Need help with controlling VLF beam width

[Steve Sands]

I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I need a range of 50 feet. Receiver will be a
tuned resonant tank with about 90 db of gain post detection. I would
like to be able to "focus" this beam but I have no idea of how one
might control the dispersion or radiation pattern of a carrier like
this.

Thank You in advance for your input

Steve

 

[Dbowey]

hybridyne posted:

<< I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I need a range of 50 feet. Receiver will be a
tuned resonant tank with about 90 db of gain post detection. I would
like to be able to "focus" this beam but I have no idea of how one
might control the dispersion or radiation pattern of a carrier like
this.

Thank You in advance for your input

At that frequency, the best you can do is use a loop antenna on the
transmitter. It won't be "highly focused," but it will be directional.

Don

 

[John Larkin]

I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I need a range of 50 feet. Receiver will be a
tuned resonant tank with about 90 db of gain post detection. I would
like to be able to "focus" this beam but I have no idea of how one
might control the dispersion or radiation pattern of a carrier like
this.

Thank You in advance for your input

Steve

To create a nonsilly em wave at this frequency, the antenna would be
huge (as in miles long) and the focus would be correspondingly fuzzy.
If the antenna is small, all you'll get is a local electric or
magnetic field, not a real electromagnetic wave of any consequence.

I'm thinking that not only is it impossible to focus a 25 KHz carrier
with a small antenna, it's probably not feasible to send any usable
amount of signal 50 feet. The magnetic field from a dipole coil falls
off as distance cubed - bad news - and electrostatic coupling is
probably worse.

What are you trying to do?

John

 

[Joerg]

Hi John,

I'm thinking that not only is it impossible to focus a 25 KHz carrier
with a small antenna, it's probably not feasible to send any usable
amount of signal 50 feet. The magnetic field from a dipole coil falls
off as distance cubed - bad news - and electrostatic coupling is
probably worse.

You can get well past 50 feet. A lot. But you are right, for a truly
focused EM field Steve would need to lease some property the size of a
national forest and order a tractor-trailers loaded with wire spools.

Regards, Joerg

 

[Ken Smith]

I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I need a range of 50 feet.

You think you need a focused beam to solve some design problem. I think
we can assume that just running a wire for 50 feet is not in the cards.

If you go with electro-magnetics:

If you wind a coil onto a large rod core, you can get more of the power to
go in the desired direction than at right angles to it. This will be
about the best you can do with anything smaller than, lets say, 50 feet
doing the radiating.


You could change to using sound: 25KHz is high enough that most people
can't hear it unless it is quite loud.


You haven't said what the design goal is.

 

[Joerg]

Hi Ken,

You could change to using sound: 25KHz is high enough that most people
can't hear it unless it is quite loud.

Please don't do that. If can be quite a torture for animals such as
dogs. Even if it's out in the woods we should mind foxes and others.

Remember those "toneless" whistles? Blow gently into them and the dog
obeys some command such as 'come' if trained. Blow hard and you get
growls and hisses.

Regards, Joerg

 

[Ken Smith]

Hi Ken,

Please don't do that. If can be quite a torture for animals such as
dogs. Even if it's out in the woods we should mind foxes and others.

Remember those "toneless" whistles? Blow gently into them and the dog
obeys some command such as 'come' if trained. Blow hard and you get
growls and hisses.

Good point.

A 50 foot twisted pair would be the best answer. It would be very
directional.

 

[Steve Sands]

To create a nonsilly em wave at this frequency, the antenna would be
huge (as in miles long) and the focus would be correspondingly fuzzy.
If the antenna is small, all you'll get is a local electric or
magnetic field, not a real electromagnetic wave of any consequence.

I'm thinking that not only is it impossible to focus a 25 KHz carrier
with a small antenna, it's probably not feasible to send any usable
amount of signal 50 feet. The magnetic field from a dipole coil falls
off as distance cubed - bad news - and electrostatic coupling is
probably worse.

What are you trying to do?

John

Thanks for all the input. I did leave out an important fact. I did
intend to use a coil wound on a form or around a either a steel or
ferrite rod depending on efficiency. I have already done some tests
and projecting a detectable carrier is in the bag so to speak. The
problem lies in shaping the dispersion so I can focus the beam into a
limited space, say 25 feet in diameter at 50 feet from the radiating
coil.

Sorry for the omission.

 

[Ben Bradley]

Thanks for all the input. I did leave out an important fact. I did
intend to use a coil wound on a form or around a either a steel or
ferrite rod depending on efficiency. I have already done some tests
and projecting a detectable carrier is in the bag so to speak. The
problem lies in shaping the dispersion so I can focus the beam into a
limited space, say 25 feet in diameter at 50 feet from the radiating
coil.

As others have said, due to the long wavelength at that frequency,
this is essentially magnetic signal transmisssion at 25kHz.
I've got an idea and I'm trying to visualize whether it would
work... Put several coils around the main coil, with enough of the
signal in them at reverse polarity to cancel the signal along the
sides. I'm hoping this would "narrow the beam" but I suspect that in
the far field it would just reduce it or cancel it out equally in all
directions. Perhaps someone has actually done something similar with
magnetic fields and can tell you more how to do it.
The basic question is, can you "focus" a magnetic field? I suspect
not, at a distance there's a "north pole" and a "south pole" and the
field goes between the poles in a characteristic way, and adding other
"magnets" around it can only change the shape of the near-field. The
far field will effectivly "see" just one magnet.

