A short 160M antenna
 

Wimpie wrote in :

There is nothing wrong with a jump to another branch of science (for
example astrophysics) to find out whether the statement is right or
wrong.


I agree with that. It looks like a question of scale, not an absolute. I
don't know enough to say much, so I haven't, but if this is like relativistic
effects in that it is real, but extremely insignificant on the scales
presented for discussion, then arguing about it is surely proportional in its
significance.

A short 160M antenna
 

El 09-11-14 19:40, Lostgallifreyan escribió:
wrote in :

What Maxwell's Equations say is a moving magnet produces a moving magnetic
field.

A moving magnetic field is not an electromagnetic field.


Never mind Maxwell, why is no-one asking "why is an electromagnet not used as
an antenna when driven by AC?" That should be a basic reality check because
I've neve heard of such a thing working, and if it did it would be widely
known.

You may know that an AC current through a loop does generate near
fields, and far (EM radiation) fields. This follows from Maxwell's
equations. The solution for radiated power from a loop carrying AC
current is:

Prad = 3.85*10^-30*(A*I)^2*f^4 [W]

The product of A (Loop surface area) and I (rms current through loop)
is the magnetic moment (m). m is used in formulas frequently.

The formula contains f^4, when you halve the frequency, the radiation
drops to 6.25%. This is the reason that at low frequency the EM
radiation is negligible in most cases. This behavior is also the
reason that you need large antennas to produce low frequency EM
radiation fields with useful efficiency.

If you take two vertically oriented loops that are perpendicular to
eachoter, and feed them 90 degrees out of phase, you create a
rotating/spinning magnetic field around the Z-axis. Because of the
orientation of the two coils, they don't interact with eachother. So
by using two oscillating magnetic dipoles, you can generate a rotating
magnetic dipole.

The summation of the two fields (our rotating field), generate a
vertically polarized EM radiation field in the XY-plane. This is like
in a normal loop antenna, but now the pattern is omnidirectional
(because of two loops instead of one). In positive and negative Z
direction there is a pure circular field.

How you generate the rotating magnetic field doesn't matter. Instead
of using two out-of-phase fed loops, I can generate exactly the same
rotating field by using a rotating permanent magnet with its N-S
direction in the horizontal plane. I can also use a rotating coil fed
with DC current. Due to mechanical limitations, you can't generate
high rev/s (that is the frequency). Because of the f^4 behavior EM
radiation is negligible in practical mechanical systems involving
permanent magnets.

In the "AC fed two coils case" energy is delivered by the sources
providing the current, in the "rotating permanent magnets case" the
energy is provided by the drive mechanism.


--
Wim
PA3DJS
Please remove abc first in case of PM

A short 160M antenna
 

El 11-11-14 12:43, Lostgallifreyan escribió:
wrote in :

There is nothing wrong with a jump to another branch of science (for
example astrophysics) to find out whether the statement is right or
wrong.


I agree with that. It looks like a question of scale, not an absolute. I
don't know enough to say much, so I haven't, but if this is like relativistic
effects in that it is real, but extremely insignificant on the scales
presented for discussion, then arguing about it is surely proportional in its
significance.

You are right, it is a matter of scale and especially rotation
frequency, but you don't need relativistic velocities to make it
happen. I posted an explanation based on two loops in the same topic.

--
Wim
PA3DJS
Please remove abc first in case of PM

A short 160M antenna
 

On 11/11/2014 7:00 AM, Wimpie wrote:
El 09-11-14 19:40, Lostgallifreyan escribió:
wrote in :

What Maxwell's Equations say is a moving magnet produces a moving
magnetic
field.

A moving magnetic field is not an electromagnetic field.


Never mind Maxwell, why is no-one asking "why is an electromagnet not
used as
an antenna when driven by AC?" That should be a basic reality check
because
I've neve heard of such a thing working, and if it did it would be widely
known.

You may know that an AC current through a loop does generate near
fields, and far (EM radiation) fields. This follows from Maxwell's
equations. The solution for radiated power from a loop carrying AC
current is:

Prad = 3.85*10^-30*(A*I)^2*f^4 [W]

The product of A (Loop surface area) and I (rms current through loop) is
the magnetic moment (m). m is used in formulas frequently.

The formula contains f^4, when you halve the frequency, the radiation
drops to 6.25%. This is the reason that at low frequency the EM
radiation is negligible in most cases. This behavior is also the reason
that you need large antennas to produce low frequency EM radiation
fields with useful efficiency.

If you take two vertically oriented loops that are perpendicular to
eachoter, and feed them 90 degrees out of phase, you create a
rotating/spinning magnetic field around the Z-axis. Because of the
orientation of the two coils, they don't interact with eachother. So by
using two oscillating magnetic dipoles, you can generate a rotating
magnetic dipole.

The summation of the two fields (our rotating field), generate a
vertically polarized EM radiation field in the XY-plane. This is like in
a normal loop antenna, but now the pattern is omnidirectional (because
of two loops instead of one). In positive and negative Z direction there
is a pure circular field.

