addyismad

why modulating a wave with a signal of very high frequency enables long distance transfer of information????/

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Russlk

High frequency is not a requirement; you just have to have an antenna that will radiate the carrier frequency. Low frequencies follow the curvature of the earth better than higher frequencies. Above 100 mHz, distance is limited by the earth's curvature. In the 15mHz to 50mHz range, signals can be bounced off the ionosphere to increase range.

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zachtheterrible

Using high frequency enables higher data transfer rates.

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akg

so that we can use efficient small antennas , the HF signal is the carrier for the low freq signal , combining these will produce a HF component .

mstechca

Now just a sec.

He asked a really good question.

You understand that the higher the frequency the shorter the wavelength. and the distance (believe it or not) is a heck of a lot smaller than lower frequencies. This is one reason why you can pick up stations from far away with low frequencies. This is at least what I think it is.

About 3 years ago, I built a cheap shortwave radio when I was learning about amplifiers. and I managed to pick up a station from Alabama, and yet, I'm still in Ontario, Canada.

When I play with my superregen at a high frequency, I can't pick up stations outside of Ontario.

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akg

U could get short wave transmission at long distance (considering earth) ..because ionosphere and ground surface act as a waveguide to propage the wave along the curvature of earth before getting absorbed, above around 100Mhz ionosphere cannot reflect the waves ,and they escape from earth's atmos .. that's y they r LOS propagation..

audioguru

Can't your "radio" pickup many FM stations in Buffalo NY that is just a stone's throw away from you? Then there are more FM stations in Syracuse and Rochester NY.
Does it get all 20 or 30 of the FM stations in Toronto? You can probably see the CN Tower from where they transmit from.

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zachtheterrible

Another point: in freespace, a lower frequency radio wave will travel farther than a higher frequency. This has nothing to do with atmospheric skip or the ionesphere or anything.

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Russlk

Zach, what is the basis for that statement? I was not aware of that.

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zachtheterrible

http://www.electro-tech-online.com/v...pace+frequency
Happy reading :lol:

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Russlk

The equation given by pyroandrew is ridiculous: The free space loss depends on how much of the radiated power is captured. If you have a sharply focused beam and capture all the radiated power, to think that the loss in one mile is 36.6db boggles the mind. Those who proposed to power a satelite using microwaves certainly did not use that equation!

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JimB

The equation given in the link is correct.
This is the attenuation between two isotropic antennae.
OK so an isotropic antenna is a theoretical concept but you have to start somewhere and then add the gain (or loss) of the practical antenna with respect to an isotropic antenna.

An isotropic antenna is one which radiates a uniform field in all directions.

JimB

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