Hi All,

Info below from the following site----
http://www.ee.surrey.ac.uk/Workshop/...x.html#bhcurve

Unlike electrical conductivity, permeability is often a highly non-linear
quantity. Most coil design formulæ, however, pretend that it is a linear
quantity.

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

My question is-

If I wind a transformer using the specified A sub L and then use that
transformer in a receive antenna where the voltages are very small, wouldn't
I be low on the curve and cause the transformer to function poorly
especially at the lowest frequency of the design?

Mike

PS Thinking about a Flag antenna, which has a small output signal.

amdx wrote:
Hi All,

Info below from the following site----
http://www.ee.surrey.ac.uk/Workshop/...x.html#bhcurve

Unlike electrical conductivity, permeability is often a highly non-linear
quantity. Most coil design formulæ, however, pretend that it is a linear
quantity.

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

My question is-

If I wind a transformer using the specified A sub L and then use that
transformer in a receive antenna where the voltages are very small, wouldn't
I be low on the curve and cause the transformer to function poorly
especially at the lowest frequency of the design?

Mike

PS Thinking about a Flag antenna, which has a small output signal.

Al is usually the value for low flux density. That is, it's the value
you'll have when the flux level is low. Permeability will drop from
there at high flux levels.

If you're making a broadband (untuned) transformer, you only need to
insure that the winding impedance is high enough. If you design it to
have adequate impedance at the lowest frequency, you should be ok for
frequencies above that. If you're making a tuned transformer, you'll
probably be using either powdered iron core or a ferrite core with a big
air gap in the magnetic path like a ferrite rod. Either will withstand
many orders of magnitude of flux density above what a received signal
will produce before there's any noticeable change in permeability.

The assumption of constant permeability is often a reasonable one.
Change in permeability with flux density is certainly nothing you have
to worry about in a receiving application unless you've got a lot of
turns and a lot of DC current in the winding.

Roy Lewallen, W7EL

I agree with Roy that the linearity of coils wound on powdered iron
cores is unlikely to be a problem for you. A while back, I had
occasion to question if some filters we use, made with small powdered
iron toroid core inductors, were causing distortion, so I built up some
similar filters with air-core coils and very good capacitors that I
knew would not distort. The result was "no change". That corresponds
in this case to third order intercepts in excess of +50dBm, which would
be considered at least pretty good by all but the fanatics for use in
receivers. I don't know how much in excess of +50dB, because that was
about the limit of what I could see in that test. Also, I know that
the broadband transformers used in the best H-mode mixers have allowed
those mixers to perform at similarly high--and higher--third order
intercepts. I suppose those transformers are transmission-line types,
wound on ferrite cores.

Cheers,
Tom

Roy Lewallen wrote:

Al is usually the value for low flux density. That is, it's the value
you'll have when the flux level is low. Permeability will drop from
there at high flux levels.


Not to nit-pick but the permeability of nearly all powdered iron
formulations actually rises with increasing flux levels (AC) and then
falls off. For #26 material (u=75), the effect is very much exagerated
with the permeability increasing nearly 300% at ~5000 Gauss and then
falling very quickly. However the permeability does drop for any value
of DC bias current and larger DC bias currents produce greater
reductions in permeability.

73, Larry Benko, W0QE

amdx wrote:

If I wind a transformer using the specified A sub L and then use that
transformer in a receive antenna where the voltages are very small, wouldn't
I be low on the curve and cause the transformer to function poorly
especially at the lowest frequency of the design?

Mike

PS Thinking about a Flag antenna, which has a small output signal.

Mike,


I think you are focusing on non-issues and not consider things that are
really important.

First, I would not use a 43 core on a low frequency receive antenna.
This is especially true with an ungrounded antenna that has
exceptionally low signal output, like a Flag.

There are very few antennas in the world that are perfectly UNbalanced
or perfectly balanced. Even what we consider an unbalanced antenna can
cause feed system problems. When the antenna has very low signal
output yet occupies a large spatial area, you are especially looking at
problems.

The flag has low common mode impedance, and fairly high differential
mode impedance. It is neither balaunced nor unbalanced, it is in that
soupy world of something that requires equal and opposite currents at
the feed without perfect voltage balance. It is not a balanced antenna,
and not an unbalanced antenna.

When that is combined with the very low signal output, you have to pay
particular attention to the transformer design.

You really can't use a transmission line transformer because it will
not have enough isolation. You need a primary-secondary transformer
with isolated and slightly seperated windings. You really don't want a
material that requires 30 or 40 turns, because extra wire will increase
stray capacitance from primary to secondary.

You almost certainly want to move into a binocular core with fairly
high permeability at the lowest frequency, like a 73 material. Unless
you have a few volts of RF from a closeby station, flux density is not
an issue.

You want to keep primary/secondary capacitance down near a dozen pF or
less if possible, and have NO direct path for common mode currents.

http://www.w8ji.com/k9ay_flag_pennant_ewe.htm

73 Tom

wrote in message
oups.com...

amdx wrote:

If I wind a transformer using the specified A sub L and then use that
transformer in a receive antenna where the voltages are very small,
wouldn't
I be low on the curve and cause the transformer to function poorly
especially at the lowest frequency of the design?

