Mick and Smithy Talk Antennas
Chapter 6
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
After a short holiday recess, Mick and Smithy reappear at their local radio club. Mick is anxious to clarify some points about the notes he had borrowed off Smithy. “First of all, these fields you mentioned, I’m not exactly sure I understand how they come into it at all.”
“Well,” said Smithy “Remember I said we were dealing with a coaxial feeder, so that the fields, in the main, will be localised within the cable. As each of the two fields reach their zero energy state in their cyclic variation, it stands to reason that the other field must hold all the energy, otherwise it would contravene the conservation of energy laws. Thus over time they have to average at each holding half the power being transferred along the feeder.”
“What is a field anyway?” said Mick, not wanting to be so easily dismissed. “Look,” said Smithy “I can’t explain everything at once, for the moment just regard a field as a region where forces exist due to electrical or magnetic energy being present.” Mick appeared to accept this - to Smithy’s relief (he had suddenly glimpsed the dilemma of schoolteachers faced with never-ending series of questions of the...but why variety?)”
“What happens to the energy when the feeder is terminated in an open circuit or a short circuit? You said the energy was continuously recycled, so it would go on forever wouldn’t it?”
“Ah - a good old trick question, Mick,” said Smithy “Well it shouldn’t surprise you to know that we haven’t found a case of perpetual motion. First of all, remember that whether we are considering a theoretical generator or an actual transmitter with an’ATU’, neither will be able to present to their end of the feed line the correct conditions to recycle all this energy back down the feeder i.e. to act as an open or short circuit. So some reflected energy is going to be dissipated in their output circuits, with a possibility of damage if the conditions are sustained. Now going back to those fields we talked about, some of the field energy will not be contained within the cable, that which escapes will be lost as radiated energy, also, because the cable must have some resistance there will be losses in this too. Eventually these losses will account for all of the energy.”
“I think we are getting a bit too theoretical here, Mick, lets get some tea and I’ll tell you about something you don’t appeared to have spotted.” Munching biscuits and sipping tea Smithy said “In those notes I tried to explain what was really happening in a feeder-antenna system, but without over-complicating it. It is a matter of common experience that operators will match the antenna system to the transceiver quite happily, the old guys used to say ’middle for diddle’, without understanding what is happening. (Here note that Smithy made no mention of automatic ATUs - well he wouldn’t would he?) There is a small complication that I didn’t mention in those notes. Obviously dividing up a’so-called ATU’ into an input side and an output side neglects the fact that there must be interconnection between the two - after all there are usually three variable controls to adjust. Well, usually there has to be variation of the coupling between these two sides, and in fact this coupling means that adjustments are interactive. A skilled operator usually has no difficulty in arriving at the best settings, often a ball-park setting up can be done in receive for maximum noise, though the club W3DZZ dipole on 80-metres SSB does seem to be an exception to this” chuckled Smithy.
Grabbing another chocolate biscuit, Smithy moved away from the hubbub around the tea area and went to retrieve the club’s MFJ analyser. Turning to Mick he said, “did you get the idea of transformed impedance on a mismatched feed line?” “Yes I understood that ok, it’s because of the variation in the summation of the forward and reflected voltages and currents that the voltage/current ratio at points along the feeder is not constant” said Mick. Smithy looked at his friend in amazement and thought to himself that it was funny how Mick had laboured over the simple idea of fields yet had understood this most important idea with apparent ease. “Well anyway I’ll rig this MFJ analyser up to show you” said Smithy.
Smithy connected up the club’s MFJ analyser to a length of large diameter 50-ohm coax . “Now Mick” said Smithy “This analyser is so useful in looking at how feeders work: to demonstrate transformed impedance, all I have to do is change the frequency of the oscillator in the analyser so that effectively we are looking at different cable lengths. Of course the PHYSICAL length of the cable doesn’t change, but its ELECTRICAL length (in terms of wavelength) does as I change the test frequency. An important thing to remember about feeders is that you must always think in terms of electrical length, in other words fractions of or whole numbers of wavelengths.” Smithy connected a small diecast box up to the free end of the coax and set its switch to 75-ohm. He then took a tape measure out of his sports bag and with Mick’s help measured the cable length. It came out at 17.27 ft after Smithy had done some paper and pencil work. “Now we need to find out what the electrical length is, remember RF travels slower in cable, so we need to reduce this length by the velocity factor of the cable, which we will assume is 0.66” said Smithy.
