Wideband Toroidal Antenna?

[Rerouter]

Hello, Working on a project that may not stray high enough to be True RF, but felt like this would be the best place to ask.

For a project I need to measure a relative conductivity change of liquid solutions of various acidity, and composition, and do not want to contaminate the measurement because my probes began dissolving or reacting.

To accomplish this I am hoping to use a toroidal coil for both RX and TX, My understanding is that the coil will get the magnetic field enlarging and reducing around the core which will induce a current through the hole in the middle, e.g. the fluid,

Then by measuring the return loss will be able to calculate the impedance of the fluid. assuming the better the conductivity the less attenuation of the original signal. (I may have this backwards)

So now for the part I need some help with, building the RX and TX coils, Ideally I would like them to be as wideband as possible e.g. 10Khz - 50Mhz, while still being reasonably smallish (hoping for under 7cm diameter), however I am going to guess these are not easy to make, and could use some pointers in the right direction,

At this stage the signal source was intended to be a differential current mode driver, but if it needs to be different I am all ears,

[T3sl4co1l]

So you're doing... what?  A one-port and measuring its impedance relative to a baseline?  A two-port and measuring its transmittance and reflectance?

Going through the problem, from the most basic level, to the final implementation, is very helpful indeed.

(A one-port is two electrical terminals, isolated from anything else.  Basically, any two-terminal component, but real components do have capacitance to ground, or to space, so it may need additional constraints.  A two-port is simply two one-ports (isolated from each other and from anything else), with some sort of coupling between, so that signals are reflected from each AND transmitted between them, with a measurable amplitude and phase shift.  A two-port, with one port terminated in a given load, reduces to a one-port in a well defined way.  Scattering parameters are most often used to measure these, but there are six other equivalent types, chosen based on what you're calculating.)

Example, if you wind two coils (one is affected, the other left as reference) and construct a nulling bridge circuit, you're measuring a one-port.  Also if you resonate it with a capacitor, or straight up measure the impedance, how ever you do it.  A transformer is a two-port.

As for design, what kinds of liquids?  Electrolytic solutions?  Liquid metals?  What temperature range?  (If temp is different from the reference coil -- a reference based method is going to be useless!)  What do you estimate the signal strength to be?  Can you design a coil that delivers a stronger signal (greater sensitivity of \$\Delta Z\$ to liquid conductivity)?  Why do you suspect it should be toroidal?  Should it have any other features, like low/minimal external field?  (Toroidal windings have the upside of being more-or-less self-shielded.)

Also, what about chemical sensitivity?  Is it okay to use boring old hookup wire, or enameled magnet wire?  Does it need to be encased in Teflon, or glass?  Exposed to vacuum and plasma?

Tim

[Rerouter]

So you're doing... what?  A one-port and measuring its impedance relative to a baseline?  A two-port and measuring its transmittance and reflectance?

I was thinking 2 port, the concept at the most basic level is the more conductive the fluid, the stronger the signal amplitude at the receive coil, its acting like a single turn between 2 separate toroid transformer cores, when its poor at conducting it cannot carry as much power and the RX coils load sees a smaller signal.

This was where i thought using a current mode output amplifer for the TX coil would help, as it would remove the resistance of the coil, and just be amp-turns,

I was planning to measure both the output signal amplitude and the received with AD8307 power detectors, as they can work down to 10KHz with capacitor sizes that are still easy to get in COG capacitors. and as a feature creep option using an AD8302 for phase recovery if it was needed to compensate the amplitude. this however is far from being set in stone.

For the antennas as I only cared about magnetic induction and not electrostatic, I am thinking the coils would need electrostatic screening, to reduce the measurement to just magnetic coupling of the induced current. however I am not sure this is required, 

As for design, what kinds of liquids?  Electrolytic solutions?  Liquid metals?  What temperature range?  (If temp is different from the reference coil -- a reference based method is going to be useless!)  What do you estimate the signal strength to be?  Can you design a coil that delivers a stronger signal (greater sensitivity of \$\Delta Z\$ to liquid conductivity)?  Why do you suspect it should be toroidal?  Should it have any other features, like low/minimal external field?  (Toroidal windings have the upside of being more-or-less self-shielded.)

The liquid would be Ionic solutions so essentially electrolytic in both water and solvents, temperature range from -70C to +180C, and pH over most of the spectrum, and both coils would be present in the fluid with at least 25cm immersed on all sides, so they will be at the same temperature. in my head this should also remove the issue with the cores changing over temperature, but i suspect i am wrong.

The signal strength is whatever it needs to be at this point, so far testing has been using a +-10mA AC source with a compliance up to 70V. but resonances and dips at various points are a problem.

The reason I am leaning towards toroidal is that it needs to induce a signal in the fluid, normal conductance is measured with a 4 wire setup with conductive probes immersed into the water. an AC current applied and the voltage measured, the toroidal approach generates that current but with no contact to the fluid required.

Also, what about chemical sensitivity?  Is it okay to use boring old hookup wire, or enameled magnet wire?  Does it need to be encased in Teflon, or glass?  Exposed to vacuum and plasma?

It will be encased in teflon, between the 2 with a hollow center. like shown in the image attached.

Hope that helps.

[chris_leyson]

I think you mean axial and not toroidal, a toroidal coil will contain most of the field within the toroid and won't be any good as an antenna. Look up Helmholtz coil, they will give you a more or less uniform magnetic field in your sample area but you need to use two identical coils, two for transmiter and two smaller coils for receiver. Also, you will never cover the frequency range you suggest with a single coil arrangment, try it out at lower frequencies first, say up to a few tens of kHz. If you don't want to wind your own coils look at the air core wire coils from Jantzen Audio they've got coils from a few uH to 100H. EDIT: You might have to arrange the coils so that the field passes across the sample area rather than along it and maybe mount the receive coils at 90 degrees.

[T3sl4co1l]

What range of ionic concentration?  Is the outside geometry guaranteed -- is the loop thickness, diameter and return path constant?  There will be a tempco due to expansion, if nothing else, that you'll have to calibrate out.

Conductivity naturally varies with temperature (because of solvent self-ionization and ionic mobility) as well.

Ionic solvents have a high dielectric constant, which may cause a lot of your signal to appear as electric rather than magnetic coupling.  There's not really a good way to shield this while maintaining loop current -- after all, if you want wide dynamic range, you need a large EMF to obtain a sensible current.

You may end up needing one turn, or even less (i.e., multiple turns of the fluid).  Instead of a helical toroidal winding, you might do a single turn (a metal tube over the teflon tube), with cores stacked over it.  This acts as a coupler or launcher, where signal goes in one end, goes out along the fluid, then into the same structure (rx) opposite.  Yes, you will probably want shields between them.

High impedance means high voltage and low current, which pretty much precludes low frequency operation.  10kHz is unlikely even with nanocrystalline cores.

At high frequencies, mind that the fluid exhibits skin effect and standing waves, depending on its conductivity (which is just imaginary dielectric constant) and k.  This can be avoided by using a more compact design, but only proportionally so; and your minimum frequency limit shares the same proportionality, so this won't buy you any more frequency range.

Tim

[Tantratron]

I'm still not sure how you plan to wireless measure the chemical concentration, do you have a picture or clearer details exactly where are the chemicals and how do you intent to probe their signature...

Have you considered some inspiration if reading induction lamp technology, maybe check this video which I've done long ago explaining the principle to transfer but also probe ionic energy, signature via near field radio engineering