Mike

PS Thinking about a Flag antenna, which has a small output signal.

Mike,


I think you are focusing on non-issues and not consider things that are
really important.

First, I would not use a 43 core on a low frequency receive antenna.
This is especially true with an ungrounded antenna that has
exceptionally low signal output, like a Flag.

There are very few antennas in the world that are perfectly UNbalanced
or perfectly balanced. Even what we consider an unbalanced antenna can
cause feed system problems. When the antenna has very low signal
output yet occupies a large spatial area, you are especially looking at
problems.

The flag has low common mode impedance, and fairly high differential
mode impedance. It is neither balaunced nor unbalanced, it is in that
soupy world of something that requires equal and opposite currents at
the feed without perfect voltage balance. It is not a balanced antenna,
and not an unbalanced antenna.

When that is combined with the very low signal output, you have to pay
particular attention to the transformer design.

You really can't use a transmission line transformer because it will
not have enough isolation. You need a primary-secondary transformer
with isolated and slightly seperated windings. You really don't want a
material that requires 30 or 40 turns, because extra wire will increase
stray capacitance from primary to secondary.

You almost certainly want to move into a binocular core with fairly
high permeability at the lowest frequency, like a 73 material. Unless
you have a few volts of RF from a closeby station, flux density is not
an issue.

You want to keep primary/secondary capacitance down near a dozen pF or
less if possible, and have NO direct path for common mode currents.

http://www.w8ji.com/k9ay_flag_pennant_ewe.htm

73 Tom

Hi Tom,
Thanks for your input, When I wrote the above I couldn't remember what
people were using on there flags so I went to one site and just copied what
that site said, maybe it was a misprint and should have said 73.
I used a 3F3 4229 pot core with 3 turns and 13 turns
on my flag, I cut a styrofoam clamshell to go between the sec and pri to
limit capacitance. I also put a grounded electrostatic shield between
windings.
I was happy with my nulls, (whatever that means;-) but wonder how efficient
my transformer was.

My questions still stand,
Question 1
How many gausse is used to measure permeability and set AL?
Question 2.
Can anyone take a stab at how many gausse in a typical FT140-43 toroid
with 8 turns on the secondary, and 34 or 35 turns on the primary used on a
flag antenna with a low level signal.
On the second question the material can be modified to reflect the material
and turns as needed.
Thanks
Mike

amdx wrote:
. . .
My questions still stand,
Question 1
How many gausse is used to measure permeability and set AL?

Essentially zero.

Question 2.
Can anyone take a stab at how many gausse in a typical FT140-43 toroid
with 8 turns on the secondary, and 34 or 35 turns on the primary used on a
flag antenna with a low level signal.

I won't bother to calculate it because the change in permeability would
be so small you wouldn't be able to measure it. This is a non-problem;
you're wasting your time worrying about it.

On the second question the material can be modified to reflect the material
and turns as needed.

If your circuit is sensitive to a change in a few parts per million of
permeability, it has serious problems. The permeability will change
several orders of magnitude more than that with modest changes in
temperature.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
amdx wrote:
. . .
My questions still stand,
Question 1
How many gausse is used to measure permeability and set AL?

Essentially zero.

Ok, So I'll use the figure of 1 gausse as where permeability is measured and
from there I can assume the inductance increases 40 percent at 2000 gausse
for the toroid specified on the Magnetics webpage. The only info I have is
from
Ferroxcube Soft Ferrites and accessories 2000 data book, and it simply says
"The initial permeabilty is measured------ at a very low field strength."

Question 2.
Can anyone take a stab at how many gausse in a typical FT140-43 toroid
with 8 turns on the secondary, and 34 or 35 turns on the primary used on
a
flag antenna with a low level signal.

I won't bother to calculate it because the change in permeability would
be so small you wouldn't be able to measure it. This is a non-problem;
you're wasting your time worrying about it.

I'm not worrying, just curious, since I've been using a large potcore to
deliver microwatts that at one time I used at near a kilowatt. Just
wondered
if we were losing some low frequency response because of a change in
permeability. It seems as though the permeabilty measurement is made nearer
the power levels of our receive antenna signals.


On the second question the material can be modified to reflect the
material
and turns as needed.

If your circuit is sensitive to a change in a few parts per million of
permeability, it has serious problems. The permeability will change
several orders of magnitude more than that with modest changes in
temperature.

Roy Lewallen, W7EL

Thanks Roy,
I appreciate the discussion and information.
Mike

On Thu, 6 Apr 2006 14:58:37 -0500, "amdx"
wrote:

Question 1
How many gausse is used to measure permeability and set AL?

Hi OM,

You will never in your lifetime escape the bare minimum of ½ Gauss
presented by the Earth's magnetic field.

73's
Richard Clark, KB7QHC