Hello all -

I built a simple ferrite antenna communication system. Unfortunately it
won't work if I set the sender more distanced than about a meter. That is
even true with different transmitter configurations.

Here the details:
Transmitter:
ferrite antenna: diameter 8mm , 50mm long
frequency is 77.5KHz, digital modulation is AM 25%
bit-rate is 1 bit/sec (0 is 100ms carrier 25%, 1 is 200ms carrier 25%)
insulated copper wire coil 10 turns

The transmitter is self-constructed and delivers a very good signal.

Receiver:
same antenna copied, but a built-in resonating capacitor.
ready-to-use WWVB 77.5KHz receiver. Demodulated signal goes to scope.

The transmission works over about one meter without any shortage.


Now the problem is that I can change the transmitter parameters but I cannot
reach a substancial greater distance. I changed:
- the coil wound times
- output current to the antenna (measured across a series resistor)
- added an antenna current sensor coil to sense the antenna current and to
see if the ferrite antenna saturizes (NO! Very clean sinusoid)


Googling around to find theoretical aspects of ferrite antenne got no good
results. I spent several hours and read all I can read.


Have someone suggestions to try or good links to read? Especially for:
- when a ferrite or iron powder rod/bar goes in saturation?
- optimal rod dimensions
- optimal coil design (I suggest single layer, resonating with good Q
capacitor, about 3 to 10 turns)
- LNA design for such a low frequency?
- antenna field theory in near-field.

If you need further details please ask.

Thanks in advance.

Regards -
Henry

On Tue, 24 Oct 2006 22:38:18 +0200, "Henry Kiefer"
wrote:

Hello all -

I built a simple ferrite antenna communication system. Unfortunately it
won't work if I set the sender more distanced than about a meter. That is
even true with different transmitter configurations.

Here the details:
Transmitter:
ferrite antenna: diameter 8mm , 50mm long
frequency is 77.5KHz, digital modulation is AM 25%
bit-rate is 1 bit/sec (0 is 100ms carrier 25%, 1 is 200ms carrier 25%)
insulated copper wire coil 10 turns

The transmitter is self-constructed and delivers a very good signal.

Receiver:
same antenna copied, but a built-in resonating capacitor.
ready-to-use WWVB 77.5KHz receiver. Demodulated signal goes to scope.

The transmission works over about one meter without any shortage.


Now the problem is that I can change the transmitter parameters but I cannot
reach a substancial greater distance. I changed:
- the coil wound times
- output current to the antenna (measured across a series resistor)
- added an antenna current sensor coil to sense the antenna current and to
see if the ferrite antenna saturizes (NO! Very clean sinusoid)


Googling around to find theoretical aspects of ferrite antenne got no good
results. I spent several hours and read all I can read.


Have someone suggestions to try or good links to read? Especially for:
- when a ferrite or iron powder rod/bar goes in saturation?
- optimal rod dimensions
- optimal coil design (I suggest single layer, resonating with good Q
capacitor, about 3 to 10 turns)
- LNA design for such a low frequency?
- antenna field theory in near-field.

If you need further details please ask.

Thanks in advance.

Regards -
Henry


The propagation mode here is pure magnetic coupling, not a proper
electromagnetic "radio" wave, because the antennas are so small
compared to a wavelength. Dipole magnetic fields fall off with the
cube of distance.

A lot more turns on the rods, and resonating with a cap, will help
some. Longer rods would help some, too, but 1/d^3 is a cruel function.

How far do you need to go?

John

The propagation mode here is pure magnetic coupling, not a proper
electromagnetic "radio" wave, because the antennas are so small
compared to a wavelength. Dipole magnetic fields fall off with the
cube of distance.

A lot more turns on the rods, and resonating with a cap, will help
some. Longer rods would help some, too, but 1/d^3 is a cruel function.

How far do you need to go?

Hi!

I remember 1/d^4 for a full EM-field here.

The receiver is a WORKING time-code receiver. Working in distance at least
2000km from the time-code transmitter with an EIRP of 30KW. The time-code
transmitter have of course a VERY BIG antenna (120m height).
So I'm a little confused of your capacitor idea. That is true?: The
transmitter is NOT sending an electromagnetic wave but the same antenna
system at the receiving end reads it as an full established EM field?

How far: Hm, several km's if possible. Maybe I should go higher in
frequency?
What other small effective antennas work here?

I think the problem is not the minimum turns because I tested it with an
original ferrite rod - the same as in the original time-code receiver. It
have a lot of turns, probably 100 or more. The same behaviour with 100 turns
AND with 10 turns. No difference!

What I understand of ferrrite antenna theory is:
That the coil is simply an impedance transformer and bandpass (with a
parallel capacitor for narrow-band reception) to couple the preamplifier to
the antenna system (= ferrite rod).

But I miss something. Maybe something with differences between transmitting
and receiving with a ferrite antenna. The antenna is not pure reciprocal -
because the ferrite (or iron powder) can be nonlinear!

