coil antenna tx/rx setup 1

[mageta]

In the attached file, the transmitter is isolated from the metal tube. The receivers are also isolated from the metal tube, but common to the transmitter. Using a differential mode on the receiver coils, would signals picked-up by the metal tube be rejected by the receivers? Also, would the transmitter signal be coupled to the receivers via the metal tube, or would the received signal be received by rf?
I am transmitting 2 mhz and 400 khz from tx to rx1 and rx2, than looking for phase and magnitude difference. I don ‘t want to receive any signal through the metal tube, just the em radiated signal.

 

[VEBill]

The 'tube' looks like a transformer.

If you use "differential mode on the receiver coils" then they'll reject both the signal carried by the transformer core tube and any direct signal.

 

[mageta]

well it is my understanding that that is what i am doing. both receive coils are used in differential mode. when the metal tube is separated where the transmitter, and both receivers are all isolated the receivers only get fr power. when the tube is connected together, most of the power on the receivers is coming through the metal tube.

when the rx1 and rx2 are isolated from the transmitter, it makes no difference, still most signal power is getting on the receivers via the metal tube.

what effect would there be on the system if the transmitter was in differential mode?

 

[VEBill]

Subject sketch for ease of reference:

 

[VEBill]

To be clear:
The metal "tube" is the grey bar that's grounded at each end?
What's the black line in-between the TX coil and the RX1 coil?

The only difference between RX1 and RX2 is that RX1 is just ever so slightly closer to the transmitter than RX2. Although your sketch is not scaled, I assume it's not monstrously huge. Therefore, at these frequencies (2 MHz and 400 kHz [note correct capitalization], very long wavelengths) there's essentially no difference whatsoever between the two - with the tube or without the tube. Therefore, if the outputs are connected out of phase and combined correctly, then the outputs will always cancel out.

The only way to get a significant delta signal would be to partially withdraw the tube so that the two RX circuits see vastly different inputs (RX1 around the tube, RX2 empty) and thus provide a net delta.

All this depends on some reasonable assumptions about system gain and so forth.

 

[mageta]

the transmitter is about 2.5 feet away from rx1, and rx2 is about 1.5 feet away from rx1. both frequencies are transmitted at about 60 vpp. both receive coils are part of one circuit system.

the metal tube is in sections, but system is used with the tube combined (electrically conductive from end to end and earth grounded).

the application is to reject all signal induced via the metal tube, and only pick up signal power via the transmitted rf.

as of now, majority of the signal is coupled to the rx1 and rx2 via metal tube since it forms a single large magnetic field.

the application properly working should result in there being very little phase difference and about 18 dB attenuation between the two receive coils, in free air. in different mediums, the phase difference will increase, and so will the attenuation.

the signal must be electromagnetically radiated, and not induced.

ps. what exactly does this mean:
"The only way to get a significant delta signal would be to partially withdraw the tube so that the two RX circuits see vastly different inputs (RX1 around the tube, RX2 empty) and thus provide a net delta."

 

[mageta]

correction regarding the metal tube. the tube is stainless steel, and therefore non magnetic. this means it should not permit magnetic flux. so this means that the system does not form a large magnetic field?

this makes me scratch my head even more, since i cannot see how the tube is coupling the signal to both, rx1 and rx2?

 

[VEBill]

Okay, I think I'm starting to see what you're trying to do...

How about this:

Only one RX circuit with a two-part coil. The first part of the coil is wound in one direction and is sensitive to both the induction-coupled (transformer or tube) and also to the radiated RF (through the air). The second part of the coil is wound in the opposite direction and is SHIELDED from the radiated RF.

So the induction-coupled RF cancels out within the two-part coil itself, while the radiated RF is detected by the open half of the two part coil.

Leave some adjustment method to balance-out the two halves of the two-part coil. Use fully shielded wiring or you will go insane trying to balance it.

The advantage of doing the balance within the coil (not including any amplifiers until AFTER the balancing act) is that is gives essentially unlimited headroom (linear region).

Is this the concept that you're after?

 

[mageta]

that could be a way to do it, but i am just trying to have the rf signal picked up by the receivers. eliminate all other sources.

this tool is used to measure resistivity of mediums by sending rf through them and measuring attenuation and phase shift. this is why all other signal paths must be either cancelled out or suppressed.

 

[VEBill]

Because I don't understand why you're constrained by the conceptual design as represented by the sketch above, it's difficult to assist.

General advice is to arrange the sensor (or sensors) to directly (or indirectly) measure what you're after. In other words, it's much better (for example) to use a clever passive bridge circuit to extract the signal, than trying to extract the signal later using some far more complicated and far less effective active post-processing.

Whatever thing you're trying to do with two coils, it's typically better to combine them before the amplifiers (unless you have a very specific reason to do it as you've drawn).

Where exactly is the material? Is in inside the tube? Is it moving along? These are the sorts of basic questions that you've not yet addressed.

 

[Higgler]

I've seen a ship antenna at your frequency created exactly as your drawing shows. Big coil at the bottom of a mast, couples to the mast inductively, radiates signals. I think you're doing the same, inducing enough current on the tube to make it radiate. You'd have to pick it up with both coils which would have different amplitudes and slight phase difference.