two antennas transmitting near each other

[jt1982]

What happens if two antennas are physically close to each other and transmitting the same signal at the exact same frequency?

Would this impact the ability of either antenna to transmit its signal? What would be the physics/technical explanation for what is going on?

Thanks.

[Rt169Radio]

If they were using the same power output I think they would both talk over each other, but if one was transmitting with more power then the other I think the one with more power would be the winner.

[mmckenna]

Co-phased antennas if you were feeding them from the same transmitter, simulcast if you were feeding them from different transmitters.

If the co-phased antennas are set up right, they produce a figure-8 radiation pattern. If they are not set up right, the pattern gets all funky.

The simulcast system, if set up correctly, would work fine. If not set up correctly, as in not timed right, it creates an awful mess. Two transmitters transmitting on the same frequency with the same audio, just slightly out of time tends to cancel out in some areas. In other areas, the capture effect comes into play, and the receiver will sort of lock on to the strongest signal.

As for the antennas, no affect on them. You could get too much TX power radiated into the other antenna, and I guess that could wash back into the other transmitter and act like a high SWR, but I could be wrong. Never had a reason to try doing that.

I know if you get a receiver and a transmitter antenna too close, the transmitter can destroy the receiver. I've seen it happen before. The transmitting radio is blasting out 50 watts or more. Some of that power is going to end up in the receiver. The receiver is designed to listen to extremely weak signals. When you hit it with too much RF, it can wipe out components in the receiver.

[n5ims]

Can you better describe your situation since the answer could be different based on what exactly is going on.

For example, if you have two independent stations (two transmitters each connected to their own antenna) you may have one set of issues. You would have to consider not only how the two antennas interact, but also take into consideration how the transmitters are generating the signals (are they exactly on the same frequency, what is the relationship between the phase of the signals to each other, etc.)

If you have a single transmitter where the signal is being split to feed the two antennas you may have another set of issues. Here you can assume that the signal feeding them are in phase up to the point that the signal is split. Now you must know how long (in wavelengths at the operating frequency generally) the feedline is between the splitter and each antenna, along with various factors of that feedline (velocity factor to name one). It's also important to know how far apart the antennas are from eachother, they design of the antennas, the counterpoise, etc.

You may get results where they act together to form a directional antenna where you get gain in one (or more) directions and loss in others. It may be that they signals interact so they cancel eachother out and minimal signal is actually radiated. Most likely you'll get something inbetween (random factors produce random results).

[jt1982]

Wow, thanks for all the great, quick, detailed responses! Allow me to give a bit more detail for this 'thought experiment'.

I have an 802.11n MIMO WiFi system with 2 transmit chains and 2 receive chains. Each transmitter and receiver pair share the same antenna. So there are 2 antennas transmitting identical signals with identical frequency content. But, assume the signals are out of phase (This is my understanding of 802.11n / MIMO).

The antennas are spaced out about 7-8cm apart.

The system is time-multiplexed, so the transmitting signal does not saturate the receive chain amplifier. In other words, at any given time, the system is *only* transmitting, or *only* receiving.

What I'm curious about is what are the physics within the antenna when it's getting blasted by essentially the same signal, and it's trying to transmit at the same time. Would the radiation pattern change? Now, I know that the aggregate radiation pattern could change (signal is bouncing off of other metal), but I'm curious about the radiation pattern for the single antenna that's getting blasted by an in-band signal.

The antenna's receive chain is 'open', so no signal would make through the receive amplifier. But, I could imagine that the antenna would have a resonance effect as a result of incoming signal, and send essentially a 'reflected wave' back through the transmit chain (since it's the same signal heading through air, a reasonably linear waveguide for this 'experiment').

So from that standpoint, the SWR theory makes intuitive sense to me. I would consequently think the efficiency of the impacted antenna would go down.

Is this more or less in line with your guys' thinking, or do you have a different opinion/approach?

Thanks again for the discussion, enjoying it.

[zz0468]

Originally Posted by jt1982

What happens if two antennas are physically close to each other and transmitting the same signal at the exact same frequency?

Both antennas would radiate.

How they radiate is subject to all the variables that you didn't provide, so the best you get is a generic incomplete answer.

Originally Posted by jt1982

Would this impact the ability of either antenna to transmit its signal?

Yes. How is subject to all the variables that you didn't provide, so the best you get is a generic incomplete answer.

Originally Posted by jt1982

What would be the physics/technical explanation for what is going on?

The best explanation of the physics behind it that I've seen is in the MIT Radiation Lab series. 28 volumes, so it's pretty complete. It's out of print, but available on-line or on CD. It's a good read , and since you seem to be an engineering student working on a homework assignment, you should should get familiar with it's contents. I'd start there if I was you.

