the “effective area” a receiving antenna presents to an enveloping RF radiation field. It is the area in which the field and the antenna interact.
“Well” started Smithy “ if we go back to basic principles and consider that those sub-atomic particles that we called photons actually constitute the radiation field it will become easier to understand the significance of this area. Incidentally, it is also called capture area and aperture. Now Mick, you remember when we discussed the transmitting antenna, I told you that such antennas are surrounded by areas where induction fields exist. The inner zone, if you remember, was called the near zone. As a start, you remember that energy in this zone was returned to the antenna, well that means that most of those pesky little photons are reabsorbed by the antenna. Likewise in the case of a receiving antenna, photons of an impinging radiation field are most likely to be captured in this same near field - hence it will define a capture area. This is not a precise picture because when we consider the behaviour of photons we are actually talking about the average effects. There is a degree of uncertainty implicit in quantum mechanics and it becomes impossible to visualise the “fuzzyness” which is reality. So some photons outside the near zone are captured whilst some within the zone escape capture by the antenna. Returning to our discussions on the transmitting antenna, if all photons were recaptured in the near zone, there would be no radiation. In fact to digress a moment, it is possible to design what are called near field antennas where true radiation fields can be made very small. OK so far?”
“I think so” replied Mick. “OK” said Smithy “Now, so far in our discussions we haven’t talked about antenna gain. You can see that if you are able to increase the capture area of an antenna then you also increase its gain. Because antenna gain is often acquired by making the antenna directional, we must stipulate that this refers to the capture area (aperture) presented in the direction where gain is maximum. This can be confusing, because at first sight, polar diagrams of high gain antennas might suggest that they collect energy over a smaller area. This is not so, they collect energy over a smaller angular arc, but their capture area in the directional plane is expanded. As I say, this can be quite confusing if you don’t bother to think about it from a first principles viewpoint. So you can also see now that the physical size and arrangement of a real antenna does have an effect on capture area - it is not simply the rough approximation I gave you for the extent of the near zone around a transmitting antenna in our previous discussions. You might think, Mick, that using power gain or aperture we can uniquely compare two antennas, but as I told you once before, there is no one characteristic which can so define an antenna as good or bad, so for example we still have to take efficiency into account i.e. how much power is absorbed or radiated by the antenna.”
“Finally” said Smithy “a good rule of thumb to remember about simple antennas such as the dipole, is that the capture area (aperture) is related to the square of its length so that as frequency is increased, such simple antennas are no longer practical as their capture area becomes progressively smaller. Mathematically, the capture area of a receiving antenna can be calculated by dividing the power delivered to a matched load by the power density of the field in which the antenna is located. This” said Smithy “is a convenient place to stop, because next time I’ll tell you how the properties of antennas in receiving and transmitting mode are inextricably linked - which I’ve always found is a fascinating aspect of antenna theory.”
For once, Mick drank his tea in complete silence as he tried to digest what he had just been told.
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.
Episode 18.
Episode 19.
next episode (Chapter 21).