Do magnetars emit audible LW AM radio waves that can be heard on receivers?

In rec.radio.amateur.space Radium wrote:
On Jul 14, 1:17 am, "George Dishman" wrote:
"Radium" wrote in message

oups.com...
..

Isn't it true that the carrier-frequency must be at least 2x the
highest intended frequency of the modulator signal?

No.

Karl Uppiano sharply disagrees.

Karl Uppiano explained in http://groups.google.com/group/sci.e...cea47a5?hl=en&
:

The highest modulating frequency for AM must be less than 1/2 the carrier
frequency. Conversely, the lowest carrier frequency must be twice the
highest modulating frequency. Period. I don't care what specific frequencies
and/or energies and/or colors you propose.

If you want to modulate at 20KHz, the carrier must be at least 40KHz. It is
no coincidence that CD audio uses a 44.1KHz sample rate. It is essentially
the same principle. If you exceed the Nyquist criterion, the sidebands
overlap the baseband (i.e., aliasing occurs) and you cannot unambiguously
decode the original modulation.

So who is right and who is wrong? I am so interested yet so frustrated
over this!

I keep getting conflicting answers about this topic. Its driving me
crazy!!!!!!!!!!!!!!!!!

WTF is going on here??????????????????!!!!!!!!!!!!!!!!!!!!?!?!?!?! ?!

WTF is going on is that you can't ask a meaningful question.

Here's some reality:

Q: Do extra terrestrial objects generate radio signals that can be heard
on Earth?

A. Yes, thousands and thousands of them. The field is called Radio
Astronomy. Google for more information.

Q: What frequency do they generate?

A: Basically, all of them. Most natural sources of RF are broad band
generators much like an electric arc.

Q: Where would one listen for signal?

A: Usually from around 1 GHz to hundreds of GHz. Some objects in the
solar system generate signals down into the tens of MHz but antenna
size and the ionosphere place a practical lower limit of around 100 MHz.

Q: Is the signal AM or FM or what?

A: None of the above. Modulation implies a carrier with information.
Natural objects generate broad band RF noise.

Q: Don't some of the sources vary in some way?

A: Some of them vary in magnitude over time, i.e. they get louder
and weaker periodically. Some sources are "bursty", i.e. most of
the time the are not there, then for some period of time they are.

Q: Do magnetars generate signals?

A: Some do, some don't seem to.

Q: Are these signals audible?

A: Depends on what you mean. If you hooked a speaker to a radio telescope,
you would hear white noise, i.e. a hissing sound much like what you
hear on an FM radio between stations.




--
Jim Pennino

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"Radium" wrote in message
oups.com...
On Jul 14, 1:17 am, "George Dishman" wrote:
"Radium" wrote in message

oups.com...
..

Isn't it true that the carrier-frequency must be at least 2x the
highest intended frequency of the modulator signal?

No.

Karl Uppiano sharply disagrees.

Karl Uppiano explained in
http://groups.google.com/group/sci.e...cea47a5?hl=en&

He is wrong. The basis of AM is that the sine wave
carrier is multiplied by another signal which can be
treated as a sum of sines. The relevant maths is:

http://www.sosmath.com/trig/prodform/prodform.html

If the carrier frequency if fc and the modulation has
frequencies up to fm then you get sidebands like
this:

http://en.wikipedia.org/wiki/Image:Am-sidebands.png

If you multiply 44.1kHz by a band from 20Hz to 20kHz,
you get an upper sideband given 44.12kHz to 64.1kHz
and a lower sideband from 44.08kHz down to 24.1kHz

The highest modulating frequency for AM must be less than 1/2 the carrier
frequency. Conversely, the lowest carrier frequency must be twice the
highest modulating frequency. Period. I don't care what specific
frequencies
and/or energies and/or colors you propose.

If you want to modulate at 20KHz, the carrier must be at least 40KHz. It
is
no coincidence that CD audio uses a 44.1KHz sample rate. It is
essentially
the same principle. If you exceed the Nyquist criterion, the sidebands
overlap the baseband (i.e., aliasing occurs) and you cannot unambiguously
decode the original modulation.

Nyquist applies to sampling.

So who is right and who is wrong?

Look at the maths, it is never wrong. Modulating fc
with fm gives a lowest frequency of fc-fm so as long
as fc fm, you don't get aliasing.

George

On Sat, 14 Jul 2007 23:36:08 GMT, Sam Wormley
wrote:

Amplitude modulation, in the communications world, has a definite
structure--I suspect that magnetar spectra don't exhibit amplitude
modulation characteristics.

A magnetar is a type of pulsar. You have a signal at some frequency (or
range frequencies) that varies in amplitude with time (as the object
spins). That's the very definition of amplitude modulation. Nearly every
radio source around shows some degree of amplitude modulation as the
result of spin. This includes objects radiating well out of the radio
band, as well. Optical binaries exhibit AM. Starspots show up as AM.
Rotating asteroids are AM. Cepheids. Cataclysmic variables. Etc.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

In rec.radio.amateur.space Sam Wormley wrote:
Chris L Peterson wrote:
On Fri, 13 Jul 2007 20:04:13 GMT, Sam Wormley
wrote:

Why would you expect a star to generate amplitude modulation?

Why would you expect it to generate anything else? These sorts of
objects are rotating at high speed, which modulates the amplitude we
receive. While there are probably other types of modulation as well, the
amplitude variation is the dominant effect. Of course, magnetars are
emitting mainly hard x-rays. I don't know that there's enough long
wavelength energy to detect on any kind of ordinary radio.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


Amplitude modulation, in the communications world, has a definite
structure--I suspect that magnetar spectra don't exhibit amplitude
modulation characteristics.

