Jonathan's Carriacou diary: Transmitter
The human guess to the constitute of things,
Is that atoms form clusters, sometimes in rings,
Electrons are part of this complex world,
Their influence is all pervading, amazing to behold.
Around an electron is a field of electricity,
When it is moved it also creates magnetism, initially,
At a flick of a switch and in the wire,
These fields ARE radio waves, spreading out afar.
At the speed of light too fast to conceive,
They travel through matter it's hard to believe,
And by changing their strength or even their vibration,
A word or news can be conveyed as information.
Marconi's dream and Maxwell's brilliance,
Has saved lives at sea, sometimes at an instance,
Inventions such as TV, radio and mobile phones -
Just a few examples that will continue to fill our homes.
Slept 11 hours last night! I think that accounts for the blues yesterday - I was exhausted.
Kathy and I take on the transmitter challenge!! Have a really great creative day today as we had loads to do. Also the good day seemed to be the same for everyone as we all seem to be on top form. Manage on first day to make the basic transmitter and Kathy almost finished the receiver. The receiver will have two types of detector and so the design has taken a little longer. We are in the nice position to start testing the two together tomorrow.
Reggie came along last thing to pick us all up in the minibus. He had a medium-wave car receiver and so we turned it on. Could just about hear the transmitter buzzing away. I was a bit disappointed as I had expected much more of a signal. By the end of the day K and I were feeling knackered. We were both picking up bradawls and trying to use them as screwdrivers and generally going a bit fuzzy with all the efforts of the day.
Another creative action packed day for the two of us. I make up a morse key to operate the transmitter. K and I set up the antenna and the transmitter and receiver on the sea front so that we can use the sea as a good connection to Earth. Razorblade detector did not seem to work but the coke did! K very happy.
By the end of the day we were in a position to try out the transmitter near to the receiver but even just ten feet between the antennas produced no result. It was near to the end of the day and as an experiment we tried out one of the three or four other alternative coils for the transmitter. The largest winding coil we found worked! So we dragged back the film crew and got them to film the first working test runs with the set-up. Great way to end the second day.
Big coil or small coil?
The transmitter we built was a spark gap transmitter. The small coils we used (i.e. using only about 100 turns of wire) made the greatest spark and so naturally we thought this would mean that it would be best for the transmitter. However, it seems that a spark is not enough - it needs to be a high-voltage spark. The smaller coils seem to produce a large spark but I would guess that it is a low-voltage, high-current spark. There's loads of power in this spark but it is mostly heat and light and it seems to have very little radio energy.
The larger coil produced a smaller spark but the coil produces a high-voltage back which makes the spark a high voltage spark. You can see that there is a difference in the colour, presumably because the high voltage breaks down the air to produce ozone and other gases that the lower voltages can't do. So the large coil won the day.
The first half of today was quite stressful. Time seemed to be going away from us and the transmitter, antennas and receiver modifications and the tens of other things to do weren't getting finished. This was partly because other filming had to be done and we could not make too much noise and also because they needed to film our steps and results. A major reason was also because we were trying again and again to get the transmitter to work better - and failing.
K tried out a step-up-transformer using the buzzer coil as a primary and a much larger secondary coil around it to produce a step-up in voltage. But all our attempts failed to step up to any reasonable degree. We were having problems with the coupling efficiency between the coils and losing all our power! Instead of getting a step of x10 or x100 we were getting x2, which was basically useless. The sun beat down and we were getting tired. Thinking slows down and we all become less efficient. The challenge of sending a message over a distance seems far off.
We try out a much larger step-up coil having 1000 to 1 step-up. This worked much better but even this was having problems. We rigged up a simple spark gap and every so often we would get a decent spark but then minutes would go by with nothing. It began to dawn on us that the humidity was partly the problem. In some ways high humidity makes sparking easier but in this case it was causing very small sparks and losses in the air well before the voltage had time to build up to the level it could do. Consequently we were missing our potential.
We all stopped for lunch to have a break and think about what could be done. I started to think about what was happening in the spark. I would guess that the buzzer starts to spark immediately, the connections separate and the screw comes away from the bending saw blade. In the first instance the gap is incredibly small, perhaps a millionth of a metre. So in this case even 1 volt across this gap would produce 10,0000V/m electric field and spark.
As the blade moves away a little more, the air is hot and electrified and passes current (due to the ions made in the initial spark) and this continues to spark as the gap increases. So it is possible to have a low voltage spark this way. Our thought was that it might be possible to add the high voltage produced by the step-up coil into the lower-voltage spark. In the end we found that we could use the vibrating saw blade not only to form the buzzer circuit but also to kick-start the high voltage spark into action. With another screw attached to the saw blade and the connections wired up carefully and in the correct way we got our high voltage spark gap transmitter to work!!
Kathy and I, Derek and John put up another large antenna for the transmitter and used the previous antenna for the receiver. They were fitted about 40m apart and they worked - just about. The signals were tiny in the earpiece and softer than the sound of the wind or the crashing of the waves.
The final scene After the time signal has been sent by the two Mikes, Kathy sends me a secret morse code message, while I sit trying desperately to hear the morse above the background noise! I could not do it. I ask Kathy to send it faster. Suddenly it was much better. I hear Y .. , then Y. S, definitely an S at the end. Then I try perhaps three or four times to hear the middle letter. Sounds like a dot which is an E. Longer letters such as Y -.- - are in some ways easier to hear because you realise the first dot or dash has passed with the quiet succession of the others. E is therefore rather difficult. Kate ends up coming and saying "Jonathan, what else can it be?" I had been concentrating so hard I hadn't even thought about the word. Of course it was YES! This is really a high. I've been learning morse since Christmas and my first ever live test was done on TV!!
Paul tells me later that all the while this was going on the religious radio programme in the background of the radio reception was talking about circumcision and damnation!! I was concentrating so hard on the morse that I missed all this, otherwise I would not have kept a straight face!
Kathy, Drew and I take a boat to Trinidad Cays - wonderful place. Green-turquoise sea filling a half mile wide stretch of sand with the odd island dotted here and there. The blue-green lagoon reflects the light so wonderfully that the sea lights up the underparts of the birds that fly over - amazing to see. The boat home from the Cays to Carriacou was great and very, very relaxing - something very mellow about a boat trip slowly making one's way home.
Copyright & revisions
Originally published: Wednesday, 26th July 2006
Last updated on: Monday, 28th January 2008
- Body text - Copyright: The Open University
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