Ferrite antenna com system
 

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

Ferrite antenna com system
 

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

Ferrite antenna com system
 

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

Ferrite antenna com system
 

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

Ferrite antenna com system
 

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

Ferrite antenna com system
 

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.

Ferrite antenna com system
 

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

Ferrite antenna com system
 

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.

Ferrite antenna com system
 

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

Ferrite antenna com system
 

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

Ferrite antenna com system
 

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.

John, that is what I have seen! I resonated the antenna coil and driven it
with it's resonance frequency. Seems that the achievable distance was a
little more than the circuit without resonating capacitor.

You say, that driving the ferrite rod into saturation will force it to leave
more power into air? Why?

- Henry

Ferrite antenna com system
 

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

Efficient antennas at that frequency are effectively very long
bits of wire. The ferrite rod is small compared to the wavelength
and very inefficient at generating a far field.

This is the antenna of the DCF77 transmitter (same frequency):
http://de.wikipedia.org/wiki/Bild:Dcf77.jpg

Kind regards,

Iwo

Ferrite antenna com system
 

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?


I tested it as resonating circuit using the original time-code receiver
antenna AND a second time without the capacitor.
Maybe I got a little more power in the air with the resonating circuit, but
it was not very distingiuable.

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 ?

I don't know the exact manufacturer of the time-code receiver ferrite
antenna but I comparable model reads:
L=900uH
bandwidth=700Hz
n=94
see original data
http://www.hkw-elektronik.de/pdfengl...00-77,5-DE.pdf
It is not the same antenna but very similar.

The original foil-capacitor is 682 labeled. I don't measured it but I think
it should be 6800pF reading.

For my second experiment I used no capacitor and turns=10.

If I would find a PSPICE model for an ferrite antenna ...


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 ?

Hm. I thought he just trying different turns value to achieve this. The
coil is the impedance transformer for the ferrite rod (=antenna). I'm wrong
here?


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 original coil is thinner than 0.3mm. If I compare it to my 0.3mm wire
maybe it is 0.18mm. The second experiment with the 10 turns coil is 0.3mm
enamelled copper wire.
I will give Litz wire a try if the system as such works...


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 ?

Good question. I series blocked DC with a WIMA MKS4 1.0uF 100VDC
high-quality capacitor. As measured the "big" capacitor is outside the
bandwidth of the antenna.

I don't think there is any DC component left. And yes, there is no magnet on
my desk laying around :-)
Is there any internal rectifiation phanomen in the ferrite possible?


- 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.
How much band noise should I expect?

- Henry

Ferrite antenna com system
 

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!)

Thanks. I will look there.

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!

Maybe the time-code receiving IC is a bad design. I don't know. It's
operating current is 500uA only. That is very small. It can receive the
time-code over 2000km with such an antenna with an transmitter EIRP of 30KW.
The receiving antenna is 700Hz bandwidth. I don't think this is super
narrow. Even if we look at the time-code receiver quartz filter with a
bandwidth of about 10Hz I can met it with my stable wave generator. It is a
PLL-design with a clock quartz. Should be typical 10ppm. I don't have a very
good frequency meter to verify it.

In my second transmitter experiment I used a not-resonated driver design. So
there are no problems with detuning the transmit antenna expected. It is
just driven by the 77.5KHz power signal.

CRT screen is off if I experiment. Otherwise I seen a very big CRT signal at
the receiver...

If the two ferrite rods will detune because of the close proximity I cannot
control it. I don't think so.

If you can hear induction heaters or something this is surely with a very
big antenna and a resonable good receiver design.

- Henry

Ferrite antenna com system
 

Henry Kiefer wrote:
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.

John, that is what I have seen! I resonated the antenna coil and driven it
with it's resonance frequency. Seems that the achievable distance was a
little more than the circuit without resonating capacitor.

You say, that driving the ferrite rod into saturation will force it to leave
more power into air? Why?

You misunderstood what I said. It was, " the most field you can
generate with the ferrite rod antenna will occur when it is
almost reaching saturation,"

If you saturate the rod, the field you generate will have lotsof 3rd
harmonic components in it, but little more of the fundamental. I was
trying to emphasize that you will need as strong a magnitic field as
possible aat the transmitting antenna, and just below saturation is
that limit, when a ferrite core is involved.

If the rod has a large lenght to diameter ratio (say , above 10) then I
think the uptimum coil arrangement on the rod also doffers considerably
for the transmitting and receiving cases, since the receiving case does
not deal with saturation.

