Fractal Antenna:
https://en.wikipedia.org/wiki/Fractal_antenna
http://www.antenna-theory.com/antennas/fractal.php
Pulsed RF:
https://en.wikipedia.org/wiki/Pulsed_radiofrequency
http://www.microwaves101.com/encyclopedias/pulsed-rf-sources
Don't use a fractal antenna. A pulsed signal needs a wide bandwidth without holes in it and fractal antennas don't have this. I don't know of any uses where fractal antennas are better than a simpler alternative. For wide bandwidth you need an antenna that is about 1/2 wavelength across or more, i.e. 7 mm at 20 GHz. This assumes that by pulsed you mean short pulses. If the pulse length is e.g. 10000 cycles or more at 20 GHz you can get away with a narrow-band antenna which can be much smaller. The Quality factor is roughly the number of 20 GHz cycles in the pulse, which is the inverse of the relative bandwidth you need.
Hi
I am not sure what you mean by a 20 Hz signal and pulsed RF. Can you be more specific. What signal do you want to send at 20 Hz? How often do you need to turn it on and off? (This defines the bandwidth). Is it just continuously on (CW) - I don't expect so as in this case it does not send any information. It is important to know these things in order to be able to specify an antenna. Do you want to send a signal at 20 Hz through the air, that goes on and off at a lower frequency (what frequency) or do you want to send a signal at e.g. 17925 MHz through that air, that goes on and off at frequencies from 0 to 20 Hz? Doing it this way would be much more efficient. A 20 Hz signal would need a very large antenna to get any kind of efficiency.
What distance do you want to be able to receive the signal at?
Hi,
Originally I wanted to transmit a 100 Watt 20 Hz continuous sinewave. I need to receive the signal at very close range (less than 1 meter). I was thinking of just using the near field. And I thought if I pulsed the signal that would reduce on the overall requirements (as it would reduce the bandwidth).
But in thinking about it, as Biological materials are non-linear, I guess I could transmit two frequencies, with the difference between them being 20 Hz. Unless I am mistaken, that would induce a frequency of 20Hz in the bio-material? I do not wish to encode any signals onto this 20 Hz signal.
If you want to send a 20 Hz signal 1 m then your best bet would be a metre diameter coil or larger. This can generate a large magnetic field nearby, but not much of an electric field.
If your signal really is CW the bandwidth is zero - actually you are going to turn it on and off so the bandwidth is not zero but roughly 1/(time it is on) but usually you need more bandwidth than this to take account of tolerances and temperature changes (so that when the antenna centre frequency isn't quite right it still works). If you pulse it then the bandwidth of the signal goes up, not down (the bandwidth is roughly 1/(pulse length)).
(Some) biological materials are non linear, so you can try to use mixing to generate the frequency you want, but you probably won't get much of it.
At 20 Hz it is very hard to actually radiate any significant power with an antenna the size you want. On page 443 of the second edition of Electromagnetic Waves by Carlo G. Someda it concludes/shows that the active power is negligible compared to the reactive (in this case inductive) power. Your best bet is probably inductive coupling, i.e. using the reactive power.
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