Research for novel: spark-gap transmitter

[liquidfrost]

(Please go ahead and move this if it's in the wrong section.)

I'm a science fiction writer doing research for my latest book and hoping someone can help. A technically competent character (far more competent than me with my long-ago high school physics) - let's call him Jake - needs to build a device that generates several EM pulses at a fairly specific frequency. I believe he needs to build a spark-gap transmitter, something I'd never heard of until I started googling today, but he's stuck on a low-tech world with renaissance-era technology.

The materials available to Jake are metal, wires, magnets, tools, etc. but no electricity or batteries. I'm happy for him to make a battery (from a lemon, perhaps?). If this no doubt odd-looking device creates dramatic sparks and smoke, all the better. I'm not worried about the receiver end of things, as that's taken care of.

Is this possible? Could you describe how to make it, what materials are needed, and what it would look like? The online instructions I've so far discovered either use materials Jake doesn't have, or create too complicated a device for Jake's purpose.

Thanks in advance.

 

[sophiecentaur]

Hi.
A battery, Inductor and Capacitor would be all he needed - plus wire.
He would need a hefty source of power so 'lemon' batteries would not be good enough, I think. Sulfuric acid was available from Medieval times (Oil of Vitriol, from a friendly Alchemist) so he could make some sort battery stack with Zinc (also available, apparently) and Copper plates in glass jars. Drawing copper wires is straightforward and he would have to wrap them in cotton insulation to insulate the turns of his copper coils (Faraday had to do this himself, too). Capacitors can be made with copper foil, in layers, separated by a good insulator - possibly shellac (also available, I think) on paper. Then a long wire for an aerial plus a metal stake in the ground for an Earth. If all he needed was some 'splats' of radio frequency signal to be picked up by his high tech rescuers (?) then he could literally 'dab' the wires onto the battery to produce sparks. The batteries would need to quite big - with plate areas of, perhaps 20cm square, because he would need to draw some considerable current for a good 'fat' spark and that type of simple cell packs up quite soon because Hydrogen gas bubbles form on the plates (polarisation) which need to be brushed off.

He should be arrested for doing this in modern times because of the interference he'd generate but it wouldn't be a problem in a Radio-less world. He would need to be pretty ingenious and to have some idea of the electrical constants and formulae that we mostly need to look up in books when we want to be sure of them.

 

[davenn]

- let's call him Jake - needs to build a device that generates several EM pulses at a fairly specific frequency. I believe he needs to build a spark-gap transmitter,

A standard sparkgap transmitter WONT have a specific freq, as Sophicentaur hinted at in his final statement, it would be very broad in freq.

to make a sparkgap transmitter with tighter freq control, you need to use a AC generator running at the frequency of interest. do some google searching on some of Marconi's early transmitters.

cheers
Dave

 

[sophiecentaur]

Problem is that you need lots of power or an amplifer if you want narrow band. But in a radioless world, you wouldn't need such a narrow band because the interference would only be Solar.

 

[liquidfrost]

Thanks for your reply, sophiecentaur. I'll do a bit of research into what that would look like. The signal only needs to travel a few meters at most, not into outerspace (essentially it's so Jake can perform a "trick") - how many lemons is that? :)

From the transmitter you described, do you know how narrow the frequencies would be? And could Jake control this to any extent? (i.e. emit a pulse within this broad band of RF or that broad band of RF).

 

[sophiecentaur]

So, if his spark transmitter can be that crude, what is he receiving the spark signals on?

If he wanted to transmit over a few metres then why not use a magnetic coil, loads of Iron and a compass needle? OR he could even use a wired circuit. No one would attach any significance to a piece of wire in a Renaissance scenario. They would only be looking for strings to pull.

 

[liquidfrost]

Would that transmitter transmit a specific frequency? Or at least a fairly narrow band?

The receiver in this case is the science fictional element of the plot - essentially nanotech robots or similar (which Jake does not have to build).

BTW he's not actually trying to hide the fact that this is technology and not magic, so the appearance of the device doesn't matter. It just needs to be easily made without machined parts.

 

[sophiecentaur]

Any spark transmitter will produce a broad band (very loosely tuned), noise-like signal. But that would be of no consequence for your nanotech robots (no problem for even a Radio Ham, in the same circumstances, with a simple bit of receiving kit). Spread spectrum communication is with us today, in a more sophisticated form. Your robots could easily detect an on-off keyed signal or short bursts of spark radiation. You are, of course, assuming that they are pretty intelligent? Or are you assuming that they need a signal with a particular narrow band frequency range in order to react?

I realize you don't want to spread your ideas everywhere. You could send me a PM if you are bothered by that.

 

[liquidfrost]

I'm prepared to assume whatever the plot requires in order to be able to write the scene I need to write. :) I'll PM you with thanks if I may pick your brain a little longer.