- Henry

On Wed, 25 Oct 2006 01:03:21 +0200, Henry Kiefer wrote:


The propagation mode here is pure magnetic coupling, not a proper
electromagnetic "radio" wave, because the antennas are so small
compared to a wavelength. Dipole magnetic fields fall off with the
cube of distance.

A lot more turns on the rods, and resonating with a cap, will help
some. Longer rods would help some, too, but 1/d^3 is a cruel function.

How far do you need to go?

Hi!

I remember 1/d^4 for a full EM-field here.

The receiver is a WORKING time-code receiver. Working in distance at least
2000km from the time-code transmitter with an EIRP of 30KW. The time-code
transmitter have of course a VERY BIG antenna (120m height).
So I'm a little confused of your capacitor idea. That is true?: The
transmitter is NOT sending an electromagnetic wave but the same antenna
system at the receiving end reads it as an full established EM field?

How far: Hm, several km's if possible. Maybe I should go higher in
frequency?
What other small effective antennas work here?

I think the problem is not the minimum turns because I tested it with an
original ferrite rod - the same as in the original time-code receiver. It
have a lot of turns, probably 100 or more. The same behaviour with 100 turns
AND with 10 turns. No difference!

What I understand of ferrrite antenna theory is:
That the coil is simply an impedance transformer and bandpass (with a
parallel capacitor for narrow-band reception) to couple the preamplifier to
the antenna system (= ferrite rod).

But I miss something. Maybe something with differences between transmitting
and receiving with a ferrite antenna. The antenna is not pure reciprocal -
because the ferrite (or iron powder) can be nonlinear!


Yes, a ferrite stick antenna works quite well for receivers, but not for
transmitters. Try winding a few dozen turns around the whole room - i.e.,
up the wall, across the ceiling, down the other wall, across the floor,
and so on. Or, you could wrap a piece of 50-conductor ribbon cable, and
make loops by soldering the ends together offset by 1. ;-)

I don't know very much about antenna theory, but I know that the bigger
the better. ;-)

Something's telling me that it's theoretically possible to transmit with
a ferrite stick, but from the kind of power you'd have to run through it,
it would probably blow up. =:-O

Good Luck!
Rich

Henry Kiefer wrote:
. . .
But I miss something. Maybe something with differences between transmitting
and receiving with a ferrite antenna. The antenna is not pure reciprocal -
because the ferrite (or iron powder) can be nonlinear!

1. Ferrite and powdered iron are entirely different materials, with
different physical and magnetic characteristics. Powdered iron isn't a
good choice for this application.
2. You're not likely to drive either one into a nonlinear region when
they're in the form of a rod because of the large air gap in the
magnetic path.

Roy Lewallen, W7EL

Henry Kiefer wrote:
Hello all -

I built a simple ferrite antenna communication system. Unfortunately it
won't work if I set the sender more distanced than about a meter. That is
even true with different transmitter configurations.

Here the details:
Transmitter:
ferrite antenna: diameter 8mm , 50mm long
frequency is 77.5KHz, digital modulation is AM 25%
bit-rate is 1 bit/sec (0 is 100ms carrier 25%, 1 is 200ms carrier 25%)
insulated copper wire coil 10 turns

The transmitter is self-constructed and delivers a very good signal.

Receiver:
same antenna copied, but a built-in resonating capacitor.
ready-to-use WWVB 77.5KHz receiver. Demodulated signal goes to scope.

The transmission works over about one meter without any shortage.


Now the problem is that I can change the transmitter parameters but I cannot
reach a substancial greater distance. I changed:
- the coil wound times
- output current to the antenna (measured across a series resistor)
- added an antenna current sensor coil to sense the antenna current and to
see if the ferrite antenna saturizes (NO! Very clean sinusoid)

In addition to what others have said, the most field you can
generate with the ferrite rod antenna will occur when it is
almost reaching saturation, and that takes a lot of ampere
turns. You can deliver more ampere turns to the rod than
your transmitter output can deliver if you resonate the coil
with a capacitor. That way, you have the current bouncing
back and forth through the capacitor added to the current
from the amplifier. If the coil-capacitor Q is, say, 100,
there will be 100 times more current through the coil than
the transmitter is delivering. This will probably take a
coil with a considerable mass of copper in it.

With this approach, you might reach 10 meters.

On Tue, 24 Oct 2006 22:38:18 +0200, "Henry Kiefer"
wrote:

Have someone suggestions to try or good links to read? Especially for:
- when a ferrite or iron powder rod/bar goes in saturation?
- optimal rod dimensions
- optimal coil design (I suggest single layer, resonating with good Q
capacitor, about 3 to 10 turns)

So there is a resonant circuit at the transmitter and not just a coil?

With such low number of turns (and hence low inductance), the
capacitor would have to be huge to resonate it at 77.5 kHz. Where do
you get high Q capacitors with such capacitances ?

