Mick and Smithy Talk Antennas
Chapter 16
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
It seems a long time since we last eavesdropped on Mick and Smithy’s conversations at their local radio club. But of course there had been the long Christmas break. In the last few shopping days before Christmas, Mick had been in the city frantically looking for presents for family members (of course he had sorted out what he himself wanted long ago - all gadgets for his station!) And so it was with some surprise that he thought he saw a familiar figure disappearing through the doorway of “Gadget Planet”. He looked again, no he wasn’t mistaken it was Smithy - after all there aren’t many people these days still wearing sports coats with leather elbow patches on!
Back at the club after Christmas, Mick mentioned this to Smithy. “Well, I only went in there out of curiosity” admitted Smithy “you see Mick, it amazes me the trivial ways people nowadays amuse themselves.” “Get any good radio books over the holiday?” queried Mick. “No I haven’t come across any lately, but I guess I’ve got most of the classic radio books and rather interestingly most of the good ones are usually of American origin” replied Smithy. “Anyway let’s get on with what I promised to tell you about. We had got so far as to deal with the fields around a transmitting antenna. Remember the fields are only a way of describing the energy flow. They do not represent the process of energy creation itself. This is by those queer little photons being emitted when electrons are accelerated. Some photons don’t have enough energy to escape and therefore return to the initiating conductor - these form the induction fields. Those photons that do escape - they form the radiation field.”
“Yes, Yes, Yes, I got all that before” said Mick, anxious to learn the secrets of the receiving antenna. “Not so fast” cautioned Smithy, “before we get on to receiving antennas, I need to tell you more about the radiation field so that you will be able to understand the effect it has on an antenna which is placed in the field.”
“We may regard a field as a map of the strength and direction of the electric and magnetic forces at all points in time and space around the transmitting antenna, stretching out to infinity. Remember we use patterns of E field lines to represent the field - a line represents a constant value of electric field strength and the line spacing shows how the field varies. The patterns move, but do not change their structure: the horizontal axis can thus represent either distance or time from the antenna. This axis is called the propagation axis of the radio wave. Once these fields have been launched from the transmitting antenna, they are self-supporting: one generates the other in their onward passage through time and space. The moving patterns of E field and H field are inseparably linked; neither can exist alone. The passage of the radio wave is a process of cause and effect: a change in the E field causes a change in the H field, which in turn causes another change to the E field and so the disturbance passes through the transmitting medium. At a distance from the transmitting antenna the E and H field lines (viewed in two dimensions) are effectively vertical and horizontal. Note that close to the transmitting antenna the field lines are spherically diverging.”
“Phew!” muttered Mick. “Now before I tell you how the field interacts with a receiving antenna, there are still some details to cover” said Smithy “but some of them will have to wait ‘till next time as I’m getting dry and need a cuppa. As the disturbance expands outwards, you must remember that the energy in this disturbance is finite (it is the transmitter output power). This means that the power in unit area (or volume if we were thinking in three-dimensional terms) must decrease as distance from the transmitting antenna increases. This is called the “spreading loss” as the wave propagates through the transmitting medium. So the values of E and H field strengths decay with distance. Next time we will consider what happens at an arbitrary point “P” in the field - the location of a receiving antenna.”
And so they migrate back to the tea area - now freshly restocked with post-Christmas chocolate biscuits. Smithy resigned himself to hearing from Mick details of his latest Christmas gadgets - details that he, of course, was not the slightest bit interested in. But at least as partial recompense, Mick bought the teas!
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.
next episode (Chapter 17).