150Khz Antenna question

I have seen those types of antennas, but I found some really neat ideas under LW loop antenna I imagine some core material would help.

greg

e

Please understand that what you are describing is not what's generally understood as an antenna. Antennas couple electromagnetic waves between their environment and electronics circuits. What you want is either a coil to couple a 150kHz magnetic field into your sample, or (effectively) a capacitor to couple a 150kHz electric field into your sample. It would also be possible to couple some of each, and since you will presumably be very much in the near-field of your transducers, you can adjust the ratio to suit your needs (presuming you understand what your needs really are). Electromagnetic waves are generally considered as fully developed only some distance from the transmitting antenna structure (the "far field," generally more than a wavelength from the antenna).

The transducer you need depends on what magnetic and/or electric field strengths you need in your sample, what the sample comprises (e.g., its permittivity, loss tangent and permeability and how big it is, how uniform you need the field to be, and perhaps some other factors.) I'd say it's way too early to be worried about the voltage out of your excitation circuit. Also, does the excitation really need to be square-wave, or can it be sinusoidal? It's far more difficult to get square wave AC magnetic or electric fields than it is to get sinusoidal fields.

Cheers, Tom

Search Google for "LowFER". These are radio stations in the 136KHz region. Much of the technology should be directly applicable.

Please note that attaching an antenna to your 150KHz square wave trash generator is a bit like constructing a jamming transmitter, with harmonics extending well into commercial and broadcast frequencies. I suggest you make an effort to limit your harmonics by using a sine wave, not a square wave.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

The efficiency of energy coupling and the controlability of field strength would increase if you could actually insert the sample into the EM field of the transmitter ~ in a field gap of a magnetic structure, or in-between the plates of a capacitive one.

RL

The 150kHz and the small antenna implies the near field coupling. Do you need to couple by the electric field or by the magnetic field?

High voltage is required for the electric field (capacitive coupling).

Coil is required for the magnetic field (inductive coupling).

In any case, the antenna has to be at resonance to be efficient.

It depends. What exactly are you trying to accomplish?

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Sure looks like it.

Back at the university I always wondered about that: Immature fields (near field), juvenile fields, adolescent fields? So where does an EM field reach puberty?

:-)

--
SCNR, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

ce

o

What you are proposing to do is illegal in most countries.

Leon

In answer to all;

I still at the begin of the project, who is highly experimental, here some answers:

I can certainly use a sine wave instead of a square wave. Coupling will be done in direct contact (space only by the plastic wall of the case, 1/4" at max). By your description, it will be electric field coupling (or capacitive). The compound is a liquid running trough a pipe install in contact with the emitter case.

Bye Jacques

What you are proposing to do is illegal in most countries.

Leon

You may have a point, I will have to make sure.

If this is the case, I will have to enclose the system in a metal box to make sure it did not irradiate everywhere.

Bye Jacques

. . . snippity snip . . .

John Kraus gave a workable definition in his book 'Antennas', that the [appropriately] normalised magnitudes of the different field components (reactive and radiation) are all equal to pi at a distance lambda/2.pi away from the 'antenna' so, closer than that, the 1/r^2 and 1/r^3 reactive terms dominate, and further away the 1/r radiation terms dominate.

So the larger value of lambda/2.pi can be taken as the bare minimum separation of two simple antennas intended to operate independently. Of course, there will still be major coupling between them on account of radiation (TX) or re-radiation (RX) and gainful antennas demand proportionately more separation in their line(s) of fire.

Chris

Hello Jacques,

What is the required strength of the field at the liquid/air transition? Is the fluid always present? Is it electrically conducting, etc? what about the liquid volume? All these have influence on the required electrical power and possible tuning circuitry.

This may be a nice challenge for an expert/professional. I think there are some professionals in this newsgroup. When you don't want to discuss the details (because of confidentiality), you may contact somebody directly.

Best regards,

Wim PA3DJS

The address is correct when you don't type abc.

PS: That final statement is 'in practice' in respect of coupling, of course. The mechanisms that give rise to directivity for the radiation field are likely to affect the reactive fields in a similar way at small distances.

In EMC the rule of thumb is usuallt (2*D^2)/wavelength, with D being the antenna width. That's where you can begin to measure real radiation patterns.

Of course Jacques seems to be well below that distance.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Could've mentioned that, but it's more about arrays than fundamental elements. I guess with a stated frequency of 150 kHz, a wavelength of 2 km, and a 'plastic case of 2" x 4" inches' there's not much chance of an array!

2D^2/lambda, as you say, is a useful criterion for minimum measuring radius for dishes, for example. I did the expt once at 11 GHz and the depth and shape of the first null was wrecked closer than 2D^2/lambda. Beyond it, the pattern was essentially independent of the distance away.

Chris

Yes, Jacques just has to decide on how much inductive coupling and how much capacitive coupling he wants. Or maybe just one of them. There is no real antenna.

And he needs to mind the law so that the mounties don't show up ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Is the liquid conductive? Is your pipe round, or can you make a flat 2" x 4" thin flowthru box for the treatment? Mike

e

I designed a circuit like you describe for 125 kHz. I used a Kaschke magnetics ferrite custom coil which was designed to manage a large magnetic field (large diameter). The application was Tire pressure monitor calibration, with a requirement of a 10A/meter field at 10 inches. Do you have a specification for your magnetic field strength?

These types of antennas are very narrow bandwidth. The most efficient way to build this system is to use a power oscillator at 150 kHz with the antenna as part of the resonant circuit. What is your frequency tolerance?

Frank

When the Daisies spread their ...

e f e

For a sine wave source you might look at the Baxandall class-D oscillator. I've got a tutorial on it on my web-site

scillator1.htm

which goes on to talk about ways of reducing the - not all that high- third harmonic content of the output by pulse-width modulating the drive. My examples are written around a 16kHz (100,000 radians per second) operating frequency, but 150kHz should be manageable. Getting exactly 150kHz might be tricky with this approach, but if you only need something in the ball-park (and the 555 isn't all that stable) the Baxandall circuit should do nicely.

-- Bill Sloman, Nijmegen