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Re: (meteorobs) Further Clarification of ELF, VLF, LF, etc.



Jim Bedient and Steve Harrison wrote:

>Here's the official titles: 
>
>Band   Frequency    Wavelength
>
>ELF     <3 Khz        >100 km
>VLF   3 - 30 kHz  100 km - 10 km  
>LF   30 -300 kHz   10 km - 1 km 
>MF  0.3 - 3 MHz   1000 m - 100 m    
>HF    3 - 30 MHz   100 m - 10 m  
>VHF  30 - 300 MHz   10 m - 1 m 
>UHF 0.3 - 3 GHz      1 m - 10 cm 
>SHF   3 - 30 GHz   10 cm - 1 cm
>EHF  30 - 300 GHz   1 cm - 1 mm

Actually, when electronics-types want to get more specific about the ELF
and VLF frequencies, we usually separate them further by using the
following terms:

0 Hertz (Hz): DC (much of the time, we "RF" types also refer to "digital"
signals as "DC" even if the digital signals are actually very, very fast
and could more-correctly be called "RF"!)

1 to 30 Hz: subaudible (below the usual human hearing frequency range)

30 Hz to 300 Hz: also subaudible (below the usual human speech frequency
range, but most humans can easily hear frequencies within this frequency
range)

300-3000 Hz: audio (because this is the usual human speech frequency range)

3 kHz-30 kHz: ultrasound (above usual human hearing range; this is the
range where your dog perks up his ears when you blow his whistle!)

---------

Hello all,

Here are a few notes on the subject which might be helpful:

** Since a VLF/ELf receiver converts its received EM frequency radiation
directly to an audio frequency (with no demodulation) , I thought it might
be helpful to compare the above given frequencies to a standard piano type
keyboard (using the modern digital tuning scheme).  This will give you an
idea what a particular received frequency would sound like.  The piano
keyboard is logarithmic, such that each linear octave represents a doubling
of frequency.

In the below, MC = middle C, and the numbers represents octaves plus or
minus from this:

MC-3 = 32 Hz
MC-2 = 64 Hz
MC-1 = 128 Hz
Middle C = 256 Hz
Mc+1 = 512 Hz
Mc+2 = 1024 Hz
Mc+3 = 2048 Hz
Mc+4 = 4096 Hz

Most good audio equipment will cover the frequency range from 30-50 Hz up
to 15-20 kHz, with some equipment going higher in order  to resolve all
overtones.  

** Note that the 60 Hz buzz from power lines will also be present in the
form of its harmonics (or overtones), at 30 Hz, 120 Hz, 180 Hz, 240 Hz, 300
Hz, 360 Hz, 480 Hz, and so on, with decreasing power on increasingly higher
frequencies.  Thus, being near power lines (even low-voltage lines) can
cause severe interference even in the VLF range.  

**  Steve McGreevy's hand-held receivers monitored the frequency range from
100 Hz to 11,000 Hz, straddling the ELF/VLF bands.  Unfortunately, he has
recently discontinued their sale, and is in the process of yanking most of
the web pages that he had up on the subject.  Thus, some of the links that
I previously listed here will no longer work.  

However, a crude, but workable ELF/VLF receiver can be constructed using a
good quality audio frequency or phonograph amplifier, connected to a
reasonably matched antenna and a good ground.  The "reasonably matched"
antenna is the more difficult part, since the wavelengths involved are on
the order of several kilometers.  One fair, but not great, solution is to
utilize a ferrite wound matching inductor designed for AM radio reception,
attached to an external long-wire antenna.  McGreevy uses some very long
loop antennae (several hundred feet of wire) for his van-based fancier
receivers.

The other catch 22 is that in order to power most phonograph and audio
amplifiers you need -- you guessed  it -- a 60 Hz power source, which comes
from local power lines, which greatly interfere with the desired signals.
The best solution to this is a 12 volt automobile system, designed to
amplify the audio signal from a cassette deck or other analog
audio-frequency source.  The primary problem with using any audio amplifier
is that these usually expect their input signals to have already undergone
some small amplification in the input device feeding it.  This is why
phonograph amplifiers  seem to be preferred, because they are designed for
a smaller input signal than tape amplifiers.  

Having said all of this, I have yet to develop such a system of my own, and
so have no experience in this area to point to, other than some light
reading.  I am sure that there are better recommendations to be found out
there, which might supply more practical solutions.  The more important
components would be:

1.  DC power supply.

2.  long-wire or loop antenna, reasonably matched to some fraction of
100,000 meters (3000 Hz, about mid-band).

3.  a good ground for the receiver, preferably a grounding stake.

4.  an impedance matching device between antenna and first stage amplifier
(may or may not be necessary).

5.  a sensitive first stage audio frequency amplifier, connected with the
necessary drivers for a speaker output, headphone output, or tape device
input.


6.  A remote location, about 1/2 mile or so away from standard power lines,
and *miles* away from high-tension lines.

Take care,

     Jim



James Richardson
Tallahassee, Florida
richardson@digitalexp.com

Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.amsmeteors.org

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