Re: Fractal antennas and Tesla coils

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


Supposing for the moment that you were trying to do the wireless power
thingy, could a broadband antenna help improve efficiency?

No, for the reasons Malcolm mentioned. With modern RF technology it's 
easy enough to generate power efficiently at one accurate frequency. 
Magnetrons and the like are not super-accurate, but the FCC etc. just 
made the ISM bands wide enough to accommodate that. ;-)

The state of the art in wireless power transmission just now (afaik) 
is a rectenna illuminated by a microwave beam from a magnetron with 
dish antenna. They have transmitted hundreds of watts with several 
tens of percent efficiency.

For those who don't know what a rectenna is: it's a rectifying 
antenna. Think of those tacky novelty things for cellphones, with LE 
Ds that light up when you make a call. They are powered by a 
rectenna. For higher voltages/currents than one RF diode can handle, 
I've seen rectenna designs made from meshes of diodes, with the diode 
leads cut to resonant length so they are the antennas. The DC outputs 
of each diode end up in series/parallel. This does end up looking a 
bit like a fractal antenna, but it's designed for optimum performance 
at one frequency.

It would theoretically be possible to build this gadget yourself- 
it's probably no harder than a Tesla coil. The hard bit is getting 
hold of power diodes that are also good into the GHz. I heard that 
they use silicon carbide schottkys, or hundreds of RF mixer diodes in 
series/parallel or such like. But the whole point of schottky diodes 
is that they have zero recovery losses, so maybe even 1N5819s would work.

The other worry is whether you're allowed to do that. I think the FCC 
allows ISM devices to leak quite a lot of RF, but this is beyond mere 
leaking- you'd be using a high-gain antenna to deliberately radiate 
all of it. Maybe you'd need a ham licence, and a morse key hooked to 
your oven magnetron so you can send your call letters before frying 
every Wi-Fi adaptor in your neighbourhood. ;-)

Steve Conner
http://www.scopeboy.com/