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Author Topic: Why cant radio antennas be point like?  (Read 2287 times)

Offline McKay

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Why cant radio antennas be point like?
« on: 28/06/2014 21:11:59 »
Why exactly radio antennas must be as large as the wavelength or noticeable fraction of the wavelength of the emitted wave? I read that very low, extremely low frequency broadcasting is difficult just because the antennas have to be huge. But I dont really understand why.
A radio wave is really just a propagating change in EM field, as I understand: electric charge at the receiving end feels, over distance, the charge (multiple differently charged points along the length of the antenna) in the transmitting antenna and reacts to it, being repelled or attracted, being jiggled around withe the frequency of the change in charge strength at the transmitting end. Isnt it like that?
Why cant, say, a van de graff machine transmit ultra-low radio waves accumulating and dissipating charge at the frequency of the wave that one want to send?


 

Offline McKay

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Re: Why cant radio antennas be point like?
« Reply #1 on: 06/07/2014 13:14:41 »
Anything?
 

Offline evan_au

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Re: Why cant radio antennas be point like?
« Reply #2 on: 06/07/2014 19:47:47 »
It's true that the 21cm Hydrogen line is emitted from a neutral hydrogen atom, which is pretty point-like compared to the wavelength. However, this emission (or absorption) event is extremely unlikely, with a lifetime of about 10 million years.
See: http://en.wikipedia.org/wiki/Hydrogen_line

Part of the challenge of designing an efficient receiving antenna is to ensure that there is a significant electric field differential between parts of the antenna. If the peak electric field intensity is (say) 5 microvolts per meter, and your antenna is (say) 0.1mm long, then  your electronics had better be sensitive to better than 0.5 nanovolts - bearing in mind that thermal noise is pretty significant too.

You can extract a bit more signal by having the antenna resonate at the frequency you are trying to detect, and by using an error-correcting code to reduce the effects of thermal noise. But overall, the antenna will make more use of the available electric field if it is at least half the wavelength.

If you can afford to build a parabolic antenna (say) 1000 times the wavelength, then you can get even greater signal strength.

The problems of building a transmitting antenna are similar to those of a receiving antenna. If you want to generate a high electric field strength, but your antenna is only a small fraction of the wavelength, then you must pump far more power into the antenna, and only a small amount of that actually gets radiated.

Designers of Extra-Low-Frequency transmitters (eg for submarine communications) sometimes pump power into high-resistivity rock, from locations many miles apart. The high resistivity pushes the conductive path far down into the rock, increasing the effective area of the antenna.

Also see: http://en.wikipedia.org/wiki/Extremely_low_frequency#Difficulties_of_ELF_communication
 

Offline syhprum

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Re: Why cant radio antennas be point like?
« Reply #3 on: 07/07/2014 03:40:33 »
Although it is necessary to feed a much higher voltage and current into a sub wavelength antenna this should only result in loss of radiated power due to the resistive loss in the materiel.
I superconductive materiel is used for its construction it should be possible to achieve higher radiative efficiency
 

Offline evan_au

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Re: Why cant radio antennas be point like?
« Reply #4 on: 07/07/2014 04:51:01 »
Quote
If superconductive materiel is used for [antenna] construction it should be possible to achieve higher radiative efficiency
Superconductors work well at DC and low frequencies, but they have problems carrying high currents at high frequencies, and tend to revert to normal resistive conductors.

However, superconductor quantum junctions (SQUIDS) have proven very sensitive at detecting small magnetic fields, see http://en.wikipedia.org/wiki/SQUID
 

Offline McKay

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Re: Why cant radio antennas be point like?
« Reply #5 on: 13/07/2014 15:48:58 »
Ah, so, from these responses, it is technically hard to do, but with some some advances in sensitivity and radiance it could be possible to send and receive ELF waves from small antennas and there is nothing fundamentally stopping it?
 

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Re: Why cant radio antennas be point like?
« Reply #5 on: 13/07/2014 15:48:58 »

 

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