Re: Why does ELF radio need more watts than MW radio?
- From: clifford wright <c.c.wright@xxxxxxxxxxxxxxx>
- Date: Fri, 31 Jul 2009 06:26:22 +0200 (CEST)
Very Good! Indeed the problem is radiating the ELF energy, not generating
it.
It is often forgotten that radio started out at VLF frequencies.
During WW1 the German's had a 24 kHz transmitter at Nauen with a vast
antenna array like a skeletal circus tent 250 metres high and covering
many hectares. The signal was generated directly at 6 kHz and quadrupled
by magnetic amplifier type circuits with over 200 kW of power.
Although hard to generate the range was very good. German agents in South
America could read the signals on a simple amplified crystal set!
In the same era the US navy used electric arc transmitters with powers up
to 500 kW in Hawaii and the Panama canal zone. These also worked at 30 to
50 kHz with vast antenna systems.
BTW the German agents actually used a collapsible frame antenna which was
only a metre or two square assembled for reception so the siganl strength
must have been pretty good!
The biggest reception problem is usually local noise sources, whistlers
and other statics.
Clifford Wright.
====================== Moderator's note: =================================
I've put your posting in front of the quoted text and adapted the units to
the SI conform conventions. HvH.
==========================================================================
rge11x <rge11x@xxxxxxxxxxxx> wrote in news:878e808f-0982-438f-a8f7-
b06970b6d358@xxxxxxxxxxxxxxxxxxxxxxxxxxx:
On Jul 27, 2:51 pm, "Green Xenon [Radium]" <glucege...@xxxxxxxxx>
wrote:
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
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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