How you generate the rotating magnetic field doesn't matter. Instead of
using two out-of-phase fed loops, I can generate exactly the same
rotating field by using a rotating permanent magnet with its N-S
direction in the horizontal plane. I can also use a rotating coil fed
with DC current. Due to mechanical limitations, you can't generate
high rev/s (that is the frequency). Because of the f^4 behavior EM
radiation is negligible in practical mechanical systems involving
permanent magnets.

In the "AC fed two coils case" energy is delivered by the sources
providing the current, in the "rotating permanent magnets case" the
energy is provided by the drive mechanism.

Excellent explanation, Wim. Many thanks for that. I learn a lot from you.

More commentary on short antennae
 

"Wimpie" wrote in message
...
The formula contains f^4, when you halve the frequency, the radiation
drops to 6.25%. This is the reason that at low frequency the EM radiation
is negligible in most cases. This behavior is also the reason that you
need large antennas to produce low frequency EM radiation fields with
useful efficiency.

Watch out for the redneck troll lurking beneath the rickety-rackety bridge!

A short 160M antenna
 

Wimpie wrote in :

Never mind Maxwell, why is no-one asking "why is an electromagnet not
used as an antenna when driven by AC?" That should be a basic reality
check because I've neve heard of such a thing working, and if it did it
would be widely known.

You may know that an AC current through a loop does generate near
fields, and far (EM radiation) fields. This follows from Maxwell's
equations. The solution for radiated power from a loop carrying AC
current is:


I did know, but I didn't think it through. :) What I had in mind was the kind
of electromagnet usually used to simulate a magnet, one with a big
lump magnetic material to guid the flux and concentrate it, etc. I imagine
the moment you do that, you get closer to a normal magnet, but it probably
makes the coil unusable as an antenna at the same time.

The maths is beyond my easy grasp, but I do have one thought... Is this one
reason an Oudin coil is using a high frequency? (High-ish... many tens of
KHz, compared to usual mains AC frequency.) They're often used to start
erratic ion laser tubes, to avoid damage to glass and seals by arcs. I'm
wondering if the combination of extreme proximity and higher frequency than
mains-AC coils might be using EM radiation to help penetrate the tube safely
in ways an arc could not, without damage, and in ways a low frequency coild
could not at all, with or without the magnetic material as a core. If this is
the case, then an Oudin coil might be a rare practical case for EM from
something far laser to a magnet than an antenna. I'm just guessing here, but
interested enough to ask.

I like the quadrature pair of loops idea. I should have thought of that at
least. I have explored quadrature generation and circular distributions in
audio, and forgot to make the connection somehow.

A short 160M antenna
 

Lostgallifreyan wrote in
:

something far laser to a magnet than an antenna

"...far CLOSER to a magnet..."
Sorry, I saw the typo in the main post before sending, then 'corrected'
something even more wrong... Can't see, been busy for 6 hours elsewhere, very
bad eyestrain today.

A short 160M antenna
 

On 11/11/2014 2:42 PM, Lostgallifreyan wrote:
Wimpie wrote in :

Never mind Maxwell, why is no-one asking "why is an electromagnet not
used as an antenna when driven by AC?" That should be a basic reality
check because I've neve heard of such a thing working, and if it did it
would be widely known.

You may know that an AC current through a loop does generate near
fields, and far (EM radiation) fields. This follows from Maxwell's
equations. The solution for radiated power from a loop carrying AC
current is:


I did know, but I didn't think it through. :) What I had in mind was the kind
of electromagnet usually used to simulate a magnet, one with a big
lump magnetic material to guid the flux and concentrate it, etc. I imagine
the moment you do that, you get closer to a normal magnet, but it probably
makes the coil unusable as an antenna at the same time.

I think there are a lot of things you don't think through, lol. Ever
hear of a ferrite core antenna? Not much different. Why would an iron
core make a poor antenna?

--

Rick

A short 160M antenna
 

rickman wrote in :

I think there are a lot of things you don't think through, lol. Ever
hear of a ferrite core antenna? Not much different. Why would an iron
core make a poor antenna?


Fair enough. I'll plead diminished responsibility though. :) I repaired an
amp during 5 hours at a house yesterday where the guy smokes. I don't, but
passive smoking does my head in sometimes. I fixed the amp just fine, but by
the time I got back here my brain was mush.

A short 160M antenna
 

On 11/12/2014 5:43 AM, Lostgallifreyan wrote:
rickman wrote in :

I think there are a lot of things you don't think through, lol. Ever
hear of a ferrite core antenna? Not much different. Why would an iron
core make a poor antenna?


Fair enough. I'll plead diminished responsibility though. :) I repaired an
amp during 5 hours at a house yesterday where the guy smokes. I don't, but
passive smoking does my head in sometimes. I fixed the amp just fine, but by
the time I got back here my brain was mush.

Most of the posts in this thread are jim and Gareth arguing so I'm going
to killfile it. If you have anything else to say I won't see it unless
you start a new thread.

--

Rick