On 2 mar, 13:58, "J.B. Wood" wrote:
On 02/23/2011 10:00 AM, RadioWaves wrote: Today I have put my homepage online with information about the Magnetic Loop
Antenna.

http://www.qsl.net/pa7nr/

PA7NR

Hmmm. *A "magnetic" loop antenna. *Must be some other types of loop
antennas as well. *Maybe there are also "electric" loop antennas.

Guess they left something out of all those antenna textbooks I have ;-)
Sincerely, and 73s from N4GGO,

--
J. B. Wood * * * * * * * * *e-mail:

Hello John,

When you cut the loop at two opposite positions, yes, you can make
your "electric" loop.

It will generate lots of E-field, you may need another coil for
matching, and it is probably less efficient then a short straight
dipole with massive capacitive disks to get larger I*delta(le)
product.

Best regards,


Wim
PA3DJS
www.tetech.nl
In case of PM, please remove abc first.

On 3/2/2011 8:40 AM, Wimpie wrote:
On 2 mar, 13:58, "J.B. wrote:
On 02/23/2011 10:00 AM, RadioWaves wrote: Today I have put my homepage online with information about the Magnetic Loop
Antenna.

http://www.qsl.net/pa7nr/

PA7NR

Hmmm. A "magnetic" loop antenna. Must be some other types of loop
antennas as well. Maybe there are also "electric" loop antennas.

Guess they left something out of all those antenna textbooks I have ;-)
Sincerely, and 73s from N4GGO,

--
J. B. Wood e-mail:

Hello John,

When you cut the loop at two opposite positions, yes, you can make
your "electric" loop.

It will generate lots of E-field, you may need another coil for
matching, and it is probably less efficient then a short straight
dipole with massive capacitive disks to get larger I*delta(le)
product.

Best regards,


Wim
PA3DJS
www.tetech.nl
In case of PM, please remove abc first.

Hello, and the not-so-subtle point is that there aren't magnetic,
electric, or any other such "types" of loop antennas. There are just
loop antennas that can further be described as shielded/unshielded,
balanced/unbalanced, electrically small or large. Just like we don't
transmit (propagate) electric (E) or magnetic (H) fields by themselves.

The purpose of an antenna is to radiate and/or intercept an
electromagnetic field. By definition energy radiated by a transmitting
antenna is not temporarily stored in the antenna's local electric or
magnetic field. It's been released into free space subject to
interception by a receiving antenna(s) or any other parasitic
structures. The receiving antenna transfers part the intercepted energy
to the load (receiver and other dissipative losses) and scatters the
rest back into free space.

By contrast, a transformer, for example, is a "magnetic" device that is
intended to transfer energy by a localized means (induction) other than
the propagation/interception of electromagnetic radiation.

To further confuse the issue, a conductor in the near (reactive) field
of a transmitting antenna will have current induced in it by the
antenna's local electric and/or magnetic fields. However, that's not
the usual purpose for which we design antennas. An exception might be
the immoboliser (PATS) system used in late-model motor vehicles that
incorporates a ring antenna embedded in the steering column that is
closely coupled at RF frequencies to the transponder chip and loop
antenna embedded in the vehicle ignition key. So is it a
transmit-receive antenna configuration or a primary coil-secondary coil
transformer configuration? Given the proximity of the inserted key to
the steering column I would guess the latter. Sincerely,

--
J. B. Wood e-mail:

On 3/2/2011 12:14 PM, J.B. Wood wrote:
On 3/2/2011 8:40 AM, Wimpie wrote:


To further confuse the issue, a conductor in the near (reactive) field
of a transmitting antenna will have current induced in it by the
antenna's local electric and/or magnetic fields. However, that's not the
usual purpose for which we design antennas.

Hello, all. I should also add that in stating the above I was only
considering nearby conductors (towers, metal on buildings, etc) and
wasn't including the local directors/reflectors that may be incorporated
into an antenna to provide the desired radiation pattern
characteristics. Sincerely,


--
J. B. Wood e-mail:

Hello John,

On 2 mar, 18:14, "J.B. Wood" wrote:
On 3/2/2011 8:40 AM, Wimpie wrote:



On 2 mar, 13:58, "J.B. *wrote:
On 02/23/2011 10:00 AM, RadioWaves wrote: *Today I have put my homepage online with information about the Magnetic Loop
Antenna.

http://www.qsl.net/pa7nr/

PA7NR

Hmmm. *A "magnetic" loop antenna. *Must be some other types of loop
antennas as well. *Maybe there are also "electric" loop antennas.

