Mick and Smithy Talk Antennas
Chapter 8
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
Back at the club once more, Mick and Smithy were discussing the World Cup. Smithy, of course, had spent no time whatever in front of the TV, but a half-heard news item on the radio had, it seemed, penetrated his anti-football defences. “You know, Mick what England needed was a good manager to weld those superstars into a team, like that Madonna fellow the Argentinians had.” Mick could only nod, his mind boggling over the possibilities of team talks in the changing room....
Anyway, matters technical took over from this nonsense. Mick has indeed made up the simplified Smith chart and proudly shows it to Smithy. Now equipped with a new understanding of feeders and SWR, Mick is anxious to pick Smithy’s brains about a multiband antenna to replace his 20-metre vertical, with which incidentally he has become so successful in working stations that he is becoming bored and feels the need for a change. But Smithy had something else on his mind, not unconnected with Mick’s idea for a multiband antenna.
“Remember last week I told you there was a final piece of nonsense about SWR” said Smithy, pulling from his bag a recent radio magazine. “Yep, I remember, go on then what is it?” replied Mick.
Smithy flipped over the magazine pages until he came across what he wanted, grunted and turned the pages around to show Mick what he had found. It was an advert for a remarkably small antenna covering the HF bands and a couple of the low VHF ones. (There were indeed many such adverts for these types of antennas.) “You see the ‘specification’ for that antenna Mick, it states a VSWR across certain bandwidths doesn’t it? Tell me Mick, what do you think that means?”
Mick suspected a trap, but he was honest enough to say exactly what he thought it meant “it’s the SWR on the antenna.”
Mick was right, it was a trap and despite all he had learnt from Smithy recently he had fallen right into it. “You know, Mick, I may be wrong (it must be admitted here that Smithy didn’t often say that) but my guess is that is exactly what a lot of people would think too. To say an antenna has such and such an SWR is just another part of the gobbledegook that has grown up around the subject. You see, Mick, this antenna, like most of the antennas we use in ham radio, is a standing wave antenna. It depends on having a high standing wave on the antenna in order to radiate, so saying it has an SWR of 1.15:1 or whatever is misleading nonsense. Remember though that like the standing wave on a feeder, the standing wave on the antenna is just a concept, the reality is that forward and reflected currents are travelling back and forth on the antenna element(s).”
Mick nodded, he realised the truth of what Smithy had said. “What do you think they mean then Smithy?” he asked. “Well” said Smithy” the only meaning it can have is that the antenna will cause the SWR on the specified feeder to be what they say. They obviously know this, but how it’s put creates entirely the wrong idea: feeder VSWR - yes, antenna VSWR - no! In any case I believe it is quite misleading to attribute a specific VSWR to an antenna as if it were a magical factor that defines how good the antenna is. If you go back and look at that list of rules for transmitting antennas I gave you some time ago, you will find that the first rule, and the most important one, was that there is no single parameter or measurement that defines how good an antenna is. Now call me cynical (Mick wanted to say here that he certainly wouldn’t, but he thought Smithy might take it in the wrong way) but putting SWR up as an indicator of a good antenna is a neat way of avoiding having to give more appropriate figures concerning its performance. Small antennas, however well they’re matched are going to be well down on large antennas where the radiating elements conform to the L/4 rule in the list I gave you. Radio is a complicated subject to try and understand at a physical level and we need to be precise in using technical terms such as SWR.”
“How far down in performance terms would this type of antenna be then judged against a full length quarter wave?” asked Mick. “Well that’s a tricky one after what I just said about being precise! In my experience even a L/4 end fed seemed to be at least two S points down on a L/2 dipole on 80 metres. When I operated a CW QRP 3W station on 80 using a dipole I can only remember once giving out a lower report than I received and that guy was using a ‘Joystick’, one of the early attempts at small multiband antennas, which incidentally eventually became something of a joke” stated Smithy. “But you should be aware that in particular if you are using low power, then one of these small antennas on HF is really going to penalise you. Let me illustrate this by saying that compared to a 100W station with a dipole, first a 6W QRP power will take you down 2 S points, that is 12dB or 16 times. Then suppose the antenna is considered as a point source, this adds another 2dB down. But worse than this, the efficiency of a small antenna is going to be much less than a dipole. At a guess you could end up 20dB down or more, which doesn’t sound too bad until you realise it means you’re a hundred times down on the guy with 100W to a dipole. Under modern band conditions of QRM you may well be below the noise level.”
“So you wouldn’t recommend one of these antennas then, Smithy?” asked Mick. “Well, I am not going to condemn them, they might be the only type of antenna you could put up on your property, I am just saying you have to bear in mind that the general rule with antennas is that you trade performance against size reduction” cautioned Smithy. Mick had become uncertain “Perhaps I’ll think about this some more before I go spending my hard earned cash.” “Ah, you poor young xxxxer” said Smithy, knocking Mick’s baseball cap over his eyes “let me buy you a tea. Next week we’ll ‘go back to basics’ about antenna theory.”
If you have missed our other episodes:
Episode 1.
Episode 2.
Episode 3.
Episode 4.
Episode 5.
Episode 6.
Episode 7.
next episode (Chapter 9).