Mick and Smithy Talk Antennas
Chapter 5
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 have a 2-metre sked when their local club is in summer recess. We join them part way through the QSO (a veil is best drawn over the start of the QSO, for it became apparent that there was some confusion between the two regarding ‘blue-tooth’ and ‘blue tongue’. In fact Smithy had been confused earlier that week over reports that Maradona’s earrings had been confiscated because of unpaid tax, re-igniting the identity question of Maradona or Madonna in Smithy’s mind).
To complicate matters further, Smithy’s home-made xtal-controlled TX had long ago drifted off channel, fortunately Mick’s state-of-the-art TXCVR was able to cope, though a casual listener might have thought Mick was working cross-band. Mick was reporting his progress in tackling Smithy’s notes on SWR. “I’ve understood everything so far, except that bit about ‘Fields’. I thought the wave propagated in the form of energy continuously going to and fro from magnetic to electric field and back again, but your description puts half the energy in each of the fields so I am mystified.”
“Right” said Smithy “there is continuous interchange of energy between the electric and magnetic fields, noting that periodically each field falls to zero. AT ANY INSTANT the total energy will be the sum of that in both fields. In considering ANY FINITE PERIOD OF TIIME lasting more than one cycle, and being a whole number of cycles, it is apparent that total energy must be split equally between the two fields, otherwise the conservation of energy law would be violated. Don’t get side-tracked on this Mick, you can understand SWR solely in terms of current and voltage on the feed line. I wanted to keep it simple, but there comes a point when you need to understand ‘fields’.”
Mick was not convinced however, but decided he would just keep quiet and think about it later. The QSO was at an end and as Mick entered his contact with Smithy in his computer log - a message flashed up on his screen “ASK SMITHY HOW THE MFJ ANALYSER WORKS?” “Oh ****, I forgot to ask him again.” It was too late, Mick knew that Smithy would have switched off immediately and scribbled his log up before partaking of his usual evening tea and biscuits.
With nothing of quality on the TV (at least Mick and Smithy agreed on this point), Mick decided to continue reading Smithy’s notes. He opened the folder, glanced at the title:
THE FEED LINE - A CONCEPT REVIEW
And quickly re-read what he’d already gone through. With elbows on the table, he bent to the task and began to read the next part.....
We can imagine a continuous RF wave (sinusoidal) to be composed of infinitesimally short pulses. At each instant in the cycle of this RF energy, as it reaches the termination, that part of the wave behaves exactly as the short pulse did. You can imagine the reflected RF energy as infinitesimally short pulses appearing and reconstituting as sinusoidal wave energy - but this is just a trick to help you understand the process. For continuous wave energy, the incident and reflected waves must exist at all points along the feeder line. The resultant of these two waves is called the standing wave. For example, an open-circuit termination, causes voltage to be doubled at appropriate points along the line. Only in the case of a load matching the characteristic impedance of the feed line will there be no standing wave on the line.
....At this point in the notes, Smithy had handwritten in red ink the following....
NOTE THAT THE STANDING WAVE IS NOT A PHYSICAL REALITY - NO SUCH WAVE ACTUALLY EXISTS. A STANDING WAVE IS A CONCEPTUAL QUANTITY - WHICH IS A REPETITIVE PATTERN DERIVED FROM SUMMING THE FORWARD AND REFLECTED VOLTAGES OR CURRENTS. ONLY THE FORWARD AND REFLECTED VOLTAGES AND CURRENTS ACTUALLY EXIST.
....Which was almost too much for Mick to take in. He groaned and went to make himself a cuppa. But it was too early in the evening to give up and so he read on....
Nowhere in our reasoning did the generator impedance or the length of the feed line appear. Mismatching at the generator end has important implications, but these do not involve VSWR on the feed line. If we are able to probe along the feed line with an RF voltmeter, we will get this pattern of the “standing waves”. The pattern repeats itself at half wavelength intervals along the feeder. If there is reactance in the load the pattern is shifted, but the shift cannot be more than a quarter wavelength along the feeder. A partial mismatch (anything other than open or short circuit) causes the maximums to be less than double the forward voltage and the minimums to be greater than zero.
Now VSWR is defined as the maximum value divided by the minimum as would be measured by probing along the line with an RF voltmeter. It is unfortunately rather a mixed blessing that a point measurement on the line may be made and the results manipulated to give the same value of VSWR as that of our basic definition. Thus the sum of the incident and reflected voltage at a point, divided by the difference of those voltages gives us the VSWR. This is how SWR meters work. Although this is convenient for ease of measurement, it creates the idea that VSWR can exist at a point - which it certainly cannot. This is the root cause of the misunderstanding of how SWR meters are used in conjunction with an ATU at the transmitter end of a feed line to the antenna.
In such cases the ATU appears to perform miracles in reducing VSWR to very low values. It does no such thing - the VSWR on the feeder will remain unaffected by any adjustment of the ATU. So what happens when we adjust our ATU for minimum VSWR at the TX output? First we must understand that the feed line “transforms” the value of a mismatched load to a new value, which then exists at the input to the feed line (this is because of the changes in voltage and associated currents along the line.) We can split the ATU into two parts: the transmitter side (input) and the antenna side (output). Controls on the input side allow the TX to deliver power efficiently to the feeder/antenna system by matching the TX to the transformed impedance of the antenna. Controls on the output side create a mismatch at the input of the feed line (it actually mirrors the mismatch at the antenna end) and so allows reflected energy on the feeder to be sent back to the antenna. There is a very small time delay, which means that this re-reflected energy is not quite in phase with its originating constituent, though this is not normally of consequence. This ‘re-cycling’ of the reflected energy is a continuous process until all is effectively accepted by the antenna.
And the SWR meter? Well it read a low ‘VSWR’ on the input side of the ATU where it monitors the match between transmitter and feed line input, but the VSWR on the feed line remains unchanged. It should be thought of as measuring the reflection coefficient of power at the TX output and certainly not as measuring some hypothetical VSWR existing between the TX and the ATU. So in conclusion, we see that neither the SWR meter nor the ATU really live up to their titles when used in this typical configuration for an amateur station.
....Mick gasped, he had understood what had previously been a total misconception about ATUs and SWR meters. He looked at the clock, it was past midnight, but even when he turned in, his brain was racing with what he had just learnt and sleep was impossible. When he did get to sleep he dreamt that trading standards officers had taken the advertisers in Radcom to court and that Smithy had been called as an expert witness. (Of course it goes without saying that in such a case Smithy would certainly have accepted an out-of-court settlement.)
If you have missed our other episodes:
Episode 1.
Episode 2.
Episode 3.
Episode 4.
next episode (Chapter 6).