Questions about superposition of two antennas

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  • #26
sophiecentaur
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Don't forget there is both series and parallel resonance. With parallel resonance, current is at minimum.
You're right that the input current is a minimum but there will be a high current flowing in the series loop (sloshing from L to C and back again). This could, I think, cause a lot more coupling.

If the antenna are not matched to each other it is easy to calibrate this out - by measuring the outputs of the two antennae in the same place and comparing them. Actually, tuning two antenna to the exact same frequency would be difficult - pretty knife edge adjustment aamof. A flatter response could be easier to deal with.

It is normal to make measurements using a 'probe' with very little coupling, so as not to disturb the system.
 
  • #27
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The receiver antenna is placed on the horizontal plane. The ideal case is that it can be oriented randomly without suffering any negative effect.

I am still not clear about the reason why a non-optimal coupling of receiver antenna can affect superposition of two antennas (I mean, when pointing two antennas horizontally). I checked the results in the case of pointing two antennas horizontally again. The superposition depends on the positions of the receiver antenna. The results present a certain pattern on different positions. Could you please tell me why it is position-dependent?

Thanks
 
  • #28
Born2bwire
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Just a quick sanity check here though.

When you take your antenna measurements, you do have both antennas present in all measurements and both of them terminated the same way through all measurements, right? When you run both antennas in the presence of each other you get coupling but as long as you keep both antennas present during measurements and the termination behavior does not change between on/off, then I would expect you to be able to properly capture the coupling and allow superposition.

The next thing is that when you measure the fields, do you orient your probe in three orthogonal directions? The fields are polarized and you need to take measurements along three orthogonal directions (and you should of course use the same orientations for all three situations) so that you get the correct field vector regardless of orientation.

Finally, if you measured the field, you should also measure the magnitude and phase and take that into account when you do your superposition. It's my recollection though that phase can be tricky to measure but you can't measure magnitudes and expect them to add up. You need to account for direction and phase to get the proper interference behavior when you sum the two measurements.

Some of these issues can be avoided by setting up the experiment to work better for you as I believe sophiecentaur has been suggesting.
 
  • #29
sophiecentaur
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It would be better if you described the orientation of antennae in terms of either the polarisation or the plane of the antenna. It's never clear what you mean. :confused:

You are much more likely to get sensible results if you have all antennas parallel and with symmetrical patterns. For a start, at least, before you have ironed out all the problems.

The only reason that there is any interdependence must be that the antennas are 'seeing each other' due to some coupling. This is why I suggest having an inefficient system (really poor coupling) with untuned antennae and with attenuators in the feeds. It is not usual to 'sample' fields with anything other than a 'probe' which is electrically so small that it doesn't interact with the sources.
The spacing of these antennas is very small in terms of wavelength and, if the are resonant, you can expect significant coupling. I think your observations confirm this.

Think of a normal Yagi antenna. You have a single tuned dipole and nearby elements that are just off resonance. These parasitic elements completely change the pattern of the antenna because of the high coupling between them all. On the other hand, think of a so-called active array of non resonant antennae, With this sort of array, you can combine the outputs of the receiving antennae in appropriate phases (actively) and get 'any pattern you want' and they don't affect each other. This is, in effect, superposition working the other way around.
What you want is to set up two fields which are independent of each other, which means that you cannot afford to have any currents flowing in one antenna due to the currents in the other antenna. That means Low Coupling is required.
 
  • #30
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Thanks for the quick reply.

The two transmitting antennas are theoretically in phase. However, there is a very little difference in phase due to limitation of the hardware.

The receiver antenna does have 3D coils which are oriented orthogonally.
 
  • #31
Born2bwire
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Thanks for the quick reply.

The two transmitting antennas are theoretically in phase. However, there is a very little difference in phase due to limitation of the hardware.

The receiver antenna does have 3D coils which are oriented orthogonally.
I'm talking about the phase of the field, not the antennas. Do you have a setup that can actually measure phase or are you limited to magnitude? Because if you are only measuring field magnitude and direction you can't expect to superposition to work unless you can figure out the relative phase difference between the fields at that point. But I am at a loss on how you could do that because the coupling of the antennas throws things off.

EDIT: Well I guess a first-order approximation of the phase difference would be to measure the relative path lengths of your measuring point between the two antennas. You could then calculate the relative phase shift roughly by the path difference but this neglects the secondary effects of the coupling between antennas. This requires of course identical antennas and that they be excited with signals that have the same phase.

EDIT EDIT: The above would work best if you are doing far-field measurements. For near-field measurements I do not think that it would work. Heck, I'm not quite sure if it would work well in the far-field since the calculations we do for far-field radiation still take into account the phase shift over distance between observation and source.
 
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  • #32
sophiecentaur
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I'm glad someone else has picked up on the practicalities of this experiment. It needs some serious RF knowhow to get over some of the basic problems that seem to be arising.
The frequency he's operating at and the distances he's using would make phase / distance pretty small.
Also, if he doesn't operate with the transmit antennae having patterns that are symmetrical in the plane of the paper, he's got yet another imponderable.
There is also a question of the receive probe. Is it a real 'probe' or is it yet another (possible) resonant antenna?
I can't be sure how much of this that ffj is taking on board. I think it's harder than he realises. Possibly his supervisor hasn't seen all the factors either. It wouldn't be the first time.
 

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