[jt1982]

zz0468 - My previous reply is quite lengthy and offered the technical detail. It is still 'pending approval' because I am a new member to the forum. My lack of response is not indicative of my lack of willingness to provide detail. My apologies if it came across that way.

At a high level, my inquiry is directly related to an 802.11n MIMO WiFi router that I'm designing & building.

I am not an engineering student doing a homework assignment (I am many years out of college). I'm a practicing engineer who's made a career switch from one industry to another, and I'm trying to understand how I need to design my RF front ends. I feel like this forum is a great place to have discussion, because it is filled with antenna experts and enthusiasts alike.

Thank you everyone for your productive feedback so far.

[wtp]

A.M. or F.M.?

[jt1982]

Sorry guys, I've now sent 3 replies pending! My tongue is being held due to being a new member... I think these are legitimate, non-spam posts appropriate for this forum.

It's neither AM nor FM; it's 802.11n WiFi MIMO. 2.4GHz & 5GHz bands. I'm a professional engineer who's building my own router.

My apologies for the confusion my short, un-detailed initial post caused for you all.. that was a mistake, but I was worried about overwhelming with a flood of detail off the bat :P .

[mancow]

Originally Posted by jt1982

What happens if two antennas are physically close to each other and transmitting the same signal at the exact same frequency?

It would just appear as one signal.

Originally Posted by jt1982

Would this impact the ability of either antenna to transmit its signal? What would be the physics/technical explanation for what is going on?

You seem to be talking about "coupling". This becomes an issue when one is hooked to a transmitter and one to a receiver. If both are transmitting generally nothing adverse will happen to either.

[beachmark]

For WiFi MIMO AP's, the 2 transmit RF streams should be independent. I.e., they are trying to transmit different info, with instantaneously different amplitides and phases (but on the same frequency). If this is like LTE MIMO (and I think it is), then the (receiving) client device will report back on how well it can demodulate both streams; if that is not working, then that link reverts to SISO with just one data stream....preferrably transmitted from one antenna so that there will be no odd phasing/pattern effects between the 2 antennas for that data stream that will cause pattern distortion.However, other client devices working on the same AP may have physical links good enough to utilize MIMO and so their 2 transmit data streams will still be on both antennas. (And I am going on some very basic knowlege of 802.11, and assuming there is no tranmsit time multiplexing of the different client device's data steams.)

What this all boils down to is that the RF from each antenna SHOULD (as I understand it) be fairly statisitically independent in instantaneous phase and amplitude. As such, the phasing of 1 signal is random compared to the the other. So the 2 antennas should not statically phase in or out with each other and produce any sort of steady figure 8 or cardiod or any other distorted, non-omni pattern. They might phase oddly for a symbol period now and then, and produce an odd pattern for an instant, but that will look like some sort of multipath fading to the receiving client device.

As for VSWR, yes, I can see one effecting the other. The matching to each antenna might have to be modified for that. And I guess there COULD be pattern distortion caused by the other antenna acting as a passive re-radiator (like in a yagi) but that would vary with how the 2nd antenna appeared 'terminated' for energy that coupled from the other antenna; I am not sure how to think of that off the top of my head. Also, the fact that these are broadband signals means that any such passive re-radiator pattern effects would probably be localized in spots in the transmitter spectrum and would not be an issue for the whole transmitted spectrum. So, my general thought (with no data!) is that is not a big issue either.

So that takes care of the pattern distortion issues; it should not be huge, as I understand things. And the VSWR can be corrected. BUT....There is the question of the transmit energy from one amplifier getting back into the other via the antenna coupling and causing some havoc between the amplifiers. (And I assume this is a modern AP which can transmit 100 mW or more on each channel, so a real amp is needed on each channel.) The energy feedback from one to the other can cause some real problems; the most common is that there will some undesired mixing of signal 1 with signal 2 at the output of signal 2's amplifier, and vice versa. Since these are on the same frequency, there should not be much of what would be considered intermod, like we commonly deal with in 2-way systems between two different transmitted frequencies.

BUT there will be what is called spectral re-growth, where the adjacent channel, and 2nd adjacent channel, and out to the xth adjacent channel's, transmitter noise levels will go up above what one of the 2 channels would transmit alone; this can get really bad, depending on the level of coupling and the amplifiers' behavior. This IS a problem, both for FCC compliance and for system performance. In that case, you are basically polluting other channels in the WiFi spectrum with sideband crud caused this RF 'crossover' feedback between amps that is being coupled back and forth via the 2 close antennas. So you need to be able to measure this as a designer, and be ready employ methods and components to limit the transmitter to transmitter coupling via the antennas (or any other internal AP path for that matter).