Coils

[Anonymous]

Hi Eric,
Whatever I read has this magical figure of 24-28 turns or a coil of around 300uh. I appreciate that we are involved with near induction fields(and where is that written up!) and not the usual radio transmission theory. But excuse me if I am extremely naive and ask if we can divorce ourselves entirely from such theories - and devote attention to L controlling current rise and C as influencing discharge periods. Are we simply concerned with the swift switching of magnetic energy, and seeking the response from a target responding to illumination by the energy within the transmitted field?
Take me back to step one please -
The coil transmitting the energy into a field is subject to what considerations i.e. what parameters define the efficiency of the transmission coil? High current and low turns pluse what frequency of repeating of excitation, or is it low current and high turns and high frequency of excitation. This figure of 300uh appears almost universal regardless of the frequency or duration of excitation. Why???
Your illustration(greatly appreciated) of SD2200 waveforms displays a surprising slow rise in current. Your comments high-light that the switching times of excitation employed do not allow full current flow to develop. Do you have any thoughts why the designer Bruce Candy followed this course of action - do you follow a similar course of "clipped" excitation yourself - or can you possibly comment on the advantages/disadvantages of utilising such an excitation.
Hurry up and write your book!
regards.
g.

 

[Anonymous]

Hi Graeme,
Probably a case of

 

[Anonymous]

Hi Eric,
Thankyou for your response. Your scope responses look much like what I am achieving.
g

 

[Anonymous]

Hi Eric,
You remarked in a previous posting (to JC,11.05.00) that that the speed of switch-off of magnetic field may affect the response from the target object. May I invite you to comment further.
g.

 

[Anonymous]

Hi Graeme,
I

 

[Anonymous]

Hi Eric,
Thankyou. I'm pulling out all sorts of wires at the moment and building various coil formers - both single and D-D pattern. My neighbours must think I am potty as I walk back and forth on the lawn stringing many strands of thin wire together.
Excuse me if I labour the topic of coil design, but a recurring recollection I have is an article a few years back (Electronics World?? - of course I cannot find the article!!) on cave-radio. The frequencies used were around 900htz - sitting around our area of interest. The aspect that interested me was that the Tx coil was tuned, and the Rx circuit also had some consideration in this regard.
Is there any relevance in P.I. design to considering tuned-circuits?
g.

 

[Anonymous]

Hi Graeme,
If you tune a PI coil (just remove damping resistor), it ceases to be PI and you are just transmitting a lightly damped train of oscillations. The receiver sampling wont like working with all these sine waves about and will just lock up.
Eric.

 

[Anonymous]

Hi Eric,Graeme and all the pulse staff, There is an very interesting article to a project using VLF for surface-to-underground communications,theorical and the construction of a troglograph for cave communication 400feet thru the rock and limestone with a fcy of 3K2768Hz .It is interesting because many things are in relation with pulsed signal and tuned coil.I realize this project for a friend firemanm many years ago and it working great.It is a 7 integrated circuits and 5 transistors project with two 19inch coil and 10 pages of text and schematics. The name of the magazine is ELECTRONICS DIGEST from Electronics today,an ARGUS specialist publication,Winter 1986 Vol7 N

 

[Anonymous]

Hi Graeme and Alexis,
One other thing that a PI is good at is finding other coils. Many times I have had ringing on a waveform that I have been trying to damp, without success, only to find that the culprit was a coil under the bench on even the other side of the room in some cases. Even reels of cable or hookup wire can cause this problem. I once thought of developing a marker system using this effect. One application would be for marking a plastic water main. You would bury ferrite cored tuned circuits at intervals alongside or on top of, and this would make it easy to relocate it in the future. The receiver would be tuned to the marker frequency and in fact you could have different frequencies for different pipes. Might also be useful underwater. From what I have seen, the range should be much greater than for an equivalent size metal object. The markers, being passive, could be encapsulated and have an indefinite life.
Now there