Mechanical vlf is it practicable

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Re: Mechanical vlf is it practicable

Post by Admin on Mon Jan 30, 2017 7:35 pm

Hello Densil,
No, it is not really practical. The Grimston transmitter only operated at 17kHz and had a maximum frequency limit of 40kHz.

You need to generate 135000 waves every second, that is two magnets per wave (+ and - half-waves) = 270000 per second. If you rotate a disk too fast then the centrifugal force alone will limit the circumference of a disk.

If you could get 6000 revs per minute (100 revolutions a second) then you would need a lot of magnets. Do the maths and you will see what I mean.

Many disks would also be a problem. If you add together several sinusoidal waves from two different sources then you would not get double the frequency, so you would need 2700 magnets on a disk rotating at 100 revolutions a second (6000 r.p.m.).

You would have to have almost perfect magnet spacing to get a good stable frequency per revolution or you would get frequency "wobbling".

In addition, you cannot escape electronics: according to your licensing conditions your transmitter frequency must be linked to a stable reference, such as a crystal-controlled oscillator or a crystal-based wave-meter.

Sorry Densil but the idea is not practical in amateur circles, unless you are prepared to throw money at it.

Best regards from Harry

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Mechanical vlf is it practicable

Post by Densil on Sun Jan 29, 2017 1:22 pm

Hi harry
i read about the grimeton 40khz transmitter and thought that it is just a big alternator. today we have motors with very high revolution rates even the motors in hard disks and such. Would it be practical to make a hamradio tx for 136khz using a high speed motor and a few magnets? Using a single motor shaft and more than one disk to mount the magnets you should easily be able to get perhaps 500 magnets (50 per disk). If you use 5mm squares of that fridge-magnet 2mm thick material you could get hundreds of magnets on one disk. Just an idea using no electronics.
/D

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