Howdy antenna aficionados!

Wanna FIGHT?

Here is another misconception propagated by "guru" W8JI, like "there is no
electrostatic shielding, the grounded piece of tubing (shield) IS the
antenna."

Let the games begin! (I will hold my horses for a while :-)

As posted on reflector:


Subject:
"The June, 2006 QST has an article about a 160m RX loop.
Comments, advice welcome."
73,
Tim K3HX


Tim, et al,
I just received the rag here. Very interesting this article, in light of
the many discussions on this reflector and antennas reflector,
concluding by our best experts that a shield contributes to NO USEFUL
function whatsoever in such a receiving loop! It just essentially
becomes electrically the loop itself; the shield's outer skin IS the
antenna. No S/N ratio advantage, no anything except adding to a very
critical balancing result. No noise discrimination, no magic. So what
gives with this mythological article?
73, Roy K6XK


W8JI wrote:

Roy and all,

It appears QST isn't as careful as they used to be years
ago.

It is a very well known physical property of a "shield" more
than several skin depths thick that essentially nothing goes
through that shield. It's a Faraday cage, and when the
time-varying electric field goes to zero so does the
magnetic field. This is explained in nearly every handbook
(even ARRL publications) and is the reason coaxial cables
have that "third path" on the outside of the shield for
common mode currents. This is why a bead balun or a coil of
coax works to stop common mode on the outside of the shield
and does not affect the stuff inside the cable.

Obviously any technical explanation that requires fields to
go through the shield isn't accurate.

It is also a very well known fact that radio waves are
electromagnetic, and the interference (unless in the
induction field area of less than a half wave or so) is a
radio wave, and like all radio waves it is neither magnetic
or electric. It has a fixed ratio or electric to magnetic
fields. It is also established that a very short distance
from a small loop the predominant field is electric, not
magnetic. Clearly a shield can't "filter" noise, since noise
is not field sensitive.

So what does the shield in a loop do? The shield is actually
the antenna element. The shield is actually what receives
(and/or transmits) the EM wave. NOT the wire inside the
shield. The wire inside the shield simply couples whatever
device is connected to the antenna (the loop's shield) to
the receiver.

If we have a poor coupling system to the loop, how we
configure the shield can certainly affect the balance of the
loop and the common mode current. Say we use an unbalanced
line or we use an unbalanced amplifier directly attached to
a loop. Now we have created a problem, the feedline feeding
the loop acts just like part of the antenna system. We have
an unbalanced feedline or amplifier tied to a balanced
antenna, and so every conductor going to that point acts
like part of the antenna system. We can couple noise and
signals picked up by the feedline shield and everything in
the house to the input of the receiver.

I go through this problem in detail at:

http://www.w8ji.com/magnetic_receiving_loops.htm

If you read carefully you'll see no has ever said the shield
and how the shield is connected won't change the system when
the system is not designed correctly. When the feedpoint is
done correctly, the presence or absence of a shield has no
effect at all on the system. All the stuff about the shield
"filtering" the fields and blocking the electric field is
nonsense. But some construction methods do result in better
loop balance.

The characteristics of an improperly done feed system are
affected by the construction of the loop, but that isn't
because the shield is necessary or that a solid shield
behaves differently than braided shield. It's because
something was more wrong with the construction in one system
compared to the other, not because of the shield quality.

73 Tom

Yuri Blanarovich wrote:
Howdy antenna aficionados!

Wanna FIGHT?

Here is another misconception propagated by "guru" W8JI, like "there is no
electrostatic shielding, the grounded piece of tubing (shield) IS the
antenna."

Let the games begin! (I will hold my horses for a while :-)

As posted on reflector:
. . .

I don't find anything incorrect with Tom's response. What did you find
in it that wasn't accurate?

