Mick and Smithy Talk Antennas
Chapter 4
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 had endured another week of evenings spent fruitlessly shouting into his microphone on 20-metres (he was no cw operator, to him cw was just QRM.) He was anxious to find out why Smithy had expressed doubts about his vertical antenna, despite the low SWR and impedance around 50ohm that he had measured on the club’s MFJ analyser. At last the pair met down at their local club to continue their discussion.
Mick has arranged to pick up some old magazines from Smithy. Mick was not his usual cheery self when he called round to Smithy, “the fact is last night I had more than enough SPAM to last me for some time” he said. “Oh we never eat that stuff now” replied Smithy “we had enough of that in the 60s.” There was an awkward silence.
Mick spoke first “I noticed that list you gave me the other night only referred to transmitting antennas, does that mean those statements don’t apply to receiving antennas then?” “Clever of you to notice that” said Smithy “ well most of them do, but actually there’s a lot more to how receiving antennas work than just inducing a signal voltage in the antenna - but we’ll leave that for another day.”
They had progressed to Smithy’s garden shack to get the magazines. As they entered something on the shelf caught Mick’s eye. “What’s that” said Mick, pointing at a rather unusual looking piece of equipment. (It was in fact an oval-shaped flat biscuit tin, with SO239 sockets either end. On the flat top of the tin was mounted another unit featuring a sloping front panel. On the panel were a meter and a row of nine LEDs. Above and below each LED was a slide switch and pushbutton switch respectively.) “That” explained Smithy “ is something I built up a couple of years ago when I gave a talk at the club about SWR. It is a network of L/C sections, arranged to represent a section of 50 ohm coaxial cable. I used it in series with a coaxial feed to a mismatched load from an HF transmitter. The LEDs and the meter show that fiddling with an ATU at the transmitter end has NO EFFECT on the SWR in the feeder or at the load.”
“But we always set the ATU for minimum SWR don’t we?” said Mick. “You know I don’t really understand this SWR business, I keep thinking it depends on the impedance seen at the feed line by the transmitter. "Well, I’m not surprised at you being confused” remarked Smithy “you see Mick, the so-called SWR bridge at the transmitter output in fact does not measure the SWR on the feeder, neither does the ATU’tune’ the antenna - so it’s little wonder people get confused. What is wrong is that both these units have a job to do, but it isn’t the jobs that their names suggest - really they have somehow or other acquired the wrong names and as time goes on the myths remain unchallenged.” “So what do they actually do then Smithy” queried Mick.
“It’s a long story and I need some tea. I’ll tell you what Mick, I’ll lend you my notes on that talk and you can see how you get on and check back with me if you don’t get any of it” suggested Smithy. “The other thing to do, is to look out for an old radio book, I recommend it; it has the best explanation of SWR I’ve ever seen in any radio book. It is volume 5 of the SERVICES TEXTBOOK OF RADIO dealing with TRANSMISSION AND PROPAGATION. It’s an old book now, it was issued in 1958 by HMSO. It’s well worth trying to get one though.”
Smithy reached up and pulled a folder down off the shelf, took some papers out and handed them to Mick. “Don’t read this last thing at night” he warned!
Despite Smithy’s warning, it was late one evening that Mick took out the notes and began to read:
THE FEED LINE - A CONCEPT REVIEW
This is a review of the working of a feed line as used in a typical amateur HF station. In order to do this we shall consider transmission only, assume the feeder is coaxial and avoid discussing baluns or systems other than the usual transmitter -swr bridge - atu- feeder- antenna combination.
The feed line is classified as a distributed parameter circuit. That means its capacitance, inductance and resistance are spread uniformly along the cable. We can use a trick to see what happens in the feeder line: we can imagine the cable to be made up of a series of very short sections of discrete components L and C whose values determine the characteristic impedance of the cable. We can use another trick to understand what happens to the RF energy passing down the feeder from the transmitter. Imagine that the RF energy you understand as being sinusoidal waves can be represented by a series of very short pulses, all of the same time duration, but each one slightly different in amplitude according to the sinusoidal wave shape.
Now, what we do to understand this SWR business is that we consider what happens to just one of these pulses on its passage down the feeder from transmitter to load. In doing this we shall consider how this pulse behaves when it encounters different types of load (which in practice would of course be the antenna).
First we shall send a fast pulse to a short-circuit at the end of a feed-line and use a’scope to see what happens. This short-circuit represents one extreme in the range of impedance that might occur as a load. It so happens that the short-circuit condition is a good way to start understanding what takes place along the feeder and at the load. As the pulse travels down the feeder we can imagine it encountering the successive L/C sections representing the cable and which delay it. The total delay is made up of all the small delays of each of these imagined sections and we can express the delay as the velocity of the pulse being a proportion (the velocity factor) of the speed of light. Although we observe the pulse as a voltage on the’scope, it is an electromagnetic disturbance, with half its energy in the E field (electric) and half in the H field (magnetic). We will look at these fields in more detail later.
Now, here comes the important bit. When the pulse reaches the shorted end of the feed line, an equal but opposite polarity pulse is generated at the short-circuit (this happens because no voltage can exist at a short-circuit but the energy of the original pulse must be conserved). So the forward pulse disappears, because its energy has been totally transferred to a reflected pulse which starts to travel back down the feeder to where the original pulse started from. This reflected pulse would appear on a’scope display separated from the original outgoing pulse by twice the distance along the feeder (the outward and reflected directions). Neat isn’t it? Now you can see why I chose a short-circuit as the first condition to look at - it’s so easy to explain.
......”I don’t know about that” muttered Mick to himself “ my head hurts”. But he was beginning to understand it so he carried on reading......
If we now look at an open-circuited feed line, the other extreme in the range of terminations, something different has to happen to satisfy the terminating conditions. Again we consider what happens to a single pulse going down the feeder, again the incident pulse is delayed and on reaching the end it is again totally reflected, thus conserving energy as it disappears. But this time, the open circuited feed line means that no current can flow at this point as the pulse arrives and this can only be satisfied by the reflected pulse having the same polarity as the incident pulse. Thus at the moment of arrival of the pulse, the voltage at the open end of the feed line momentarily doubles. We see that energy which cannot be accepted by a load has to be returned to the generator to comply with the conservation of energy law.
If we terminate the feed line with a load equal to its characteristic impedance no energy needs to be reflected as it is all absorbed by the load. We can regard this as the cross-over point in circuit behaviour. For terminating loads with lower impedance (extreme case being a short-circuit) the reflected pulse is opposite polarity. For terminations having a higher impedance than the cable characteristic impedance, the voltage pulse is the same polarity (extreme case being an open-circuit). Each side of the cross-over point, the proportion of energy reflected increases, until at the two extremes, all is reflected.
......At this point Mick had had enough, there were still several pages left, but he felt confident that he had understood the important principles and the rest could wait. Foolishly for so late in the evening he finished off with a toasted cheese supper. The inevitable happened - a nightmare of being chased down never-ending tunnels by ghostly shapes that overtook him and then suddenly came back towards him in horribly distorted fashion with pieces breaking off them. He woke sweating, feeling more stressed even than when he had tried learning morse.
If you have missed our other episodes:
Episode 1.
Episode 2.
Episode 3.
next episode (Chapter 5).