Guess they left something out of all those antenna textbooks I have ;-)
Sincerely, and 73s from N4GGO,

--
J. B. Wood * * * * * * * * *e-mail:

Hello John,

When you cut the loop at two opposite positions, yes, you can make
your "electric" loop.

It will generate lots of E-field, you may need another coil for
matching, and it is probably less efficient then a short straight
dipole with massive capacitive disks to get larger I*delta(le)
product.

Best regards,

Wim
PA3DJS
www.tetech.nl
In case of PM, please remove abc first.

Hello, and the not-so-subtle point is that there aren't magnetic,
electric, or any other such "types" of loop antennas. *There are just
loop antennas that can further be described as shielded/unshielded,
balanced/unbalanced, electrically small or large. *Just like we don't
transmit (propagate) electric (E) or magnetic (H) fields by themselves.

The purpose of an antenna is to radiate and/or intercept an
electromagnetic field. *By definition energy radiated by a transmitting
antenna is not temporarily stored in the antenna's local electric or
magnetic field. *It's been released into free space subject to
interception by a receiving antenna(s) or any other parasitic
structures. *The receiving antenna transfers part the intercepted energy
to the load (receiver and other dissipative losses) and scatters the
rest back into free space.

By contrast, a transformer, for example, is a "magnetic" device that is
intended to transfer energy by a localized means (induction) other than
the propagation/interception of electromagnetic radiation.

If in your opinion there do not exist antennas that generate a
dominant magnetic or electric field (in the near field), then you are
contradicting yourself, as you can't transfer energy with a magnetic
field or electric field only. So your transformer also involves
electric fields. Maybe you should look into the Poynting theorem.

To further confuse the issue, a conductor in the near (reactive) field
of a transmitting antenna will have current induced in it by the
antenna's local electric and/or magnetic fields. *However, that's not
the usual purpose for which we design antennas. *An exception might be
the immoboliser (PATS) system used in late-model motor vehicles that
incorporates a ring antenna embedded in the steering column that is
closely coupled at RF frequencies to the transponder chip and loop
antenna embedded in the vehicle ignition key. *So is it a
transmit-receive antenna configuration or a primary coil-secondary coil
transformer configuration? *Given the proximity of the inserted key to
the steering column I would guess the latter. *Sincerely,

--
J. B. Wood * * * * * * * * *e-mail:

When a noise source is about 5..10m away from an 3.6 MHz antenna, the
coupling of that noise source towards a "magnetic" loop antenna may be
different from the coupling towards an "electric" antenna, though
both antennas may produce the same far field radiation. This is not
from a textbook, but from experience (I am also working in power
electronics).

I fully agree with you on the far field statements, but when you live
in an apartment (where significant spurious emission from home
equipment are in the near field of your 3.6 MHz antenna), a so-called
magnetic loop antenna may behave different (w.r.t. a short "electric"
dipole). It can be worse or better. Many radio amateurs know this from
experiments, without knowing the EM theory behind it.

I have no problems when people talk about a "magnetic loop antenna".
It shows me that they are discussing an antenna with a circumference
0.2 lambda. When people talk about a "loop antenna", it can be
anything.

Best regards,


Wim
PA3DJS
www.tetech.nl

On Wed, 2 Mar 2011 12:56:23 -0800 (PST), Wimpie
wrote:

When a noise source is about 5..10m away from an 3.6 MHz antenna, the
coupling of that noise source towards a "magnetic" loop antenna may be
different from the coupling towards an "electric" antenna, though
both antennas may produce the same far field radiation. This is not
from a textbook, but from experience (I am also working in power
electronics).

Text books would enlarge that volume to one half to several
wavelengths for the "near field." The text books would further
clarify this with math (yes, I know, professional and academic
discussion in light of this being an amateur forum is anathema) and
define the difference with the terms Fresnel diffraction (near-field)
and Fraunhofer diffraction (far-field). The operative physical length
of the antenna becomes meaningful, but this is getting ahead of what I
call the "benchmark" method below.

