Why does ELF radio need more watts than MW radio?

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Green Xenon [Radium] - 27 Jul 2008 19:51 GMT

Hi:

I've remember reading somewhere than ELF [Extremely Low Frequency]
radio transmission is inefficient because it requires to much power.

If that is the case, wouldn't MW [Medium Wave] radio transmission
require even more power?

MW and ELF are forms of electromagnetic radiation in the RF spectrum.

An photon [or electromagnetic wave] of a higher-frequency has more
energy than a photon of a lower-frequency.

Let's say there are there are two radio transmitters, one emits 2 GHz
waves while the other emits 2 kHz waves. If the two radio transmitters
use the same modulation scheme [AM/FM, etc.] and emit the same amount
of photons-per-second-per-square-meter, the 2 GHz transmitter will be
using more watts than the 2 kHz transmitter -- because a 2 GHz photon
requires more power to generate than a 2 kHZ photon. Right?

So how would transmitting a lower-frequency radio wave require more
power than transmitting a higher-frequency radio wave?

Thanks,

Radium

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rge11x - 28 Jul 2008 04:13 GMT

On Jul 27, 2:51 pm, "Green Xenon [Radium]" <glucege...@gmail.com>
wrote:

> Hi:
>
[quoted text clipped - 22 lines]
>
> Radium

Because the signal energy the receiver captures is proportional to its
antenna size, and for a mobile receiver it is very difficult to make a
large enough antenna to be practical Other things being equal the
antenna size is roughly proportional to its wavelength. At 1kHz
carrier frequency the wavelength is 300km, at 1GHz it is 30cm. In the
70's the US Navy built a country-size antenna farm for ELF to link to
its strategic submarines because ELF can penetrate sea water. The
antenna that the submarine was towing was obviously tiny compared to
the transmitter's. The other issue is that the lower the frequency the
more atmospheric and other man-made external noises enter the receiver
along with the signal, thus there really is not much point ot building
large receiver antennas because the signal to noise ratio will be
dominated by how much you tranmsit and atmospheric noise, and their
ratio is independent of the receiver antenna size. Then the only thing
you can do is to increase the transmit power. But you still have to be
able to radiate it out, all of it, and that takes an antenna whose
scale is a reasonable fraction of the wavelength, hence these are
usually one-way links such as AM broadcast radio around 1MHz.

Above around 50MHz or so, internal receiver noise starts to dominate
and then the larger the receiver antenna the better the reception is,
above 300MHz most of the receiver noise is internally generated in the
first amplifier and in the following RF hardware.

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Natural Science Forum / Physics / Research / July 2008

Why does ELF radio need more watts than MW radio?