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Old December 23rd 03, 12:39 AM
dclapp
 
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Default Magnetic field and loop antennas

I've read that loop receiving antennas capture more of the magnetic
field -- and less of the electrical field (true?) -- than other forms
of antennas. Why is that?? Thanks in advance for any enlightenment!

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Old December 23rd 03, 08:06 AM
Reg Edwards
 
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There is only one field electromagnetic field.

It has two components, electric and magnetic.

One component cannot exist without the other.

As the field travels through space at the velocity of light, one component
generates the other, and vice-versa.


The magnetic field, as it sweeps by at high speed, induces a current in a
short wire bent into the form of a loop.


The electric field, as it sweeps by, induces a voltage between the ends of a
short straight length of wire (a dipole).


On a long length of wire of any shape, an appreciable fraction of a
wavelength or longer, both appreciable currents and voltages will exist.


To sum up, a small loop is most sensitive to the magnetic component as in an
electricity generator, and a straight wire with its ends is most sensitive
to the electric component. But both effects MUST act together. They cannot
be considered in isolation. As in any other electrical circuit Amps and
Volts are always related by a resistance which depends on the length and
shape of the wire.

And there are many more ways of describing it.
---
Reg.


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Old December 23rd 03, 12:51 PM
Maarten Hagg
 
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Hello group,

This is ONLY true for 'far field' sources.
In the Far Field ( 1 wavelenght) according to maxwell's law
the E-field and H-field are mutual dependent.
(natural impedance is : 120*Pi=377 Ohm)
In the Near Field ( 1 wavelenght) this is NOT true.
The natural impedance drops for H-field and raises for E-field.
This is why loop antenna's are less sensitive for local interference,
like light dimmers, TV sets ect.

Maarten Hagg
the Netherlands


"Reg Edwards" wrote in message
...
There is only one field electromagnetic field.

It has two components, electric and magnetic.

One component cannot exist without the other.

As the field travels through space at the velocity of light, one component
generates the other, and vice-versa.


The magnetic field, as it sweeps by at high speed, induces a current in a
short wire bent into the form of a loop.


The electric field, as it sweeps by, induces a voltage between the ends of

a
short straight length of wire (a dipole).


On a long length of wire of any shape, an appreciable fraction of a
wavelength or longer, both appreciable currents and voltages will exist.


To sum up, a small loop is most sensitive to the magnetic component as in

an
electricity generator, and a straight wire with its ends is most sensitive
to the electric component. But both effects MUST act together. They

cannot
be considered in isolation. As in any other electrical circuit Amps and
Volts are always related by a resistance which depends on the length and
shape of the wire.

And there are many more ways of describing it.
---
Reg.




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Old December 23rd 03, 02:48 PM
Tom Bruhns
 
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If you ask "why" deeply enough, we'll get to a point where we have to
say, "Because that's just the way it is." But a bit more explanation
to what Reg wrote... If the loop is balanced and oriented properly,
the voltages induced by the electric field will be equal amplitude and
the same polarity, or very nearly so, and cancel out at the feedpoint,
but the EMF induced in the loop by the magnetic component of the
electromagnetic field will be there in full force. Look up "Faraday's
Law of Magnetic Induction." One advantage of a small loop at low
frequencies is that local "noise" sources are commonly predominantly
electric-field...they have not fully developed at that distance into
electromagnetic fields, and boundary conditions constrain them to be
predominantly vertical near the ground, so if the loop is properly
balanced and oriented in that field, it can reject much of the local
"electrical" noise. There are also possibilities for using the
directional "nulls" of the loop to find the direction of an incoming
signal, if the loop is small, and there are advantages there, too, in
having it respond to the magnetic component only. You can find more
complete explanations in antenna books...for instance, King, Mimno and
Wing, "Transmission Lines, Antennas and Waveguides."

Cheers,
Tom

(dclapp) wrote in message . com...
I've read that loop receiving antennas capture more of the magnetic
field -- and less of the electrical field (true?) -- than other forms
of antennas. Why is that?? Thanks in advance for any enlightenment!

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Old December 23rd 03, 04:35 PM
Maarten Hagg
 
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The way Tom tells us is a good way to tell it.

And for those who wonder are there any on the market?
There will be one soon .

See URL : http://home.hetnet.nl/~maartenmiriam/index.html
For a full colour brochure, drop me an email.

Maarten


"Tom Bruhns" wrote in message
m...
If you ask "why" deeply enough, we'll get to a point where we have to
say, "Because that's just the way it is." But a bit more explanation
to what Reg wrote... If the loop is balanced and oriented properly,
the voltages induced by the electric field will be equal amplitude and
the same polarity, or very nearly so, and cancel out at the feedpoint,
but the EMF induced in the loop by the magnetic component of the
electromagnetic field will be there in full force. Look up "Faraday's
Law of Magnetic Induction." One advantage of a small loop at low
frequencies is that local "noise" sources are commonly predominantly
electric-field...they have not fully developed at that distance into
electromagnetic fields, and boundary conditions constrain them to be
predominantly vertical near the ground, so if the loop is properly
balanced and oriented in that field, it can reject much of the local
"electrical" noise. There are also possibilities for using the
directional "nulls" of the loop to find the direction of an incoming
signal, if the loop is small, and there are advantages there, too, in
having it respond to the magnetic component only. You can find more
complete explanations in antenna books...for instance, King, Mimno and
Wing, "Transmission Lines, Antennas and Waveguides."

Cheers,
Tom

(dclapp) wrote in message

. com...
I've read that loop receiving antennas capture more of the magnetic
field -- and less of the electrical field (true?) -- than other forms
of antennas. Why is that?? Thanks in advance for any enlightenment!





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Old December 24th 03, 10:11 PM
Joe Strain
 
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DIDNT SEE ANYTHING IN SPRING 2003 so I thought the product was vaporware. I
am a prospect, but I am a SWL and dont need & wont pay for a product set up
for transmitters

The Wellbrook LA5030 is on my wish list but I don't think it's in my budget.
Notice how cleverly they avoid stating COST?


=============================

"Maarten Hagg" wrote in message
...
The way Tom tells us is a good way to tell it.

And for those who wonder are there any on the market?
There will be one soon .

See URL :
http://home.hetnet.nl/~maartenmiriam/index.html
For a full colour brochure, drop me an email.

Maarten



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Old December 30th 03, 03:45 AM
Chris Trask
 
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"Joe Strain" wrote:


DIDNT SEE ANYTHING IN SPRING 2003 so I thought the product was
vaporware. I am a prospect, but I am a SWL and dont need & wont
pay for a product set up for transmitters


You might want to look at my 2-part series on active receiving loop
antennas that appeared in QEX earlier this year.

Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo\ Sonoran Radio Research
(__)\ _ P.O. Box 25240
\ \ .' `. Tempe, Arizona 85285-5240
\ \ / \
\ '" \ IEEE Member #40274515
. ( ) \
'-| )__| :. \ Email:
| | | | \ '.
http://www.home.earthlink.net/~christrask
c__; c__; '-..'.__

Graphics by Loek Frederiks










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