Sorry for the omission.

The real omission is why you want to do this, and then perhaps we
can come up with alternate means of doing it (presuming things line a
25-foot-long twisted pair is unacceptable).

 

[Rich Grise]

Thanks for all the input. I did leave out an important fact. I did
intend to use a coil wound on a form or around a either a steel or
ferrite rod depending on efficiency. I have already done some tests
and projecting a detectable carrier is in the bag so to speak. The
problem lies in shaping the dispersion so I can focus the beam into a
limited space, say 25 feet in diameter at 50 feet from the radiating
coil.

Sorry for the omission.

The omission doesn't make any difference. Because of the way EM waves
are generated, and propagate, any scheme that you could come up with
to "focus" that kind of frequency in that kind of area would require
some kind of device that's bigger than the area in question. It's
like an ant in a microwave. It just doesn't intercept any, because
it's such a small fraction of a wavelength. At 25 KHz, the wavelength
is 12 Km. The smallest antenna you could make that makes any sense
at all would be over 1 Km long.

Two loops, tuned to the same freq., is the best you're going to do.

Make them resonant. And big.

Good Luck!
Rich

 

[Joerg]

Hi Ken,

A 50 foot twisted pair would be the best answer. It would be very
directional.

Yes, those are very directional.

Your hint using ultrasound is actually a good one. It just would have to
be well above the hearing range of animals. I don't know but maybe bats
are the ones with the highest "spectrum allocation". Then there is IR
and all kind of other options but Steve would have to share some more
about the application.

Regards, Joerg

 

[Ken Smith]

be well above the hearing range of animals. I don't know but maybe bats
are the ones with the highest "spectrum allocation". Then there is IR

I think bats go up to something like 100KHz. The bug catching bats need
high frequencies to get good locations. The flying fox doens't need
anywhere near as good of accuracy.

 

[Rich Grise]

As others have said, due to the long wavelength at that frequency,
this is essentially magnetic signal transmisssion at 25kHz.
I've got an idea and I'm trying to visualize whether it would
work... Put several coils around the main coil, with enough of the
signal in them at reverse polarity to cancel the signal along the
sides. I'm hoping this would "narrow the beam" but I suspect that in
the far field it would just reduce it or cancel it out equally in all
directions. Perhaps someone has actually done something similar with
magnetic fields and can tell you more how to do it.
The basic question is, can you "focus" a magnetic field? I suspect
not, at a distance there's a "north pole" and a "south pole" and the
field goes between the poles in a characteristic way, and adding other
"magnets" around it can only change the shape of the near-field. The
far field will effectivly "see" just one magnet.


The real omission is why you want to do this, and then perhaps we
can come up with alternate means of doing it (presuming things line a
25-foot-long twisted pair is unacceptable).

Maybe the guy just doesn't realize what can be done with wire. I had
about a 75' 4-conductor (cat 3?) wire running 10 (or maybe 100) base
whaddayacallit, TCP/IP, just fine. It was actually a 100' wire, with
about 25' in a pile at the one end.

Cheers!
Rich

 

[Rene Tschaggelar]

I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I need a range of 50 feet. Receiver will be a
tuned resonant tank with about 90 db of gain post detection. I would
like to be able to "focus" this beam but I have no idea of how one
might control the dispersion or radiation pattern of a carrier like
this.

Focussed onto a spot of say 10 x 10 mm, a finger nail ?

A dipole antenna, say lambda-quarter is in the order of 2.5km.
A focus won't be much smaller than this, provided that you
have a directional antenna such a 10 element Yagi. It is going
to take you a valley.

The other option, making more sense, is to have a magnetic antenna
and work in the near field. This is a simple as calculating a
magnetic loop. A 10 x 10m loop with say 1 Amp should be detectable
50 feet away.

Rene

 

[Ken Smith]

A dipole antenna, say lambda-quarter is in the order of 2.5km.
A focus won't be much smaller than this, provided that you
have a directional antenna such a 10 element Yagi. It is going
to take you a valley.

I think a stacked rombic is the way to go. The upside would be a beam
width of only about 3 degrees. The downside is needing to put it in orbit
because there isn't enough room on the earth.

 

[Country Loon]

As others have said, due to the long wavelength at that frequency,
this is essentially magnetic signal transmisssion at 25kHz.
I've got an idea and I'm trying to visualize whether it would
work... Put several coils around the main coil, with enough of the
signal in them at reverse polarity to cancel the signal along the
sides. I'm hoping this would "narrow the beam" but I suspect that in
the far field it would just reduce it or cancel it out equally in all
directions. Perhaps someone has actually done something similar with
magnetic fields and can tell you more how to do it.
The basic question is, can you "focus" a magnetic field? I suspect
not, at a distance there's a "north pole" and a "south pole" and the
field goes between the poles in a characteristic way, and adding other
"magnets" around it can only change the shape of the near-field. The
far field will effectivly "see" just one magnet.


The real omission is why you want to do this, and then perhaps we
can come up with alternate means of doing it (presuming things line a
25-foot-long twisted pair is unacceptable).

If you want to stear a beam you need a beamformer. An array of antennas
which are steered in a number of ways.The simplest is to introduce phase
shifts and and amplitude scaling.They are supposed to be half a wavelenght
apart which I make 6km.You may want to try this up in space where there is
more room!

Tom