Roy Lewallen, W7EL

Tom, this is well written and devoid of any antagonism towards anyone.
If anybody wants to dispute any point then all relavent data is in
place in your posting and thus
forces all who disagree to stay on subject without the need for
extraneous data when debating their differences. There will ofcourse,
be some that will be more interested in a fight or profanity in the
absence of comunicable knoweledge , but you are well positioned to just
stand by your posting without retaliating in kind.
Well done
Art

Tom, this is well written and devoid of any antagonism towards anyone.
If anybody wants to dispute any point then all relavent data is in
place in your posting and thus
forces all who disagree to stay on subject without the need for
extraneous data when debating their differences. There will ofcourse,
be some that will be more interested in a fight or profanity in the
absence of comunicable knoweledge , but you are well positioned to just
stand by your posting without retaliating in kind.
Well done
Art

"art" wrote in message
oups.com...
Tom, this is well written and devoid of any antagonism towards anyone.
If anybody wants to dispute any point then all relavent data is in
place in your posting and thus
forces all who disagree to stay on subject without the need for
extraneous data when debating their differences. There will ofcourse,
be some that will be more interested in a fight or profanity in the
absence of comunicable knoweledge , but you are well positioned to just
stand by your posting without retaliating in kind.
Well done
Art




Here is the exchange on the subject from TopBand reflector:
K3BU (...)
and W8JI responses:

Tom is confusing Faraday shield with Electrostatic shield
and whole reasoning that the grounded shield of small loop
antenna is THE antenna is all wrong. Wire loops inside the
electrostatic shield are perfectly OK to receive the RF
and ARE the antenna.

It's a very well known property that nothing passes through
the walls of a shield more than several skin depths thick.
This is because skin effect keeps the current in the outside
layers and the core of the shield wall is dead. This is the
very thing that allows our coaxial lines to behave like
three conductors, a center conductor, a inner wall, and an
outer conductor. The physical behavior of a shield does not
change with application.


Electrostatic shield in small loop antennas reduces the
interference, electrical noise locally generated (prevalent
electrical fields).

Not so Yuri.

First an electrostatic field by definition is a non-changing
field. Static is stationary or unchanging, and things that
aren't changing can't make RF noise.

(Here he is confused about electrostatic shield, "electrostatic field" and
electrical field and just like with loading coils case, confusing the issue
with behavior of ALL shields, Faraday, Electrostatic, coax, etc. applied to
a wrong case. - Yuri)

The field from an accidental transmitter (noise source) is
just like the field from any intentional signal source like
a transmitter. There is absolutely nothing that says the
field has a high field impedance (electric field dominant).
Even if it was a high impedance at the source, just 1/10th
wave or so from the source the field would change to a low
impedance.

We can't filter noise by virtue of field impedance or a
shield. Even if we could, the noise source in the nearfield
would randomly field dominant depending on distance and
source charateristics.

The only thing the shield can do at radio frequencies is
change the system balance.

73 Tom

Yuri,

It seems to me that when "W8JI" is associated with something, you
assume immediately that it is wrong. If you were to read Ronold W. P.
King's explanation about small loop antennas in "Transmission Lines,
Antennas and Waveguides", would you be any more apt to believe it? How
about Glenn S. Smith's discussion of them in Johnson and Jasik's
"Antenna Engineering Handbook" (second edition)? Each of those begins
with a reasonably detailed description of an "unshielded" loop and
moves on to a "shielded" loop.

In addition, can you expain to us how the current on the wires on the
inside of the shield is NOT balanced by an equal current in the
opposite direction on the inside surface of the shield? Please tell us
in detail just what currents are where on the shielded loop. If you
are going to try to tell us that some explanation is in error, please
provide us with enough detail that we can make up our own minds. So
far, all I've seen here is some vague reference to confusion about
shields.

The descriptions in each of the two references I gave above are far
more detailed than what you have posted here, either of your own or of
W8JI's, and I find them both enlightening--they are slightly different
from each other--but both detailed enough that you can make up your own
mind about what's really going on, and not have to read ranting
generalities or statements with nothing to back them up.

Cheers,
Tom

"K7ITM" wrote in message
oups.com...
Yuri,

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. I express my opinion based on what I know or believe. I
could be wrong and I gladly get educated. Mostly, if I see, measure or touch
something, I believe it to be right. Mumbo-jumbo "scientwific explanation",
taking off on tangent to justify the fallacy don't cut it with me.

If you were to read Ronold W. P.
King's explanation about small loop antennas in "Transmission Lines,
Antennas and Waveguides", would you be any more apt to believe it? How
about Glenn S. Smith's discussion of them in Johnson and Jasik's
"Antenna Engineering Handbook" (second edition)? Each of those begins
with a reasonably detailed description of an "unshielded" loop and
moves on to a "shielded" loop.


