Mick and Smithy Talk Antennas
Chapter 19

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 had not met up at their local radio club for a couple of months. This was particularly unfortunate as Smithy had just started on an explanation of how a receiving antenna extracts energy from a RF radiation field. But it could not be helped as Mick had been working away during this time. Smithy had once asked Mick what exactly his job was. The explanation had completely bewildered Smithy - “I head up the ‘Blue Skies Department’” said Mick. “My people prepare ‘mission statements’ for our clients, who use us to outsource facilities and personnel. We have to maintain a dynamic client relationship to take account of ongoing structural changes to their business. This means developing and refining all the procedures and tools that are part of their core strategies.” Smithy had just nodded, recalling the word that would have been used to describe this during his days in industry! Then, a couple of months later the company that Mick worked for, received an ‘award’ from the local paper. Smithy had seen the photos of the award ceremony and the dinner and had noted to himself that he had never seen such a bunch of smarmy-looking people. “Thank goodness I never had to work with people like that” had been the thought going through Smithy’s mind.

“Right” said Smithy, “Taking up where we left off, Mick, you remember I described how the RF radiation field around a receiving antenna was modified by the presence of the antenna, even though the antenna was isolated from any receiver. Well, as you might imagine, connecting a receiver to the antenna means that as before, currents flow in the antenna, but in addition to re-radiating, some current is now delivered to the receiver. This re-radiation modifies the field around the antenna as I have previously described to you. If we match the receiver to the antenna, we can extract maximum power from the E field and in such a condition, half of the transferred energy is still re-radiated. Notice Mick that I refer to the E field for this is the convention in radio engineering. Because of the self-sustaining characteristic of RF radiation, there are corresponding changes to the H field, but we cannot extract signal power from both fields simultaneously. That is, an antenna can be erected to interact with the E field component or the H field component, but not both at the same point in space.

So, the two extreme conditions are defined by a matched antenna re-radiating half of the energy transferred from the field, and the other by an unloaded antenna re-radiating all the energy it receives from the field (scattering). Between these two extremes, the proportion of signal power developed in the antenna of that extracted from the incoming field will vary from a half (matched) to zero (scattering). We use the contrivance of a fictional ‘radiation resistance’ assumed to be in series with the receiver load to account for the power lost as re-radiation. Finally the signal current flowing in the wire also sets up the static and induction fields we discussed previously. These fields do not result in any power loss in the received signal. They do however, explain why receiving antennas lose their directivity for sources in their near/intermediate zones.”

“How about that ‘effective area’ you mentioned way back?” asked Mick. To which Smithy replied that it would have to wait for another time, as the Christmas buffet was about to start.....

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.
next episode (Chapter 20).