Yuri Blanarovich wrote:

"K7ITM" wrote:
It seems to me that when "W8JI" is associated with something, you
assume immediately that it is wrong.

That's what might seem to you, but I point out gross misinformation, when I
come across it.

What gets missed quite often in these discussions is that
everyone agrees on 99 44/100 percent of the technical issues
and we tend not to discuss those issues. We only discuss the
56/100 percent of the issues upon which we disagree. It is
akin to the arguments between Einstein and Bohr. I suspect
that no two people here on r.r.a.a are in 100% agreement
on everything.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Yuri Blanarovich wrote:

"K7ITM" wrote:
It seems to me that when "W8JI" is associated with something, you
assume immediately that it is wrong.

That's what might seem to you, but I point out gross misinformation, when I
come across it.

What gets missed quite often in these discussions is that
everyone agrees on 99 44/100 percent of the technical issues
and we tend not to discuss those issues. We only discuss the
56/100 percent of the issues upon which we disagree.

Make no mistake about it, I disagree with everything Yuri has posted
about the physics behind a "shielded loop".

I certainly don't want to be considered to be 99% in agreement with
anyone who thinks a time-varying magnetic field can pass though a
highly conductive copper wall, or any wall that is several skin depths
thick, just as I don't want to be 99% in agreement with anyone who
thinks a loading coil "replaces" or has the phase shift or "current
drop" of missing electrical degrees.

The basic physical properties have to be understood before I'd be
largely in agreement. If basic building blocks are wrong, our idea of
how the worlds works must also be very distorted.

73 Tom

wrote:
Cecil Moore wrote:
What gets missed quite often in these discussions is that
everyone agrees on 99 44/100 percent of the technical issues
and we tend not to discuss those issues. We only discuss the
56/100 percent of the issues upon which we disagree.

Make no mistake about it, I disagree with everything Yuri has posted
about the physics behind a "shielded loop".

That's part of the 56/100 percent of the issues upon which you
disagree.

... I don't want to be 99% in agreement with anyone who
thinks a loading coil "replaces" or has the phase shift or "current
drop" of missing electrical degrees.

Just proves that no individual is right 100% of the time. 75m
bugcatcher coils obey the laws of physics and thus suffer a
delay in the real world. Hint: Contrary to the lumped circuit
model, *everything* has a delay in the real world. The only place
coils don't have a delay is in your mind where miracles and
magic are possible.

The loading coil causes a phase shift in accordance with the
laws of physics. The stinger causes a phase shift in accordance
with the laws of physics. The impedance discontinuity between
the coil and stinger causes a phase shift in accordance with
the laws of physics. There are NO missing degrees.

When you comprehend how an electrical 1/4WL stub can be 19 degrees
of 450 ohm line plus 18 degrees of 50 ohm line and be physically
0.1 WL long, then you will have comprehended reality.
--
73, Cecil http://www.qsl.net/w5dxp

Roy wrote, "It's not hard to demonstrate that the (time-varying)
magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that
a time-varying magnetic field will produce a current on a nearby
conductor."

Yes, it's all easy to demonstrate. It's used in practice all the time:
the shielding in a transmitter, the aluminum shield cans around IF and
RF coils, the copper strap around a power transformer (used
specifically to lower the external magnetic field around the
transformer, so it won't couple into low-level audio circuits or affect
colors on a color CRT).

And indeed it all agrees with theory. For this one, you need little
more than Faraday's Law of Magnetic Induction.

It's fine with me if there are people who don't want to be bothered
with theory, but if they profess that something works by means
different from the theory that I understand and which agrees with the
observations I make, they shouldn't expect me to believe them without
putting some very serious effort into explaining why the accepted
theory is wrong. I believe Yuri when he tells me his antenna works.
But I'm not buying into his explanation of HOW it works.

Cheers,
Tom

wrote:

The experiment I did years ago used a copper wall in a one or two foot
square thin sheet.

I made a link on one side through a high power 50 ohm load resistor.
This allowed me to run significant known current through the resistor
(and link) from a regular HF transmitter.

Using two probes, one a sensitive low voltage dial light bulb with two
prongs soldered on and the other a multiple turn loop with diode/meter
indicator, I could roughly check current in multiple places on both
sides of the sheet.

It was very easy to see where the magnetic field (and current) goes. It
does not pass through the wall. It's quite amazing to see how even a
direct soldered connection on one side will not produce current (and a
magnetic field) on the other side.