The resonant circuit impedance levels are quite low in this
configuration (small L/large C), how do you effectively couple power
from the transmitter to this low impedance level at the resonant
circuit ?

The skin depth at this frequency is about 0.25 mm, so any wire thicker
than 0.5 mm will not utilise the full copper wire, so some kind of
Litz wire with separately insulated strands could be used to keep the
coil resistance low.

The inductance of some ferrites varies if there is some DC field
present. This inductance change could detune the resonant circuit and
drop the radiated power. Are you sure that the transmitter coil is not
carrying any DC components or some even harmonic distortion, which
would cause an unbalanced magnetic field in the ferrite rod ?

- LNA design for such a low frequency?

The band noise is the dominant (compared to "white" amplifier) noise
when listening to the band with your transmitter switched off, the
receiver noise performance should be adequate.

Paul OH3LWR

Henry Kiefer wrote:
I built a simple ferrite antenna communication system. Unfortunately it
won't work if I set the sender more distanced than about a meter. That is
even true with different transmitter configurations.

Here the details:
Transmitter:
ferrite antenna: diameter 8mm , 50mm long
frequency is 77.5KHz, digital modulation is AM 25%
bit-rate is 1 bit/sec (0 is 100ms carrier 25%, 1 is 200ms carrier 25%)
insulated copper wire coil 10 turns

The transmitter is self-constructed and delivers a very good signal.

Receiver:
same antenna copied, but a built-in resonating capacitor.
ready-to-use WWVB 77.5KHz receiver. Demodulated signal goes to scope.

The transmission works over about one meter without any shortage.


Now the problem is that I can change the transmitter parameters but I cannot
reach a substancial greater distance. I changed:
- the coil wound times
- output current to the antenna (measured across a series resistor)
- added an antenna current sensor coil to sense the antenna current and to
see if the ferrite antenna saturizes (NO! Very clean sinusoid)


Googling around to find theoretical aspects of ferrite antenne got no good
results. I spent several hours and read all I can read.


Have someone suggestions to try or good links to read? Especially for:
- when a ferrite or iron powder rod/bar goes in saturation?
- optimal rod dimensions
- optimal coil design (I suggest single layer, resonating with good Q
capacitor, about 3 to 10 turns)
- LNA design for such a low frequency?
- antenna field theory in near-field.

I can't really help you with ferrite antennas for transmitting, but can
tell you that if you google around for "lowfer" and the Longwave Club
of America http://www.lwca.org/ you will find a lot about antenna
designs that are suitable for this band. They will also might have
recommendations for frequencies of operation that are legal for
transmission in your home country (I don't even know what that is!)

LNA isn't really applicable here because there is so so so much
man-made and natural noise in this band.

I'm a little surprised that your achieved range was so small from a
ferrite rod antenna, actually. Did you really tune both antennas, in
place and in circuit, for resonance? The resonance is so so super
narrow that strays between design and circuit make a big difference. I
mean, CRT screens with flybacks, and faulty flourescent lamp ballasts,
and incadescent dimmers radiate all sorts of crap around the LF
spectrum for blocks, and they aren't even trying to be intentional
transmitters! And don't get me started about induction heaters and
welding machines, those can be heard across several states!

Tim.

But I miss something. Maybe something with differences between
transmitting
and receiving with a ferrite antenna. The antenna is not pure
reciprocal -
because the ferrite (or iron powder) can be nonlinear!


Yes, a ferrite stick antenna works quite well for receivers, but not for
transmitters. Try winding a few dozen turns around the whole room - i.e.,
up the wall, across the ceiling, down the other wall, across the floor,
and so on. Or, you could wrap a piece of 50-conductor ribbon cable, and
make loops by soldering the ends together offset by 1. ;-)

You made my day
BTW: Your idea with the ribbon cable gives you a very easy made
transformator if using clamping connectors. This works very good. I
practiced it 10 years ago.

I heart it several times that a ferrite stick antenna cannot work as a
useful transmitter antenna. But why?????


I don't know very much about antenna theory, but I know that the bigger
the better. ;-)
Sure, for reasonable antennas. But if the antenna is very VERY big in
relation to wavelength it even cannot work! Read somewhere.


Something's telling me that it's theoretically possible to transmit with
a ferrite stick, but from the kind of power you'd have to run through it,
it would probably blow up. =:-O
I found no saturation state but I have not enough power at the moment to
drive it very powerful. Something I try later ...

Regards -
Henry

1. Ferrite and powdered iron are entirely different materials, with
different physical and magnetic characteristics. Powdered iron isn't a
good choice for this application.

Powdered iron should work better because of the higher permeability even
under heavy load in comparision to ferrite. I think so in theory - not
tested.

2. You're not likely to drive either one into a nonlinear region when
they're in the form of a rod because of the large air gap in the
magnetic path.

Can you explain this more detailed Ron? What will happen with the air gap?
The losses in the air gap radiates and that is the antenna function?

- Henry