Mick and Smithy Talk Antennas
Chapter 12
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 gets worse, first fields and now zones!
Smithy was wondering just how much of their last discussion Mick had taken in. He decided that if they were to go on, he would have to limit the amount of the stuff he was talking about each session. As they meet up again at their local club, Smithy decides to start off by describing the feeder/antenna interface as he had previously promised. He thought Mick would have no difficulty with this.
“Mick, we’ll start with RF power from the transmitter being present in the feeder, and we’ll assume that the feeder connects to a half-wave dipole. If you remember from our SWR discussions, what happens is determined solely by the load that the feeder ‘sees’. In this case the load is the antenna, and the antenna itself can be regarded as an opened-out feeder, open-circuited at its ends. RF current from the feeder travels along the antenna wires and as it reaches the far ends of the antenna elements, the open circuit condition forces its total reflection, with the current returning to the feeder/antenna interface. When the antenna element is λ/4, the return current will have fallen to zero when it reaches the feed point. These return currents and the forward currents entering from the feeder result in a standing wave pattern on the antenna. You remember that?”
“Yep, another thing that’s purely imaginary” muttered Mick. “OK” said Smithy “yes, remember the physical reality is that forward and return RF currents are flowing in the antenna and these produce a combined maximum on the element at the feed point. If the antenna element is not quite λ/4, then return current at the feed point will enter the feeder and also be re-reflected back along the antenna. That is the antenna will be off-tune, but it will still radiate with no significant difference, although there will be a higher SWR on the feeder. Now, the result of this ‘standing wave’ on the antenna elements is that such a build up of current along the antenna is just what we want for those queer little photons to be emitted. So remember, it’s RF current flowing that is the basic mechanism for RF radiation, maximise the current and you maximise the radiation. And we describe this photon emission, viewed on a large scale as radiated field energy. Our half-wave dipole is a standing wave antenna, as so many antennas are in amateur radio.
Now if you were to compare the behaviour of our ‘opened out feeder’ to a normal feeder section of the same length, the input currents and current distribution are almost identical, being roughly sinusoidal. But power lost from these two examples differs greatly. The only significant power lost to a normal feeder section of this length would be due to heating effects in the conductors. In the antenna case, apart from a similar heating loss, the whole power supplied is lost due to radiation from the ‘opened out’ sections. The ratio of these two losses is about one to a hundred, or one percent.”
“Phew, I got it” grinned Mick “let’s get some tea whilst I mull it over.”
Over tea, talk turned to the EMS-14 and the CSW201G that Mick had wangled for his Christmas presents. “Well, what I am going to tell you next Mick, is quite important in assembling a station and connecting up bits like those you had.” (Smithy had taken the opportunity to look this equipment up in the latest magazine.) “How’s that then, surely there’s no problem in connecting them up?” said Mick.
“Right” replied Smithy, leaving the question unanswered for the moment “time to return to the fields around a transmitting antenna. Now, a point often forgotten in amateur radio, is that those fields around the antenna almost certainly encompass you, your station equipment and wiring; that is unless you’re operating a remote antenna somehow or are sitting in a screened room.” Smithy continued “last time we considered what happens when energy is radiated from an antenna and becomes a field moving in space. This is not quite the end of the story however, because there are also what are called induction fields around the antenna. These induction fields, both electric and magnetic are generated just as they are in motors and transformers, but the important point to note is that although these two fields change with respect to time, they do not move outwards in space as the radiation field does. Unless you understand that a moving radiation field differs from stationary induction fields, you have got hold of the wrong end of the stick! And there have been people in the radio game, even at professional level, who seem not to have understood this.” (At this Mick felt suitably impressed and puffed himself up a bit). Smithy continued “the induction fields, though they change with time, following the cyclic variation of the current source, only store field energy momentarily before they return it to the source during another part of the cycle. The two induction fields do not cause any net loss of power from the source. Next time I’ll tell you about the complications these fields cause in the immediate vicinity of the antenna where different zones of influence are formed.
“I think that’s quite enough for now.” A statement that Mick was in full agreement with, though he had taken Smithy’s point about himself and his equipment being in the field around the antenna.
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.
next episode (Chapter 13).