Is it possible to create a directional VLF radiator?
 

I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I am aware of part 15 and will not be using
this for any other reason than for research. 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 being
in optics I have no idea of how one might control the dispersion of a
carrier like this. In other words in my field I can collimate or focus
a light beam but dealing with this wavelength I have not a clue!

Thank You in advance for your input

Steve Sands

In message , Steve
Sands writes
I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I am aware of part 15 and will not be using
this for any other reason than for research. 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 being
in optics I have no idea of how one might control the dispersion of a
carrier like this. In other words in my field I can collimate or focus
a light beam but dealing with this wavelength I have not a clue!

Thank You in advance for your input

Same principles, just scale up!

Mike

PS. I don't think you can do it. At 50 ft you will still be inside the
near-field for both electric and magnetic components of the RF. The wave
hasn't yet assembled itself into an electromagnetic wave.
--
M.J.Powell

Not at that frequency for that range.
a half-wave at 25 khz would be longer than the distance to the receiver.
(thinking aloud)- let's see - 27mhz is known as '11 meters' - so 27 khz
would be 1000 times longer - or 11 kilometers so an antenna would be about
half that - or around 6 kilometers - then you would need reflectors and
directors - and would need to mount it a half-wave high for propogation.
Don't they use vlf for talking to submarines?
I would try to pick a much higher frequency for experimenting. Around 150
mhz or so would make the antenna easier to work with .
focusing a light beam is easier because the wavelength is measured in
nanometers - not kilometers.
use coax to carry the signal for 25 khz.

"Steve Sands" wrote in message
om...
I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I am aware of part 15 and will not be using
this for any other reason than for research. 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 being
in optics I have no idea of how one might control the dispersion of a
carrier like this. In other words in my field I can collimate or focus
a light beam but dealing with this wavelength I have not a clue!

Thank You in advance for your input

Steve Sands


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.772 / Virus Database: 519 - Release Date: 10/1/2004

"Hal Rosser" wrote in message et...
Not at that frequency for that range.
a half-wave at 25 khz would be longer than the distance to the receiver.
(thinking aloud)- let's see - 27mhz is known as '11 meters' - so 27 khz
would be 1000 times longer - or 11 kilometers so an antenna would be about
half that - or around 6 kilometers - then you would need reflectors and
directors - and would need to mount it a half-wave high for propogation.
Don't they use vlf for talking to submarines?
I would try to pick a much higher frequency for experimenting. Around 150
mhz or so would make the antenna easier to work with .
focusing a light beam is easier because the wavelength is measured in
nanometers - not kilometers.
use coax to carry the signal for 25 khz.

"Steve Sands" wrote in message
om...
I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I am aware of part 15 and will not be using
this for any other reason than for research. 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 being
in optics I have no idea of how one might control the dispersion of a
carrier like this. In other words in my field I can collimate or focus
a light beam but dealing with this wavelength I have not a clue!

Thank You in advance for your input

Steve Sands

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.

I need to create an AM modulated carrier of 25 Khz that is highly
focused in beam width. I am aware of part 15 and will not be using
this for any other reason than for research. 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 being
in optics I have no idea of how one might control the dispersion of a
carrier like this. In other words in my field I can collimate or focus
a light beam but dealing with this wavelength I have not a clue!

Thank You in advance for your input

Steve Sands

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.
************************************************** ************
*** Yes - that is the problem - and it is unsurmountable for that frequency
at that range
************************************************** *****************
Sorry for the omission.
*********************************************
The omission makes no difference -
You will not be able to focus a beam at that distance using that frequency.
you will be able to detect the carrier - but if you walk around the
transmitter in a circle at that range with your detector (receiver), you
will notice that it was not a beam at all - the signal strength will not
vary as you might be expecting.
again - I would use a much higher frequency to experiment with.
- but what the heck - try it anyway - then you will see for yourself that
you should have used a higher frequency.


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.772 / Virus Database: 519 - Release Date: 10/1/2004