I encourage Yuri to add this experiment to the ones I suggested, or just
do this one. Understanding what happens to the currents and fields in
the presence of a shield is essential to understanding "shielded" loop
operation, and his statement about magnetic fields and non-ferrous
shields shows that he lacks this understanding. Any explanation based on
the mistaken idea that a non-ferrous shield is transparent to a magnetic
field is bound to be seriously flawed.

Roy Lewallen, W7EL

"K7ITM" wrote in message
ups.com...
Roy wrote, "It's not hard to demonstrate that the (time-varying)
magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that
a time-varying magnetic field will produce a current on a nearby
conductor."

Yes, it's all easy to demonstrate. It's used in practice all the time:
the shielding in a transmitter, the aluminum shield cans around IF and
RF coils, the copper strap around a power transformer (used
specifically to lower the external magnetic field around the
transformer, so it won't couple into low-level audio circuits or affect
colors on a color CRT).

And indeed it all agrees with theory. For this one, you need little
more than Faraday's Law of Magnetic Induction.

It's fine with me if there are people who don't want to be bothered
with theory, but if they profess that something works by means
different from the theory that I understand and which agrees with the
observations I make, they shouldn't expect me to believe them without
putting some very serious effort into explaining why the accepted
theory is wrong. I believe Yuri when he tells me his antenna works.
But I'm not buying into his explanation of HOW it works.

Cheers,
Tom

I am not selling explanations how it works.
I understand your and Roy's points. I am not claiming to try to formulate
the infinitesimal theory of wasaaaap and I didn't try that with loading
coils. Ensuing discussions helped me to better understand the mechanaism of
how things work, the theory and how can I better apply them. I thank you for
that.

What I have problem with someone claiming shield is not a shield (Why do
they bother calling it shield or shielded loop?), when I saw the shielding
properties of it in the vicinity of the local interfering signals. It
performs as a shield to the antenna that is wound inside. Tom categorically
denies SHIELD, it IS the ANTENNA he claims. (Like there is no current drop
along the loading coil! - The gospel from the all-knowing guru.)
What I have problem with someone claiming the small loop antenna (three plus
one turn) is not the antenna, but when I remove the shield, the "not
antenna" is still THE ANTENNA.

I am not arguing the mechanics or theory behind how the shield works, it may
be transparency to magnetic field, it may be the voltage generated in the
gap, bla, bla...
Based on my experience with the said antenna, I concluded that wire loops
are THE antenna, shield works as an electrostatic shield.

I know that if I stick oscillator inside of 10' of 1/2" tubing, I will get
hardly or no signal out. I know if I bend that tubing into a circle with gap
and stick wire loop antenna inside, I can get signals out of that "shielded"
antenna and can attenuate close by interfering signals. Shielding doesn't
MAKE my antenna work (it works without shield too), shield enhances its
rejection/shielding properties in near fields.
I know there are small loops and there are small shielded loops and they
work and I have proved it.

Just don't tell me it is called shield because it is antenna, or that
antenna inside the shield doesn't work, or shield doesn't shield from
electrostatic fields, or that my antenna I described doesn't work as I
described.

Tom may pontificate his ideas to his worshippers, but I don't swallow that.
I point out my, and who else cares, disagreement, especially when I see his
"ideas" migrating into ham literature.

Go ahead with your but, but, butts.....

73
Yuri Blanarovich, K3BU, VE3BMV

Cecil,
Now we have special case of biiiig coils being antenna here.
Let the games begin!

Yuri

"Cecil Moore" wrote to W8JI:

When you comprehend how an electrical 1/4WL stub can be 19 degrees
of 450 ohm line plus 18 degrees of 50 ohm line and be physically
0.1 WL long, then you will have comprehended reality.
--
73, Cecil http://www.qsl.net/w5dxp

"Roy Lewallen" wrote in message
...
I haven't gone through this in detail yet, but one misconception is
glaring:

Yuri Blanarovich wrote:
. . .
I used 1/2" copper water tubing (non ferrous material passing the
magnetic field) for circular loop about 4 foot diameter. . .

If you believe that, it's no surprise that you're having difficulty
understanding how a shielded loop works.

It's not hard to demonstrate that the (time-varying) magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that a
time-varying magnetic field will produce a current on a nearby conductor.
Simply put an oscillator or signal source into a copper box -- you can
solder one op out of PC board material. Run some wires all around the
inside which carry the oscillator signal, putting them as close to the
shield wall as you like. Put a battery inside the box to power the
oscillator and seal the box up. Then sniff around the outside of the box
with any kind of magnetic field detector you can devise. If you have a
little potted oscillator of some kind, you should be able to do this in a
couple of hours at most.

