Mick and Smithy Talk Antennas
Chapter 17
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. Smithy has promised to explain to Mick how a receiving antenna works -in terms of its interaction with a radiation field.
“Right” said Smithy “Let’s get down to business then Mick. I’ve told you about the field last time we had a session. We shall assume a vertically polarised signal (meaning the E field occupies the vertical plane) and also that we have erected a vertical wire antenna in this plane. Once we have erected our antenna, we can almost forget about the E and H fields sweeping by it and just imagine the field strength at the wave front varying with time as determined by the transmitter output.”
“What do you mean - wave front?” asked Mick. “OK” said Smithy “ for our vertical wire, the wave front is the vertical plane in which the antenna lies and which extends at right angles to the direction of propagation. Incidentally Mick, we assume that the antenna is in “free space” - a place not to be confused with outer space (Smithy knew Mick was an avid viewer of’Red Dwarf’). Free space is just a hypothetical region where there are no obstacles or complications to the uniform expansion of the radiation fields (except in this case of course - the wire antenna we have erected). Everywhere in the plane of the wave front, at any instant, the value of the E field is constant and we can think of the E field lines as vertical. In this plane the value of the H field is also constant at any instant. Mick, just imagine that we have “stopped” the expanding radiation field for a moment at the plane in which the antenna lies whilst we consider the effect the antenna is going to have on the field - and the field on the antenna.”
Smithy looked at his friend. Mick certainly looked a little glazed. But Smithy pressed on - “we can consider the energy in the field at this point in space by defining the total power flowing through a unit area of the wave front, in watts per square metre or a suitable subdivision. This is the power density at the wave front. Remembering that E field strength is measured in volts per metre or a suitable subdivision, we might expect that the signal voltage developed at the receiving antenna is just the antenna length multiplied by the field strength.” Mick nodded at this. But Smithy went on “this is however not the case - although the signal voltage is proportional to the field strength. The reasons for this, put in simple terms, are:
- Although we may only intend to use the antenna we have erected for receiving, it proves impossible to prevent it also acting as a transmitting antenna and re-radiating a proportion of the signal energy extracted from the field.
- The antenna only extracts signal energy from the field in a certain area. This area is not the simple area determined by the geometric dimensions of the antenna, but a more complex factor called the “effective area” - and this is related to the signal wavelength. It is the area in which the antenna and the field are assumed to interact. Note also Mick, that the induced voltages are distributed along the antenna and cannot just be added together.”
Smithy ended ”Mick I think that’s enough for tonight. Next time we’ll go into these complications a bit deeper and I’ll tell you how the presence of the antenna modifies the field in its vicinity.”
“Phew” said Mick “and I thought understanding VSWR was difficult!” “Yes” admitted Smithy “this is a difficult subject - we are in a rather specialised area here Mick, a sort of no-mans-land between electrical engineering and applied physics.” And with that the pair make their way over to the tea area, where Mick is able to get a life-saving transfusion of 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.
Episode 14.
Episode 15.
Episode 16.
next episode (Chapter 18).