In the receiving case, the end sections of the rod act as flux
collectors, and only the middle thirs or so has almost all the
collected flux passing through it, so this third is the optimum place
for the coil. /in the transmitting case, the rod has a tendency to
saturate at the center, first, with this arrangement, and you want
essentially the whole rod to approach satuation at the same ampere
turns. This will produce a field that acts as if it has been produced
by the full length of the rod. You can achieve something close ot this
by spreading the turns out, all over the rod, with an extra
concentration (a second or third layer layer, perhaps) at the ends.
Something like this (shown in cross section. View with fixed width
font i.e. Courier, so charcters are on grid pattern):

* = wire in cross section
# = rod

*** ***
****** ******
************************
##########################
************************
****** ******
*** ***

Ferrite antenna com system
 

Efficient antennas at that frequency are effectively very long
bits of wire. The ferrite rod is small compared to the wavelength
and very inefficient at generating a far field.

This is the antenna of the DCF77 transmitter (same frequency):
http://de.wikipedia.org/wiki/Bild:Dcf77.jpg

OK Iwo. But why a small receiving ferrite antenna works here? A
non-saturated (=linear, and that means the superposition theorem works)
antenna system is reciprocal as antenna theory predicts. So you should
explain where the difference is!

- Henry

Ferrite antenna com system
 

If you saturate the rod, the field you generate will have lotsof 3rd
harmonic components in it, but little more of the fundamental. I was
trying to emphasize that you will need as strong a magnitic field as
possible aat the transmitting antenna, and just below saturation is
that limit, when a ferrite core is involved.

I understand that. I added a second coil on the ferrite rod to measure the
antenna current and set it just below the point where I saw harmonics (or
say non-sinusial) waveform on the scope.


If the rod has a large lenght to diameter ratio (say , above 10) then I
think the uptimum coil arrangement on the rod also doffers considerably
for the transmitting and receiving cases, since the receiving case does
not deal with saturation.

In the receiving case, the end sections of the rod act as flux
collectors, and only the middle thirs or so has almost all the
collected flux passing through it, so this third is the optimum place
for the coil. /in the transmitting case, the rod has a tendency to
saturate at the center, first, with this arrangement, and you want
essentially the whole rod to approach satuation at the same ampere
turns. This will produce a field that acts as if it has been produced
by the full length of the rod. You can achieve something close ot this
by spreading the turns out, all over the rod, with an extra
concentration (a second or third layer layer, perhaps) at the ends.
Something like this (shown in cross section. View with fixed width
font i.e. Courier, so charcters are on grid pattern):

That is a very interesting configuration. Never seen such a design. I read
about a old-fashion remote controller system having a ferrite antenna
transmitter. There someone wrote, the transmitter antenna was a mignon
battery-shaped ferrite rod. e.g. much shorter but wider than mine. So an
optimum ferrite transmitter antenna is maybe more like a fat battery shaped.

- Henry

Ferrite antenna com system
 

On Wed, 25 Oct 2006 17:57:21 +0200, "Henry Kiefer"
wrote:


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.

The problem with loop antennas made of ribbon cable (or other
multiconductor cable connected this way) is the stray capacitance
between turns. The self resonance frequency (without external
capacitor) may be below the band of interest, so you can not resonate
such antenna with an external capacitor.

Paul OH3LWR

Ferrite antenna com system
 

The problem with loop antennas made of ribbon cable (or other
multiconductor cable connected this way) is the stray capacitance
between turns. The self resonance frequency (without external
capacitor) may be below the band of interest, so you can not resonate
such antenna with an external capacitor.

I found it very useful for design of hard-driven Power MOSFET driver
transformator till 400KHz without problems.

- Henry

Ferrite antenna com system
 

Henry Kiefer wrote:
Efficient antennas at that frequency are effectively very long
bits of wire. The ferrite rod is small compared to the wavelength
and very inefficient at generating a far field.

This is the antenna of the DCF77 transmitter (same frequency):
http://de.wikipedia.org/wiki/Bild:Dcf77.jpg

OK Iwo. But why a small receiving ferrite antenna works here? A
non-saturated (=linear, and that means the superposition theorem works)
antenna system is reciprocal as antenna theory predicts. So you should
explain where the difference is!

At LF, rarely is the problem "not enough amplitude of received signal",
so past a certain point there isn't much need to make the receiver
antenna more efficient. The problem is always "too much noise!". So
antenna designs are usually built around nulling out local noise, and
loop antennas will get rid of the mostly electric-field local noise.
And they have some directionality (notably sharp nulls) which can help
get rid of specific further-away noise.

Tim.

