christofire wrote:
Magnetic antennas are sometimes considered beneficial for mobile reception
of medium/lomg-wave signals because they can be made insensitive to electric
fields, ...

A magnetic antenna was used in all of the California
75m mobile antenna shootouts that I attended. I was
told it was to keep the close-by human bodies from
having an effect on the strength of the received signals.

Which leads me to a question: Most of us OFs have
witnessed the effects of human bodies on analog VHF
TV signals being received using rabbit ears. If we
used "magnetic rabbit ears", would the problem go
away? Is it only the electric field that varies when
an EM signal passes through a non-magnetic medium
like a human body - or a tree?
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
christofire wrote:
Magnetic antennas are sometimes considered beneficial for mobile
reception of medium/lomg-wave signals because they can be made
insensitive to electric fields, ...

A magnetic antenna was used in all of the California
75m mobile antenna shootouts that I attended. I was
told it was to keep the close-by human bodies from
having an effect on the strength of the received signals.

Which leads me to a question: Most of us OFs have
witnessed the effects of human bodies on analog VHF
TV signals being received using rabbit ears. If we
used "magnetic rabbit ears", would the problem go
away? Is it only the electric field that varies when
an EM signal passes through a non-magnetic medium
like a human body - or a tree?
--
73, Cecil http://www.w5dxp.com


I understand it becomes increasingly difficult to create a purely-magnetic
antenna as the frequency rises, and ferrite with the required properties
becomes progressively more expensive! Some VHF pagers used ferrite rods,
and one or two-turn coils. Screened one-turn loops are used in the
short-wave bands, by some amateurs as well as by the military (e.g. British
Royal Navy).

Chris

On Jan 21, 12:12*pm, "christofire" wrote:


I understand it becomes increasingly difficult to create a purely-magnetic
antenna as the frequency rises, and ferrite with the required properties
becomes progressively more expensive! *Some VHF pagers used ferrite rods,
and one or two-turn coils. *Screened one-turn loops are used in the
short-wave bands, by some amateurs as well as by the military (e.g. British
Royal Navy).

Chris

The way I look at it, there is no such thing as a "magnetic" antenna.
As an example, some call shielded single turn loops "magnetic"
antennas. They claim special properties such as lower noise reception.
But this is not the case. They receive the same s/n ratio as any other
single turn loop.
The only advantage the shield provides is inherently good balance.
Good balance improves the depth of the nulls.
But you can construct plain wire single turn loops to have just as
good balance if you use good construction.
I've side by side compared the two, and came to the conclusion
most of the theories about shielded or so called magnetic loops
to basically be a myth.

wrote:
The way I look at it, there is no such thing as a "magnetic" antenna.

Given that a transmitting dipole and a receiving dipole
transfer maximum signal when oriented in the same plane,
how does one explain a ferrite loop antenna receiving
maximum signal in a plane orthogonal to the transmitting
dipole?
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
wrote:
The way I look at it, there is no such thing as a "magnetic" antenna.

Given that a transmitting dipole and a receiving dipole
transfer maximum signal when oriented in the same plane,
how does one explain a ferrite loop antenna receiving
maximum signal in a plane orthogonal to the transmitting
dipole?
--
73, Cecil http://www.w5dxp.com


Ampere's circuital law and the well-known 'right-hand rule'.

Chris

On Jan 21, 3:11*pm, Cecil Moore wrote:
wrote:
The way I look at it, there is no such thing as a "magnetic" antenna.