I don't have the King's book, in Jasik's the treatment of small loops and
shielded loops is dealing with some "medieval" designs. The closest to my
version is his Fig. 5.23a showing balanced shielded loop. But then the
5-23bdoesn't make much sense to me, having small loop on the front of
reflector, when the small loop has the minimum of radiation along the axis
through the loop, and he places the reflector in the minimum - null
direction? The way they show the loops, half of loop solid wire, half coax
line, creates confusion what is antenna, what is shield, or perhaps combines
them. I have not used those designs.

In addition, can you expain to us how the current on the wires on the
inside of the shield is NOT balanced by an equal current in the
opposite direction on the inside surface of the shield? Please tell us
in detail just what currents are where on the shielded loop. If you
are going to try to tell us that some explanation is in error, please
provide us with enough detail that we can make up our own minds. So
far, all I've seen here is some vague reference to confusion about
shields.

The descriptions in each of the two references I gave above are far
more detailed than what you have posted here, either of your own or of
W8JI's, and I find them both enlightening--they are slightly different
from each other--but both detailed enough that you can make up your own
mind about what's really going on, and not have to read ranting
generalities or statements with nothing to back them up.

Cheers,
Tom


I will not get tangled into currents, phasors, but describe my design of
small shielded loop antenna that I used on 160m and this should perhaps shed
some light on the controversy.

I used 1/2" copper water tubing (non ferrous material passing the magnetic
field) for circular loop about 4 foot diameter. At the top the loop had gap,
at the bottom it was mounted in small metallic box. Loop, box and mast were
all DC connected and grounded. Mast was about 5 ft high, with Ham-m rotor at
the base to rotate the contraption. This formed Electrostatic shield for the
antenna.

From the connection box I threaded three turns of electrical house wire #12
and across the ends connected mica trimmer capacitor C1 (abt 1200 pF?) to
resonate the three wire loop antenna at 1.830 kHz). Not connected to
anything else, nor ground or loop.
Then I threaded one turn of the same #12 wire as a coupling turn. One end
was connected to the coax braid, the other end through another mica trimmer
capacitor C2 to the center conductor of the coax. Floating, not grounded or
connected to other loop or tubing.
I tuned the C1 to resonate the three turns at the desired frequency and C2
to provide 50 ohm match to coax. Circuit wise this mirrors the LC parallel
tuned circuit with link coupling and provide better signal than other
published designs.
I tried version of this without copper tubing shield and with. I had local
AC power line noise (within fractions of wavelength) and shielded loop
attenuated the local noise.
The way I see this works, the three turns were the antenna, it was tunable
across the band. The "link" coupling allowed to keep the symmetry of antenna
and provided some isolation for common mode currents between the antenna and
coupling (well known in LC tuned circuit with link coupling.). The copper
tubing was ELECTROSTATIC SHIELD which let's the EM waves pass through.
If the copper tubing IS the antenna, then how does it work? Short, grounded
in the center bent dipole? Then the radiation pattern should have maximum
perpendicular to the plane of the loop/dipole. But the antenna has NULLS in
that direction, corresponding to the properties of the 3 plus 1 wire loops.
You scientwists can play games with theories how it should behave, but the
reality again shows how it behaves. Anyone can build the antenna as I
described and VERIFY it. Wire loops without electrostatic shield tubing
still work the same way as with the shield. So which IS antenna?

Another description of the subject antenna is at
http://www.tpub.com/content/antennaa...-352-14_31.htm

73 Yuri Blanarovich, K3BU, VE3BMV

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.

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.

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

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

Nice try Yuri, but you can see no one is buying into your theories.

A shield works the way a shield works. The time-varying fields, once
the frequency is high enough so the shield is several skin depths
thick, really isolates everything from passing through the shield.

The primary coupling is via the voltage across the gap and the current
flowing around that edge.

The shield is the actual antenna. Not the conductor inside the shield.

Noise is NOT any particular field impedance. There is nothing that says
noise is electric field dominant at the radiator, and if it was just a
few feet away (about 1/10th wave) it would change anyway.

If you disagree with how a shield works or if you think electrical
noise is a high field impedance or electric field dominant, then you
should say why or how that is true.

73 Tom