That is called Faraday shield and does not function as Electrostatic shield.

Or, just connect your rig to a good dummy load with some double shielded
coax and sniff around the outside of the copper coax shield. If you put
the detector just outside the shield, the current on the inside of the
shield will be much closer to the detector than the current on the center
conductor. So if the shield is transparent to a magnetic field, your
detector should go wild. (Make sure the rig is very well shielded, though,
so no common mode currents make their way from the rig to the outside of
the shield.)

Alternatively, if you'll spend some time with a good electromagnetics text
learning about eddy currents and the like, you'll understand why you'd be
wasting your time with those experiments.

I learned about shieldings, Faradyas, I use them, in equipment design, in RF
and harmonics suppression, I built shielded room for university. But I also
know the difference between the Farady shield and Electrostatic shield and
seen them work. Maybe lumping all shields is as no good as lumping all coils
ain't no good?

Once you're convinced that the shield blocks the magnetic as well as
electric field, you'll have to revise your theory on how a shielded loop
works. And you'll find that Tom's explanation is correct.

Roy Lewallen, W7EL

Roy,
I have magnetothermia machine which is about 200 W push-pull power generator
at around 27 MHz. It uses single turn, shielded loop, made of coax, about 30
inch in circumference. Loop wire, antenna (center conductor of coax) is fed
from the plates of two tubes, shield is open at the far end and grounded at
the exit from the enclosure. I get those 200 W heating my body tissue with
magnetic field. Maybe it has something to do with shielding being a fraction
of a wavelength distance from the radiator and the properties of the
magnetic and electric components in the antenna reactive near field region?

I know that this loop radiates along its circumference, not just from the
gap in the shield. What's yer theory? Or it don't (ooops, can't) woyk?

You seem to associate and stick to wrongos and I am sorry you find their
explanations correct, for the reality proves them wrong.

73 Yuri Blanarovich, K3BU

Yuri,

Just once, and I'm done with this.

Someone somewhere along the line mistakenly called it a shield. They
didn't understand how it works and what's important. Get over it.
Look at just the "shield" with no wires inside. Isn't that exactly a
single turn loop antenna? Isn't the feedpoint the gap in the loop?

If you put a wire (or several wires) through the inside of that tube
you used to call the shield, they just pick up the signal from the
feedpoint. Consider a single wire through the tube. There is a
voltage across the gap, the feedpoint of the loop. Since there is
essentially no voltage drop along the wire in the center, across the
distance of the gap in the tube, then the voltage across the gap must
appear as transmission line voltage across the coaxial feedlines which
are made up of the wire and the inner surface of the tube. If you've
arranged things symmetrically, then the total gap voltage will divide
equally between the two. Then it's just standard coaxial lines from
there to where you connect your receiver, or where you put a tuned
tank.

Or if you have multiple wires through the tube, the net transmission
line current divides among them. And you can resonate them with a
capacitor, but that doesn't make them have antenna currents on them.

If you have another way to analyze it accurately, fine. I don't care.
My way works for me, and it does not disagree with the _performance_
I've seen you post about. It does disagree with the _theory_ you've
suggested.

As for WHY adding the "shield" helps get rid of local e-field noise
(from sources less than a few wavelengths away, which at VLF might be
kilometers), and why the nulls are more perfect, it's because symmetry
is CRITICAL for that performance, and adding the outside tube allows
you to make a more perfectly symmetrical loop than you can practically
accomplish with just wires and all the tuning stuff you hang off it.
If you are VERY careful to keep things symmetrical, you can also do it
without the tube. But it takes amazingly little imbalance to screw
things up.

Dat's it in a nutshell.

Cheers,
Tom

Yuri Blanarovich wrote:
. . .
Tom may pontificate his ideas to his worshippers, but I don't swallow that.
I point out my, and who else cares, disagreement, especially when I see his
"ideas" migrating into ham literature.
. . .

But Tom's explanation is correct. It's consistent with theory; alternate
explanations aren't. If you're really interested in learning how a
"shielded" loop works and won't accept Tom's explanation because it came
from Tom, you can find a similar explanation in a number of reputable
texts. I'll gladly provide references, if you ask before I leave for
Dayton. Once you gain an understanding of some basic electromagnetic
principles, the correctness of the explanation will be obvious.

Oh, and don't worry about Tom's ideas migrating into the literature.
They were already in the literature well before any of us were born.

Roy Lewallen, W7EL