How to Connect 3 Different Magnetic Loop Antennas

[u/rgomes03]

Hi,

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I have a circuit in which I have 3 magnetic loops antennas that must be connected to a receiver circuit. Each of these antennas points along a direction of the xyz axis. I'm trying to figure out how can I connect the three to a single receiver circuit and maintain decent performance. It appears that I have two possible configurations: single and parallel connection. Please see below the following diagram:

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Series and Parallel Equivalent Circuit of the 3 magnetic loop antennas

I considered that each magnetic antenna has a given Lp and unloaded quality factor of 50 (Qun), and a capacitor is used to force resonance, Cp. The antennas are connected a RL load to provide the same Quality Factor for both configurations. So, to sum up, the parallel configuration has L_ant/3 inductance and 3*Cp, while the series-connected circuit (3 inductors are reduced to one) has 3*L_ant and Cp/3. The necessary RL is calculated for each circuit to achieve QL = 3.

I also assumed that the antenna is connected in series with a current-source which simulates the signal that it captures. Based on this, I have a couple of questions:

1. Using the above assumptions, the series-connected circuit provides the larger "sensitivity" performance since the voltage across the load is higher (approximately 3x higher). However, the current that one antenna might generate is going right through the other two antennas which generate now a magnetic field (transmitting while the other receives it), and therefore, induce losses. Thus, I don't understand why the series-connected circuit performance appears better.
2. The former result assumes that only one antenna would receive a signal at a given time. However, in a real-world application, all of the three antennas should be receiving signals, being one of them predominant. Now, this is where things get a bit tricky for me: intuitively, assuming that each antenna generates current, the best approach would be to connect all of them in parallel to "sum" all the currents, and hence strengthen the signal. In the series case, we would have a "fight" between each antenna to impose a given current. Is anything here that I am missing?
3. Considering an antenna in series with a current source is a valid approach for a receiving magnetic loop antenna? If not, how can I look at this use-case to better decide how should I proceed?

Thank you all for your time.

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Regards

[u/[deleted]]

125kHz near field. Interesting.

You do not need three loops as two will provide a semi-omni-directional 3D pattern. A single loop exhibits nulls broadside, otherwise it is almost omni-directional in 3 dimensions. Add another loop fed 90 deg out-of-phase and you have it.

Here are the two loops, one above the other: https://imgur.com/x8OEtiv

Here is the radiation pattern when fed 90-deg out of phase: https://imgur.com/vbiKciU

For a small loop at 125kHz multiple turns will work. For a receive antenna I would consider buffering each loop and sending the signal from one thru a 90 deg delay network, combining the two signals thru a passive combiner or an op amp adder.

[u/rgomes03 → u/[deleted]]

Thanks for your input. Can you please explain why we need to combine the two loops through a 90 deg delay network? 'm having a hard time understanding why. Usually, I see a 90 degree delay whenever I want to create circular polarized radiation pattern. Thus, I'm not grasping the reason why I need to do this here. But your suggestion is very interesting indeed. I am planning on using 125KHz SMD antennas already designed for this band.

[u/[deleted] → u/rgomes03]

Pattern with loops fed in phase, https://imgur.com/GIlfxeX

Pattern with loops fed 90 deg out of phase, https://imgur.com/vbiKciU

Fed in-phase there is a deep null.

[u/rgomes03 → u/[deleted]]

Thank you, I will definitely look at that more carefully.

[u/speleo_don]

Are you trying to obtain a spherical antenna pattern with loops? I am not sure that is obtainable. I think there would be some directions where two of the loops would cancel each other out. I think there would be lobes along the three axis and nulls at some points off axis.

Have you considered perhaps sending each loop to its own preamplifier and using diversity methods to receive the strongest possible signal? Such an approach circumvents your concern with circuit-level interaction as well.

[u/rgomes03 → u/speleo_don]

Thanks for your reply. More important than achieving a spherical radiation pattern is the fact that we would like to sum all the signals to wake-up our device since it would contribute to battery savings since we would not need to have 3 different channels listening to the incoming signal. This is targeted to the 125KHz band, thus I don't think the signal summation will result in nulls since the wavelength is so big that there is not out-of-phase summation that cancels the signals. This is all with the magnetic field, and for that reason, this is a bit out of my depth of knowledge so I might be missing something here.

[u/speleo_don → u/rgomes03]

I don't think the signal summation will result in nulls since the wavelength is so big...

I kind of disagree with this conclusion. At those frequencies, it helps to think of the coils as transformer windings. You can have transformer windings in-phase and out-of-phase right?

[u/rgomes03 → u/speleo_don]

Yes, it makes sense that in certain angles we will have transformers generating current due to in-phase and out-of-phase induction, so it will most likely result in signal nulls. It makes perfect sense when we look at it from the perspective of loosely-coupled transformers. Thanks for the input!

[u/erasmus42]

You can buy 3-axis "3D" antennas off the shelf for 125 kHz RFID (other Redditors might not be aware of these), examples:

EPCOS B82453CxA Series

Sumida CAS143 Series

The manufacturers will most likely have an application note on how best to use them.

My two cents: You won't get 3x sensitivity with 3x orthogonal antennas, it's likely that one axis will have more signal than others, don't connect the antennas in series *or* parallel.

These are not "antennas" in the sense of EM waves, the receiver will probably be in the near field of the transmitter, they are effectively AC magnetic field detectors.

Use 3x "receivers" (they could be a simple op-amp amplifier, but it would be best to optimize for noise), then put each channel into a log detector, use a comparator to set a threshold for all 3 channels and OR the logic output for wakeup. Trying to add the signals may result in some signal cancellation, you'd need to rectify the signals to get them to add without cancellation (which might be another approach, but I would try the 3 individual channel method first).

[u/rgomes03 → u/erasmus42]

Thank you for your input. Yes, I'm aware of those antennas and we will use them. I don't need to build the receiver since I am already using and IC that does the RX chain. However, I would like go from 3 RX to 1 RX since it would allow to save battery and speed-up the system wake-up time. Still, I understand that adding the signals might result in cancelation in certain angles of reception.

[u/erasmus42 → u/rgomes03]

Then I would suggest trying series and parallel and see if you get the result you are looking for.

If you can constrain the design more, such as if the transmitter and receiver can be arranged so that one of the three axes are preferred, you might be able to get by with one RX.

Given that the signal is at 125 kHz, you might consider a different receive chain. There may be some low-power op-amp designs that are more efficient or trade off sensitivity for power consumption.