However, I see, that amplitude modulation is appropriate in
astrophysics--for example...

Double Mode Cepheids with Amplitude Modulation
http://sait.oat.ts.astro.it/MSAIt770.....77..563M.pdf

The definitions for all types of modulation involve a carrier frequency.

Since natural phenomena generate broad band noise, it is arm waving
at best to call the variations in amplitude "amplitude modulation".

What it is is a broad band source that periodically varies in signal
strength.

--
Jim Pennino

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On Sun, 15 Jul 2007 00:55:01 GMT, wrote:

The definitions for all types of modulation involve a carrier frequency.

Since natural phenomena generate broad band noise, it is arm waving
at best to call the variations in amplitude "amplitude modulation".

What it is is a broad band source that periodically varies in signal
strength.

There's nothing that defines how narrow a band need be to qualify as a
"carrier". Many modern communication systems are spread spectrum, which
means the carrier may be very broad. Such systems are certainly
modulated. Also, many astronomical sources are not broadband at all, but
radiate across a narrow spectrum.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Jul 14, 4:11 pm, "George Dishman" wrote:

Look at the maths, it is never wrong. Modulating fc
with fm gives a lowest frequency of fc-fm so as long
as fc fm, you don't get aliasing.

So an fm of 10 KHz would work on an fc of 10 KHz?

In rec.radio.amateur.space Chris L Peterson wrote:
On Sun, 15 Jul 2007 00:55:01 GMT, wrote:

The definitions for all types of modulation involve a carrier frequency.

Since natural phenomena generate broad band noise, it is arm waving
at best to call the variations in amplitude "amplitude modulation".

What it is is a broad band source that periodically varies in signal
strength.

There's nothing that defines how narrow a band need be to qualify as a
"carrier". Many modern communication systems are spread spectrum, which
means the carrier may be very broad. Such systems are certainly
modulated. Also, many astronomical sources are not broadband at all, but
radiate across a narrow spectrum.

Spread spectrum technology uses discrete frequency hopping, not a
broad band signal as a carrier.

If I have a transmitter hooked to an antenna swaying in the breeze
such that the received signal strength is varying, would you call
that AM?

If the side lobes of a search radar are big enough, you can receive
them no matter where the radar points. The signal strength goes up
and down and goes up dramatically when you are swept by the main
lobe. Would you call that AM?

--
Jim Pennino

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In rec.radio.amateur.space Radium wrote:
On Jul 14, 4:11 pm, "George Dishman" wrote:

Look at the maths, it is never wrong. Modulating fc
with fm gives a lowest frequency of fc-fm so as long
as fc fm, you don't get aliasing.

So an fm of 10 KHz would work on an fc of 10 KHz?

What part of "as long as fc is greater that fm" are you too blazingly
stupid to understand?

Is 10 KHz bigger than 10 KHz?

Idiot.

--
Jim Pennino

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On Sun, 15 Jul 2007 03:45:01 GMT, wrote:

Spread spectrum technology uses discrete frequency hopping, not a
broad band signal as a carrier.

That's one spread spectrum method. Not the only one. But regardless, it
still presents as a broad band carrier.

If I have a transmitter hooked to an antenna swaying in the breeze
such that the received signal strength is varying, would you call
that AM?

Absolutely.

If the side lobes of a search radar are big enough, you can receive
them no matter where the radar points. The signal strength goes up
and down and goes up dramatically when you are swept by the main
lobe. Would you call that AM?

AM is a variation in amplitude of some signal- any signal- with time.
Yes, your radar is a type of amplitude modulation. If it were a deep
space signal, it is the modulation of amplitude that would most catch
our attention.

Certainly AM carries a somewhat narrower meaning when discussing
communications that it does when looking at astronomical signals. But
the same underlying theory works for analyzing any signal that varies in
intensity with time.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

In rec.radio.amateur.space Chris L Peterson wrote:
On Sun, 15 Jul 2007 03:45:01 GMT, wrote:

Spread spectrum technology uses discrete frequency hopping, not a
broad band signal as a carrier.

That's one spread spectrum method. Not the only one. But regardless, it
still presents as a broad band carrier.

Nope, all spread spectrum is based on discrete frequencies with
frequency hopping of some sort.

It is only "broad band" if you integrate over multiple hops.

The carrier at each frequency is quite conventional.

Google for it.

If I have a transmitter hooked to an antenna swaying in the breeze
such that the received signal strength is varying, would you call
that AM?

Absolutely.

If the side lobes of a search radar are big enough, you can receive
them no matter where the radar points. The signal strength goes up
and down and goes up dramatically when you are swept by the main
lobe. Would you call that AM?

AM is a variation in amplitude of some signal- any signal- with time.

Nope, mathematically AM is defined as a single carrier frequency
multipled by the modulation frequency. That you get a variation in
amplitude is an effect, not a definition.

It is a bit of a stretch to call a signal comprised of every frequency
over a 100 GHz span AM.

Yes, your radar is a type of amplitude modulation. If it were a deep
space signal, it is the modulation of amplitude that would most catch
our attention.

Certainly AM carries a somewhat narrower meaning when discussing
communications that it does when looking at astronomical signals. But
the same underlying theory works for analyzing any signal that varies in
intensity with time.

I'm afraid my background IS communications so I have to say astronomers
are arm waving when they call astronomical signals AM unless ET is
phoning home.

--
Jim Pennino

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