Given that a transmitting dipole and a receiving dipole
transfer maximum signal when oriented in the same plane,
how does one explain a ferrite loop antenna receiving
maximum signal in a plane orthogonal to the transmitting
dipole?
--
73, Cecil *http://www.w5dxp.com

I'm not sure without looking into it, but I notice this with
both shielded, and unshielded loops.
As an example, my 44 inch per side 5 turn diamond
loop prefers to be fed horizontally vs vertically
when receiving MW stations which use a vertical
transmit antenna.
IE: I feed it at the middle lowest corner.
If I feed it at a side corner, which would be vertically fed,
I seem to remember it not working near as well.
Do you consider an open small loop "unshielded" a
"magnetic" antenna? Some do, but I tend not to.
They act the same as the shielded loops that many
seem to call "magnetic" antennas.
The ability to respond to mostly the magnetic field
vs the electric field only pertains to the very near field
within about 1/10 wavelength.
Within 1 wavelength they often respond more to the
electrical wave. In the far field they should respond
to both fields the same as any other antenna.
Or this is my current understanding anyway.. :/
So using any type of "magnetic" antenna for the OP's
purpose would seem to be a waste of time unless
they are trying to reduce noise pickup that is within
1/10 of a wavelength away.
I know myself that these small loops are still quite
capable of picking up local noise, just like most
any other antenna. The only advantage are the sharp
nulls which you use to get rid of said noise.
If your mobile tests were within 1/10 of a wave,
maybe it makes more sense. But I'm not sure if
I can see any advantage to trying to receive a far
field signal vs any other antenna unless the nulls
are useful.

On Wed, 21 Jan 2009 17:46:36 -0800 (PST), wrote:

IE: I feed it at the middle lowest corner.
If I feed it at a side corner, which would be vertically fed,
I seem to remember it not working near as well.

Hi Mark,

That would seem to indicate you seriously unbalanced the antenna by
the side feed which is geometrically unbalanced. For instance, the
line doesn't lead off horizontally for any great distance, does it?

Of course, this all hinges on what you mean by not "near as well."

73's
Richard Clark, KB7QHC

On Jan 21, 8:59*pm, Richard Clark wrote:


Hi Mark,

That would seem to indicate you seriously unbalanced the antenna by
the side feed which is geometrically unbalanced. *For instance, the
line doesn't lead off horizontally for any great distance, does it?

Of course, this all hinges on what you mean by not "near as well."

Not sure.. I don't the the position of the feed line should
have been a problem, but it's been so long since I tried it,
I forgot what happened. I just seem to remember trying it
one time, and stuck with the bottom feed.
I'll have to try it again later. I could use my circular 16 inch
dia loop. It's small enough I can easily hold it and shift the
polarization just by rotating it.

wrote:
On Jan 21, 3:11 pm, Cecil Moore wrote:
Given that a transmitting dipole and a receiving dipole
transfer maximum signal when oriented in the same plane,
how does one explain a ferrite loop antenna receiving
maximum signal in a plane orthogonal to the transmitting
dipole?

I'm not sure without looking into it, but I notice this with
both shielded, and unshielded loops.

I'm not talking about coaxial loops. I'm talking about
coils of wire wrapped around a ferrite rod typical of
AM radios. Seems pretty obvious it is responding to the
magnetic field when it needs to be at right angles to
the transmitting monopole (or dipole).
--
73, Cecil http://www.w5dxp.com

On Jan 22, 6:32*am, Cecil Moore wrote:


I'm not talking about coaxial loops. I'm talking about
coils of wire wrapped around a ferrite rod typical of
AM radios. Seems pretty obvious it is responding to the
magnetic field when it needs to be at right angles to
the transmitting monopole (or dipole).
--
73, Cecil *http://www.w5dxp.com

I don't see how you could be receiving the magnetic
field if say you are 150 miles away from the station.
I looked around on the web for other opinions, and
ran across a page from W8JI.
http://www.w8ji.com/magnetic_receiving_loops.htm
I tend to agree with him.
Here is one quote that seems to fairly well explain
the position.

From W8JI web page..
"Acceleration of charges causes a very unique force
on other charges in the Universe. We call this effect
electromagnetic radiation. It is a totally different effect,
and it is independent of induction fields. This is the only
effect or force that works to move charges at a very
large distance, and it cannot be created by mixing
induction fields. "

Anyway, that's about as good an explanation as I
can find as to why I don't believe in "magnetic" antennas,
except for the properties at very close distances.
At any greater distance beyond about 1/10 wavelength,
it all goes out the window.
That's the way I see it on January 22, 2009 at 7:55 in
the PM. :/