Ferrite antenna com system
 

"Paul Keinanen" wrote in message
...
The problem with loop antennas made of ribbon cable (or other
multiconductor cable connected this way) is the stray capacitance
between turns. The self resonance frequency (without external
capacitor) may be below the band of interest, so you can not resonate
such antenna with an external capacitor.

Ob-flippant remark: Sure you can, you just need a negative capacitor! ;-)

I suppose if one actually built a negative capacitor out of, e.g., a gyrator,
the noise performance would be pretty much shot? Might be OK for a
transmitting antenna, though?

Ferrite antenna com system
 

Henry Kiefer wrote:
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.

The effective permeability of a rod is dictated largely by the air gap
in the magnetic path, which is a function of the length/diameter ratio
of the rod. Powdered iron in general has very low permeability compared
to ferrite. If you really wanted to apply a huge amount of power to a
rod antenna, powdered iron might be a better choice because of its high
saturation flux density. But I doubt you could get the Q of a ferrite
rod antenna at the frequency in question, so it would be considerably
less efficient. You'd probably end up with less power radiated than if
you ran less power to a ferrite rod antenna, and a less efficient
antenna would impact your received signal. You'd have to crunch some
numbers or make measurements to find out for sure.

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?

The presence of even a small air gap has the effect of reducing the
effective permeability of the core and therefore the inductance of the
winding. It also dramatically reduces the core flux density for a given
number of winding amp-turns. This makes it very hard to saturate.
Inductors used for power applications commonly have a small core gap for
this reason. A rod has a very large air gap in the path -- from one end
of the rod, curving around outside the rod, to the other. And for many
ferrites used at radio frequency, the material loss is high enough that
the core would be hot enough to explode well before you reach a flux
level anywhere close to saturation. This isn't true of all materials at
all frequencies, of course.

The radiation takes place from the field outside the core, i.e., in the
air gap. If you didn't have a gap, you wouldn't have any significant
radiation.

And it's Roy, not Ron.

Roy Lewallen, W7EL

Ferrite antenna com system
 

Sorry "Roy" -

The effective permeability of a rod is dictated largely by the air gap
in the magnetic path, which is a function of the length/diameter ratio
of the rod. Powdered iron in general has very low permeability compared
to ferrite. If you really wanted to apply a huge amount of power to a
rod antenna, powdered iron might be a better choice because of its high
saturation flux density. But I doubt you could get the Q of a ferrite
rod antenna at the frequency in question, so it would be considerably
less efficient. You'd probably end up with less power radiated than if
you ran less power to a ferrite rod antenna, and a less efficient
antenna would impact your received signal. You'd have to crunch some
numbers or make measurements to find out for sure.

Surely.


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?

The presence of even a small air gap has the effect of reducing the
effective permeability of the core and therefore the inductance of the
winding. It also dramatically reduces the core flux density for a given
number of winding amp-turns. This makes it very hard to saturate.
Inductors used for power applications commonly have a small core gap for
this reason. A rod has a very large air gap in the path -- from one end
of the rod, curving around outside the rod, to the other. And for many
ferrites used at radio frequency, the material loss is high enough that
the core would be hot enough to explode well before you reach a flux
level anywhere close to saturation. This isn't true of all materials at
all frequencies, of course.

The radiation takes place from the field outside the core, i.e., in the
air gap. If you didn't have a gap, you wouldn't have any significant
radiation.

I'm not so theory funded to explain why a gap in the core transmits
electromagnetic wave. That is an open question for me.
I even don't understand why a ferrite rod will accumulate magnetic field
lines. There is the explanation of "shorten" the field lines way. Some
mysterical!

If you talk about explosion: What is the power level you mean?
Currently I have a ferrite rod of 9mm diameter and 50mm length, driven by
40mA against 2Vpp.
Do you think it is impossible to pump maybe 5watts into the rod?

- Henry

Ferrite antenna com system
 

OK Iwo. But why a small receiving ferrite antenna works here? A
non-saturated (=linear, and that means the superposition theorem works)
antenna system is reciprocal as antenna theory predicts. So you should
explain where the difference is!

At LF, rarely is the problem "not enough amplitude of received signal",
so past a certain point there isn't much need to make the receiver
antenna more efficient. The problem is always "too much noise!". So
antenna designs are usually built around nulling out local noise, and
loop antennas will get rid of the mostly electric-field local noise.
And they have some directionality (notably sharp nulls) which can help
get rid of specific further-away noise.

The problem is to pump enough energy in the air to overcome the atmospheric
noise seen by the receiver.

BTW: The mentioned antenna array picture is NOT the DCF77 sender. DCF77 is
exactly two antennas there only. And they have a very good performance
thanks to there big outline. They radiate about 30KW out of the 50KW output
power of the final amplifier. If I get 1% performance I think I'm good.

