Mick and Smithy Talk Antennas
Chapter 10
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
Of junk and photons.....
Once again the pair meet up at their local radio club. Due to the programme of talks, it is a week or two since their last discussion and Mick was curious to see what Smithy had meant when he had mentioned a ‘back to basics’ look at antenna theory. But this week it is the club junk sale and time for discussion will be strictly limited. Mick asked Smithy to explain.
“Well, we could talk about antennas ‘till the cows come home” replied Smithy, with his attention distracted by stuff being brought in and displayed on the benches ”and there would always be more to say. It’s a curious thing about amateur radio, but my experience is that the vast majority of people in the hobby regard the antenna simply as an electrical device - something that just connects to their transceiver. Then provided the electrical requirements are all met and it works OK, they will be perfectly satisfied. Unfortunately, Mick, understanding how antennas work is probably one of the trickiest subjects in radio, in fact if you go back to the physics of antennas and radiation it becomes mind boggling. You don’t have to do this of course, as I said you can just regard the antenna as an electrical device which is the final part of our stations. Even at this level, eventually experience will teach you a great deal about antennas, but it can be a bit hit and miss.”
Smithy was not concentrating on their conversation however, for he kept noticing interesting items being brought in. He had, of course, already brought his own stuff in, whilst Mick, with totally modern equipment had no such need for junk, even if he had known what to do with it! But in a thoughtful mood for once, Mick asked “How do the professional radio engineers tackle it then Smithy?” “Well” replied Smithy, almost for the whole of the last century engineers were taught to understand it in mathematical terms. It was called ‘classical electromagnetic field theory’ and some basic equations called ‘Maxwell’s equations’ were used. Engineers had to think in terms of mathematical models for interpreting the behaviour of radio energy. They were very successful, as the vast amount of knowledge that’s been accumulated about radio shows only too well. There is a fiendish branch of maths called ‘Vector Calculus’ which is used a lot in field theory; we used to have a chap in the club who did it for fun! But anyway, even early last century, some physicists realised that field theory, whilst it could be used to DESCRIBE Radio Frequency behaviour, could not really EXPLAIN it in fundamental terms. The first fly in the ointment was what is called the transmission medium or how the energy is propagated.”
“Yes, that’s always puzzled me” admitted Mick. “At the start of the last century there was a school of thought which relied on a magical, mysterious substance filling space that would transmit radio waves” continued Smithy “but eventually physicists came to accept that quantum mechanics would have to be used to explain the fundamentals of RF behaviour. The trouble with this is that to people like us, Mick, it’s counter-intuitive.”
“What do you mean by that Smithy?” “I mean that if we try and use our experience of how everyday physical laws operate, then quantum mechanics seems weird nonsense” said Smithy. “I still don’t understand what you’re getting at” said Mick with a puzzled look on his face. He took his baseball cap off and scratched his head in exasperation.
“OK” said Smithy “enough of this philosophical stuff, If I explain the process of radiation from an antenna in terms of quantum mechanics, I think you will understand what I’ve been driving at. Photons are the elemental sub-atomic particles that are used to explain radiation. Photons have no mass in a state of rest or inactivity - so now you see what I mean about it being counter-intuitive! Anyway these photons can carry energy by having momentum. Incidentally a photon can also be referred to as a quanta. You can have quanta of different energy types but in electromagnetic energy terms they are usually referred to as photons. The wavelength of a photon, which is the distance between consecutive energy peaks depends on the medium in which it travels - this explains our ELECTRICAL wavelength that we talked about in feeders. The wavelength also depends on the frequency of the source that produced it. A photon will be produced if an electron in a piece of wire (our antenna) is accelerated or decelerated. This acceleration/deceleration of the electron is brought about in our antenna by the rapidly changing RF currents in the wire. The photons so produced will all have an amount of energy which they can later relinquish at a receiving antenna, where they cause electrons to accelerate and decelerate, that is they cause current to flow in the receiving antenna.”
“Now Mick, do you realise exactly what this means? It means that photons are emitted over the whole range of frequencies that occur when currents change in wires. But because of the necessarily larger dimensions an antenna would need to have at low frequencies it is more difficult to produce radiation at these frequencies, such as the mains supply of 50Hz. So there we are, there will always be some radiation even down at low frequencies such as 50Hz, which we wouldn’t normally think of as radio frequencies. I think that’s enough for tonight Mick, but next time I’ll get back to the conventional treatment of antennas in terms of fields. This sort of background information might not seem very relevant to the practical side of amateur radio, but it does sometimes enable you to understand something which might otherwise remain a puzzle. RF behaviour can sometimes seem inexplicable!” All Mick could say in response was his usual “my head hurts.” This was the signal Smithy had been waiting for, it was time to abandon this discussion and get on with viewing the junk at close quarters.
We should say here that over the years junk sales have changed considerably, both in the matter of junk and the people attending. For instance, Smithy and his contemporaries had all turned up at the sale bringing junk to dispose of. They also had no intention of buying any of other members’ junk - they already had enough of their own. And of course Mick would not entertain the thought of anything scruffy or home-made appearing alongside his array of transceivers. So there was the usual stalemate period at the start of the sale during which very little changed hands. But the chairman was wise to the ways of the “Smithy gang” and knew there was a sure way to get the stuff moving. He would auction the stuff off, inviting bids at knockdown prices. He knew that Smithy and his ilk would not be able to resist paying 25p, 50p or a pound for stuff. And this is what happened, Smithy became the owner of a Heathkit servicing ‘scope for a pound and several lesser items for 25p or so.
Someone even bought an unidentified item that was announced to be “a mystery item”. In years gone by another mystery item had frequented club junk sales. No one had ever identified what it was. It was quite heavy, about 9inches in each of its dimensions with some black potted compound on one surface. This had changed hands frequently, and unsuspecting members often found themselves in possession of it at the end of field day weekend, when they had innocently left their car boot lids open.
And so they wandered back over to the tea area for a final cuppa. As they did so Mick (now fully aware of the nonsense surrounding VSWR and ATUs) was telling Smithy how indignant he had become when he now saw pages of adverts in the magazines for ‘Antenna Tuners’ that do not actually tune the antenna at all. “It wants stopping” he said, “it’s misleading.” “Look” said Smithy “we are living in the age of bullshit, which when you examine it closely, often turns out to mean something other than what you first thought. Advertising people, estate agents, insurance people and such like have been doing this for years, you have to accept it Mick.” As the pair move out of our ear-shot Smithy is grumbling about the extent that the ‘celebrity culture’ had taken over the country: .....”Mick, if only fools wouldn’t put money their way we’d soon be rid of the lot of them and the TV and papers would be better for it”.....
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.
next episode (Chapter 11).