To give the magnetic loop aficionados the benefit of this, all local
noise within 100 feet would be susceptible to interfering and it
wouldn't be nullable (which is a characteristic only observed in the
far-field) except by polarization which is very haphazard in the
near-field. I have never seen a magnetic loop mount with the
necessary degrees of freedom to employ this method of "nulling." As
such, the vaunted characteristic is elusive and thus becomes legendary
rather than fulfilled.

However, the term "near-field" is rather vague. The more appropriate
discussion is found in "reactive near field" and "radiative near
field." The discussion of loop coupling to magnetic (while ignoring
electric) fields would suggest "reactive near field." In this regard,
the 80M volume of reactive interference is still roughly 100 feet in
all directions. The "radiative near field" would encompass a volume
out to 80 meters (roughly 250 feet). In either case, apartment living
finds no panacea in loop antennas.

There is another, non-textual (at least to the casual reader),
benchmark that such issues are measured by the physical spread of the
antenna itself (this usually attends discussion of capture area to
many's frustration). Here, I am returning to the allusion above of
Fresnel diffraction (near-field) and Fraunhofer diffraction
(far-field). The math (non-techs, turn your eyes away) is as simple
as:
2·D²/lambda

Let's work some examples from the sublime to the ridiculous on 80M.

The traditional half-wave dipole antenna that exhibits the traditional
usage for distinguishing between near and far:
2·40²/80 = 40 meters
a smaller quarter-wave dipole antenna
2·20²/80 = 10 meters
a tenth wave dipole antenna
2·8²/80 = 1.6 meters
a fortieth wave dipole antenna
2·2²/80 = 10 centimeters

Let's see where discussion follows in this regard.

73's
Richard Clark, KB7QHC

On 03/02/2011 03:56 PM, Wimpie wrote:

If in your opinion there do not exist antennas that generate a
dominant magnetic or electric field (in the near field), then you are
contradicting yourself, as you can't transfer energy with a magnetic
field or electric field only. So your transformer also involves
electric fields. Maybe you should look into the Poynting theorem.

Hello, and that is correct. The Maxwell equations apply in all these
cases. When solving such problems, especially when dealing with
antennas, the total E-M field contains both reactive
(electric/capacitive & magnetic/inductive) and radiative components,
although certain components predominate depending on distance from the
excited structure.

When dealing with A.C. circuit problems where dimensions are a fraction
of a wavelength, one can usually ignore the the radiative/propagation
components. Why solve a problem with a sledgehammer when a small claw
hammer is adequate? Wouldn't you rather use Ohm's law in such case
rather than dealing with E and H fields? For example, the behavior of
A.C. power power distribution lines operating a 60 Hz can certainly be
modeled using transmission line equations but unless they're very long
(implying a propagation delay), a lumped-element/circuit approach is
much more easily dealt with (lumped lines. And yes, I'm intimately
familiar with the Poynting theorem and its derivation. (The designers of
the CFA obviously weren't).

When a noise source is about 5..10m away from an 3.6 MHz antenna, the
coupling of that noise source towards a "magnetic" loop antenna may be
different from the coupling towards an "electric" antenna, though
both antennas may produce the same far field radiation. This is not
from a textbook, but from experience (I am also working in power
electronics).

There's no such thing as "magnetic" and "electric" antennas. The
marketing departments of antenna vendors or others can call these
whatever they want but they can't change the laws of physics. Now, if
one dimensions a loop antenna or dipole antenna small enough (compared
to a wavelength, one obtains a magnetic or electric dipole,
respectively. Such dipoles (note the absence of the word "antenna") are
a theoretical concept but can be applied in practice to those structures
having electrically small radiators/interceptors.

I fully agree with you on the far field statements, but when you live
in an apartment (where significant spurious emission from home
equipment are in the near field of your 3.6 MHz antenna), a so-called
magnetic loop antenna may behave different (w.r.t. a short "electric"
dipole). It can be worse or better. Many radio amateurs know this from
experiments, without knowing the EM theory behind it.

Hey, I'm a fellow Ham and well aware of the contributions over the years
by hams to antenna design. Many times, however, established
electromagnetic theory is distorted to match the perceived observation.
In the case of noise immunity I would discuss the size of the victim
antenna, the antenna type (e.g. loop or dipole), antenna dimensions,
orientation and proximity wrt the offending noise source, and whether
the victim antenna is shielded and balanced. Unless one is referring to
an electrically small antenna treated as a magnetic or electric dipole,
typing an antenna as "magnetic" or "electric" is meaningless.

--
J. B. Wood e-mail: