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--- In loopantennas@..., Barry Savage <sofistic@s...> wrote:

Hello All:
For some time now, I have been wondering about the electromagnetic

properties of common materials, and find these things rather hard to
discover. The few times I have asked about them on different
electronics-oriented groups, most people have dismissed them as not
workable. I don't want to get too off-topic, but I hope some of you
can point me in a productive direction. I also wanted to ask some
rather nieve questions that I hope someone can either answer or point
me in the right direction. So, here goes.

QUESTIONS:
1. Would it be possible to make a VLF loop-stick out of rebar?

First thing I can think of is that the remnance of rebar would be a
problem, but nowhere have I been able to find the magnetic properties
of such a readily available and cheap material. What about its
permeability, its coercivity? What does its B/H curve look like?

The problem is that it is conductive, and so you'd have I2R losses in
the induced current within the rebar. That is why cores are made out
of nonconductive ferrite, and transformer cores are made of thin
sheets of iron insulated from each other by layers of laquer.

If you look up old projects like building your own Model T spark coil,
or building one of those ring launchers with a bundle of welding rods,
note that they all require you to paint the rods to insulate them from
each other.

So- how about welding rods? Get a pound of the smallest diameter you
can find. Carefully paint them, then bundle them when dry.

2. If a "square" coil is considered optimal, why is an air-core

loop antenna so far out of square? I know aperture is an issue here,
but what are the trade-offs in terms of Q and lack of symmetry between
diameter and coil-depth?

I thought a round coil was optimal. It's just easier to build a square
coil. For the same circumference, a round coil has a larger area
enclosed than any other shape.

BTW, some ham in the state of Washington (don't have the link

offhand, but I can look it up if anyone is interested) has a
spreadsheet called "professor Coyle" that is very good for modeling
coils (both solenoid and spiderweb). I have modified it to include
wire resistance tables and a few other bells and whistles.

Where? Let us see it!

HARE BRAINED IDEAS:
1. What if you built a "steerable null" antenna using two identical

loop antennas at the end of a rotatable beam, and, in turn (heh) each
of the loops were also rotatable? Something Bear said awhile back
triggered this idea off, and I remembered experimenting with a crystal
set from Gollum's Crystal Radio site that used two spider-web coils,
one for the antenna and one for the crystal detector. The coils were
coupled (face to face) at a distance of about 6-inches, but I could
get very responsive results even at a distance of 12-inches apart, so
I played around with rotating one with respect to another and found
the nulling very sharp. I haven't done any measurement of this
arrangement, so this is only an intuitive observation.

BTW, I also built a "box" loop using a wooden "in-basket" I got at a

Michaels craft store, and used the ears of the in-box to mount it on a
base (also from Michaels) so it became an altazimuth loop. It works
very well with my little Radio Shack DX-370 at a distance of about
6-inches. The bottom of the in-box is used as the face of the loop,
the edge of the box is where the coil is wound, and a little poly
variable cap is mounted at the center of the "bottom" of the in-box.

2. Is a helical considered a loop? What if you built a large

helical antenna and treated it as if it were a loop antenna? I mean,
a helical is just a streached-out loop, isn't it? The only difference
I can see is that the distributed capacitance drops and so does the
inter-winding inductance. Humm, what am I missing here?

I seem to recall that someone (QRPBear?) tested this idea with loops
of the same diameter and turns. And found that for an air core, it
made little difference.

The problem is that the further each turn is from the next, the less
they are coupled inductively to each other. So a loop spread out into
a helix may lose efficiency. Hopefully someone with more theory and
experience under their belt can tell us.

Some spread of the turns will reduce interwinding capacitance, but you
quickly reach a point of diminishing returns.

Of course if it is on a ferrite core, that is different. How exactly I
can't say. :'/

3. A few times on this list someone has mentioned mobius-strip

loops; what is the theory behind them and how well do they work? What
are their properties in a vertical and horizontal plane of
orientation? (oops, that was a question, not a HBI)

Mobius strip loops? Hmm... I'm curious too. I can see an easy way to
"wind" one. Use the method of using a ribbon cable- only give it one
turn before you offset and solder. No, that won't work, I just gave it
a mental try and you end up with a one turn loop with a bunch of two
turn shorted turns and a one turn shorted turn. Does the same if you
don't offset. So what is a mobius loop antenna?

Steve Greenfield

Steve et Barry,

I vaguely recall seeing somewhere a description of an ELF loop(stick)
antenna using rebar as the core. Mebbe on <>. I
don't remember for sure.

The helix antenna is a unidirectional antenna commonly used in arrays
of four or more for EME work. It has the same kind of high gain and
narrow beamwidth as long yagis but has circular polarization. Part of
the problem with EME is the return signal from the moon spins in the
opposite direction from the uplink. If you don't select the correct
spin polarity you don't get the downlink signal.

The helix is fed at one end against a screen reflector and the other
end is unconnected. Pitch (spacing between turns) defines the gain
and beamwidth. Since the whole length is active there is no need to
have a decreasing radius unlike a yagi where only the driven element
is active and the directors decrease in length the further they are
from the driven element.

Re the mobius loop. Actually it is not a mobius. If you take a strip
of paper formed into a loop and let it lay on the table you will see
that it roughly defines two loops in a figure 8 when given an
additional half twist. One of the regular Antennex contributors is
working on a loop like this with their planes at 90 degrees to each
other. Seems to me that it could also be done with the planes in
line. Coupling via open-wire or 300 ohm twinlead attached to each
mobius half at the crossover point. It may be, though, that phase
cancellation could occur with the planes in line. Where to insert the
resonating capacitor is a question that remains to be answered as
does whether each half of the mobius should have its own capacitor.
Now that I think of it, if such a creature was fed with a balanced
transmission line the tuning point could be the ATU and no capacitors
required at the antenna.

The other idea I plan to try when I can get the materials and time is
a dual stacked loop sharing a common element... something like a 2:1
rectangular shape with a central shared member creating two 1:1
rectangles. Each square with its own capacitor opposite the common
member and transformer coupled with the usual 1/5th circumference
coupling loop adjacent to the common member or a 50 ohm gamma match
on the central member. The primary idea here being to double the
effective loop area while simultaneously creating a double-tuned
bandpass circuit for increased bandwidth. This would be for a fixed-
tuned transmitting loop for the 30-meter band which is 50 KHz wide.
I'm thinking in terms of a structure that would be about 3' wide and
6' tall. Still fairly compact even for indoor use.

Gotta take a shower and be off to work... hopefully without the
headache I have right now. See y'all in 8 hours or so.

73,

'Bear' NH7SR

--- In loopantennas@..., "Steve" <alienrelics@y...> wrote:

--- In loopantennas@..., Barry Savage <sofistic@s...>

wrote:

Hello All:
For some time now, I have been wondering about the electromagnetic

properties of common materials, and find these things rather hard to
discover. The few times I have asked about them on different
electronics-oriented groups, most people have dismissed them as not
workable. I don't want to get too off-topic, but I hope some of you
can point me in a productive direction. I also wanted to ask some
rather nieve questions that I hope someone can either answer or

point

me in the right direction. So, here goes.

QUESTIONS:
1. Would it be possible to make a VLF loop-stick out of rebar?

First thing I can think of is that the remnance of rebar would be a
problem, but nowhere have I been able to find the magnetic

properties

of such a readily available and cheap material. What about its
permeability, its coercivity? What does its B/H curve look like?

The problem is that it is conductive, and so you'd have I2R losses

in

the induced current within the rebar. That is why cores are made out
of nonconductive ferrite, and transformer cores are made of thin
sheets of iron insulated from each other by layers of laquer.

If you look up old projects like building your own Model T spark

coil,

or building one of those ring launchers with a bundle of welding

rods,

note that they all require you to paint the rods to insulate them

from

each other.