- Henry

Ferrite antenna com system
 

Henry Kiefer wrote:
. . .
If you talk about explosion: What is the power level you mean?
Currently I have a ferrite rod of 9mm diameter and 50mm length, driven by
40mA against 2Vpp.
Do you think it is impossible to pump maybe 5watts into the rod?

Dunno. If it had to dissipate most of the 5 watts (which is likely), it
would get as hot as a resistor that size dissipating the same power, and
that would be pretty hot. Just how hot depends on how well insulated it
is, how good the air flow is around it, and how much heat is conducted
away through the wires or any other physical connection. The first thing
I'd check would be the Curie temperature of the ferrite. If you reach
that temperature, the material will lose its magnetic properties, so the
antenna impedance will abruptly and dramatically change. At some higher
temperature, the ferrite will fracture, maybe violently.

If that gets to be a problem, a lower loss ferrite might be necessary.
That usually means lower initial permeability, and probably a lower Q
inductor. You'll have to determine what the optimum trade would be.

Shouldn't be any trick to feed some power to one and watch its
temperature with a thermocouple.

Roy Lewallen, W7EL

Ferrite antenna com system
 

On Wed, 25 Oct 2006 18:36:48 +0200, "Henry Kiefer"
wrote:

- 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.
How much band noise should I expect?

When listening at the signal e.g. through an SSB receiver, it is quite
easy to know the difference. The equipment noise is more or less
constant "hiss", while the band noise is mainly through numerous
distant lightnings.

Some field strength measurements made in England at 73 kHz during the
summer, using a calibrated meter indicated 25 uV/m in 200 Hz
bandwidth, which would produce about 120 dB more power from a full
sized (2 km) dipole than a single matched resistor at the receiver
input.

Thus, even if the actual antenna efficiency was -100 dB and the LNA
noise figure as bad as 10 dB, the band noise would still be stronger
than the amplifier noise.

While an antenna with -100 dB gain would be usable for receiving, such
antenna would be useless for transmitting.

Paul OH3LWR

Ferrite antenna com system
 

Hi Paul -

Some field strength measurements made in England at 73 kHz during the
summer, using a calibrated meter indicated 25 uV/m in 200 Hz
bandwidth, which would produce about 120 dB more power from a full
sized (2 km) dipole than a single matched resistor at the receiver
input.

My receiver bandwidth is about 10Hz because of the 77.5KHz quartz crystal
filter. So the sensitivity is better. Is 10*lg(200/10) here correct?


Thus, even if the actual antenna efficiency was -100 dB and the LNA
noise figure as bad as 10 dB, the band noise would still be stronger
than the amplifier noise.

While an antenna with -100 dB gain would be usable for receiving, such
antenna would be useless for transmitting.

It is interesting to learn, that antennas can be noise limited. So
reciprocal theorem is not all to know.

Thanks -
Henry

Ferrite antenna com system
 

Hello Roy -

I will try your suggestions when I build a more powerful power amp.

As the modulation is AM with known 25% there is a interesting measurement
methode possible:
Ramp the amplifier from low to high power and make a curve of the AM
modulation depth at the receiver or just with a measurement coil at the
transmitter ferrite rod. It should show a saturation if the ferrite goes in
saturation. I will try this.
There is an open question what a time constant the ferrite will have if it
goes into saturation or out of it. And if this is very different if iron
powder is used.

- Henry


"Roy Lewallen" schrieb im Newsbeitrag
...
Henry Kiefer wrote:
. . .
If you talk about explosion: What is the power level you mean?
Currently I have a ferrite rod of 9mm diameter and 50mm length, driven
by
40mA against 2Vpp.
Do you think it is impossible to pump maybe 5watts into the rod?

Dunno. If it had to dissipate most of the 5 watts (which is likely), it
would get as hot as a resistor that size dissipating the same power, and
that would be pretty hot. Just how hot depends on how well insulated it
is, how good the air flow is around it, and how much heat is conducted
away through the wires or any other physical connection. The first thing
I'd check would be the Curie temperature of the ferrite. If you reach
that temperature, the material will lose its magnetic properties, so the
antenna impedance will abruptly and dramatically change. At some higher
temperature, the ferrite will fracture, maybe violently.

If that gets to be a problem, a lower loss ferrite might be necessary.
That usually means lower initial permeability, and probably a lower Q
inductor. You'll have to determine what the optimum trade would be.

Shouldn't be any trick to feed some power to one and watch its
temperature with a thermocouple.

Roy Lewallen, W7EL