Benches at the club seemed to be fully occupied these days with members doing all sorts of work. A few feet away from Mick and Smithy a group had gathered around a home-made HF receiver. Dick, Rik and Vic had been trying for some weeks past to make this RX produce some signals, but signal generators and oscilloscopes notwithstanding, they had had little success. Smithy pulled a monster calculator out of his bag and connected it up to the mains. A reddish glow appeared on Mick’s face as the jumbo sized LEDs lit up. From down the bench came a woosh of noise from the recalcitrant receiver followed by shouts of jubilation and frantic prodding of test probes. Smithy appeared not to notice, he was absorbed in working out the electrical length of the cable which he announced to be 11.4 ft. As he switched the calculator off, the noise from the other end of the bench suddenly stopped, followed by groans of dismay. (It should be noted that Smithy kept this calculator not just because of its large display, he had indeed found that the pulsed LED display made an excellent wideband RF noise generator).
Smithy had a data book with him and using this he was able to see that the 11.4 ft of cable was a half wave on 43.85 MHz. He switched the MFJ analyser on and set it to 52 MHz. “Now as I come down in frequency, effectively we are looking at the transformed impedance of cables, with this particular termination, then progressively shorter than a half-wave down to the quarter wave frequency and finally well below it.” As they tuned the oscillator over this range of frequencies Smithy called out the readings and Mick wrote them down.
| MHz | SWR | R | X |
| 52 | 1.3 | 56 | 12 |
| 50 | 1.2 | 48 | 10 |
| 45 | 1.3 | 46 | 11 |
| 43 | 1.4 | 54 | 16 |
| 42 | 1.4 | 61 | 19 |
| 40 | 1.4 | 67 | 14 |
| 38 | 1.5 | 75 | 12 |
| 36 | 1.6 | 64 | 25 |
| 27 | 1.6 | 37 | 19 |
| 24 | 1.7 | 30 | 6 |
| 18 | 1.2 | 59 | 6 |
| 10 | 2.4 | 25 | 29 |
| 7 | 4.2 | 12 | 30 |
| 3.5 | 9.0 | 13 | 83 |
| 2.0 | 13.2 | 19 | 128 |
“From these results Mick, you can see that around the half-wave frequency the resolved components, R and X, of the impedance agree pretty well with what we know is the terminating impedance of 75 ohm. It’s not exact, the MFJ is not a laboratory instrument and there will be other errors in our method. So if you want to know what impedance your antenna presents to a feeder you can use an electrical half-wave or multiples thereof. Around the quarter-wave frequency you note how drastically the impedance has changed, with R down to about 30 ohm, so usually it is best to avoid this length. Then look at that reactance at 2.0 MHz, it’s 128 ohm.” Mick was impressed and thanked Smithy for the demonstration.
As they walked over to get a refill of tea Smithy asked “Oh how did you get on with those old magazines I lent you?” “It seems to have been rather a different hobby in those days, not so much commercial stuff to buy, but I was amazed by the number of constructional articles, most people seemed to be building stuff in those days and not so many columnists - more stuff written by ordinary members. What I really liked though was an article on National Field Day as it used to be” replied Mick. A glazed look came over Smithy as he remembered halcyon days of whole summer weekends spent at NFD:- erecting the tent - erecting the aerial - moving the tent - moving the aerial - that time he drove over the feeder - the’social tent’ - arm signalling to Jack at the generator - the old guys twanging away with mechanical bug keys - skulking behind the caravan during aerial erection.
“Yes” said Mick, “I laughed and laughed over those NFD photos - some of those guys were wearing sports coats and flannels and one was even wearing a tie.” As he said this he glanced at Smithy. Smithy was wearing a sports coat and flannels. Mick had never noticed this before. Even worse, Smithy was wearing a club tie, though admittedly the club had long ago modified its name from that on Smithy’s tie. Mick gulped - Oh **** just when I was going to ask him how the MFJ analyser worked.
Oh dear, is this the end of their friendship and of our eavesdropping on conversations between the excitable young duffer and the dull old buffer?
If you have missed our other episodes:
Episode 1.
Episode 2.
Episode 3.
Episode 4.
Episode 5.
next episode (Chapter 7).