So- how about welding rods? Get a pound of the smallest diameter you
can find. Carefully paint them, then bundle them when dry.

2. If a "square" coil is considered optimal, why is an air-core

loop antenna so far out of square? I know aperture is an issue

here,

but what are the trade-offs in terms of Q and lack of symmetry

between

diameter and coil-depth?

I thought a round coil was optimal. It's just easier to build a

square

coil. For the same circumference, a round coil has a larger area
enclosed than any other shape.

BTW, some ham in the state of Washington (don't have the link

offhand, but I can look it up if anyone is interested) has a
spreadsheet called "professor Coyle" that is very good for modeling
coils (both solenoid and spiderweb). I have modified it to include
wire resistance tables and a few other bells and whistles.

Where? Let us see it!

HARE BRAINED IDEAS:
1. What if you built a "steerable null" antenna using two

identical

loop antennas at the end of a rotatable beam, and, in turn (heh)

each

of the loops were also rotatable? Something Bear said awhile back
triggered this idea off, and I remembered experimenting with a

crystal

set from Gollum's Crystal Radio site that used two spider-web coils,
one for the antenna and one for the crystal detector. The coils

were

coupled (face to face) at a distance of about 6-inches, but I could
get very responsive results even at a distance of 12-inches apart,

so

I played around with rotating one with respect to another and found
the nulling very sharp. I haven't done any measurement of this
arrangement, so this is only an intuitive observation.

BTW, I also built a "box" loop using a wooden "in-basket" I got

at a

Michaels craft store, and used the ears of the in-box to mount it

on a

base (also from Michaels) so it became an altazimuth loop. It works
very well with my little Radio Shack DX-370 at a distance of about
6-inches. The bottom of the in-box is used as the face of the

loop,

the edge of the box is where the coil is wound, and a little poly
variable cap is mounted at the center of the "bottom" of the in-box.

2. Is a helical considered a loop? What if you built a large

helical antenna and treated it as if it were a loop antenna? I

mean,

a helical is just a streached-out loop, isn't it? The only

difference

I can see is that the distributed capacitance drops and so does the
inter-winding inductance. Humm, what am I missing here?

I seem to recall that someone (QRPBear?) tested this idea with loops
of the same diameter and turns. And found that for an air core, it
made little difference.

The problem is that the further each turn is from the next, the less
they are coupled inductively to each other. So a loop spread out

into

a helix may lose efficiency. Hopefully someone with more theory and
experience under their belt can tell us.

Some spread of the turns will reduce interwinding capacitance, but

you

quickly reach a point of diminishing returns.

Of course if it is on a ferrite core, that is different. How

exactly I

can't say. :'/

3. A few times on this list someone has mentioned mobius-strip

loops; what is the theory behind them and how well do they work?

What

are their properties in a vertical and horizontal plane of
orientation? (oops, that was a question, not a HBI)

Mobius strip loops? Hmm... I'm curious too. I can see an easy way to
"wind" one. Use the method of using a ribbon cable- only give it one
turn before you offset and solder. No, that won't work, I just gave

it

a mental try and you end up with a one turn loop with a bunch of two
turn shorted turns and a one turn shorted turn. Does the same if you
don't offset. So what is a mobius loop antenna?

Steve Greenfield

Barry,

Hmmm... does anyone have a good source for Mu-metal at all?

It's been in _very_ short supply ever since Mu sank beneath the
waves!

<GD&R>

But doesn't Mu-metal tend to resist flux changes??

'Bear' NH7SR

--- In loopantennas@..., Barry Savage <sofistic@s...>
wrote:

Bear, et al:

Here is something I found on the Public Seismic Network archives.

"New article in the May '96 issue of Scientific American (pg 98)

describes

construction of VLF antenna. Their plans use a 24" piece of rebar

for

the core of the coil. Anyone have a good source for Mu-metal cores

in the

2' range???"

Guess I will look up the SA archives & see what is there.
Barry