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Antennas- Multiple/Simultaneous Frequencies

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dknguyen

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Okay, so I know that a single antenna can be designed to transmit/receive multiple frequencies. But can an antenna send/receive multiple signals at the same time? I don't mean send and receive at the same time, I mean receive or send multiple frequencies at the same time. Like if I wanted to send two signals, would I need two antennas? Or could I gang two properly designed radio circuits to the same antenna?

EDIT: Oh, I also don't mean mixing two different data signals so that fits into a single radio signal. I mean physically transmitting two different frequencies simultaneously with the same antenna.
 
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Mikebits

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That sounds like frequency division multiplexing.
 

kchriste

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Look into duplexers and multicouplers. This can get quite bulky unless the frequencies are relatively far apart.
 

dknguyen

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Hmm, maybe I am being misunderstood. There is a line of radio receivers for RC hobby. They are marketed as receiving the same data on multiple frequencies simultaneously for redundancy in case one channel gets jammed. THese are two radio electromagnetic waves that are completely independent of each other. There's no multiplexing or time slicing or anything like that going on to merge two sets of data so it can be sent with a single radio wave.

But from my looking at the number of antennas on MOST, but not ALL of the receivers, there seem to be some oddballs indicating that perhaps not all receivers of the line do this. I am trying to figure out if looking at the number of antennas is an accurate way of determining this since the company is being vague.
 
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microtexan

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DK, how close together are the frequencies. If they are rather close, the single antenna probably has enough of a bandwidth to receive the two frequencies without much loss of signal.
 

Mikebits

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The problem I see is isolating TX1 and TX2. Since this is for an RC I suspect the power levels are pretty low, so 20 to 25 dB isolation should suffice. So you might get away with using a splitter/combiner as port isolation is in the range of > 20 dB. Mini circuits make combiners. A resistive splitter would not work, you would need a balun type.
 
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dknguyen

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They are transmitting redundant data using DSSS on two channels of the 2.4GHz ISM band simultaneously. So, by definition the same antenna definately works for each channel. My question is whether or not a single antenna can be used to receive both channels simultaneously, and if is possible how much extra circuitry is needed. I'm assuming they would take the simplest, cheapest route. I know they use RF modules from another company rather than making their own and the modifications or additional circuitry required to use a single antenna for simultaneous reception of two frequencies would go a long way as to figuring out whether they would take that route.

I know for a fact that the transmitter itself is transmitting two different frequencies at the same time. There appears to be only one antenna sticking out of the transmitter, but it has a thick plastic shell so there might be more than one antenna inside. I can't tell.

I'm trying to dig past the marketing literature and terminology. Things like how all devices in the line are implied to have a certain feature even though only most of them have it. Or it's obvious some higher end devices have that feature so it's just not listed in the feature list while it's listed for lower end devices that do have it...it gets hard to tell what actually has it and what doesn't. There's other confusing things like the fact that the company refers to the number of "receivers". The word "receiver" means different things when talking about the base receiver or the satellite receivers that plugs into the base unit. FOr base units it seems to refer to the actual number of radio modules inside, while for the satellite units it seems to refer to the number of satellite units themselves. (They use the same word for the final packaged product they sell and the primary component inside, and it gets confusing since a single final product might have more than one of these components).
 
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Mikebits

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Since the rx units are frequency selective, yes you can use same antenna, but you must maintain a good impedance match. A splitter would do this but a hit on RX noise figure.
 

Mikebits

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Since the rx units are frequency selective, yes you can use same antenna, but you must maintain a good impedance match. A splitter would do this but a hit on RX noise figure.
Thinking about this more, you could put a low gain, low Noise figure amp after the antenna and before the splitter which would maintain overall Noise figure. Thus the splitter would add no significant noise figure.

Like so. Antenna----->low noise Amp---->splitter---=rx1 and rx2

As seen from noise figure formula. First stage has most impact on overall noise.

 
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dknguyen

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So it is possible then?

Well I've looked at photos inside the receiver and it seems that each radio board is connected to one and only one antenna. I know the radio boards can only handle one frequency to begin with so it looks like it's just one frequency per antenna.

Each antenna's length also seems to correspond to 1/4 wave. Before, I wasn't sure if the two antennas were just two poles of a half wave dipole antenna. But since each antenna it connected to it's own board, I guess it's using 1/4 wave antennas.

They do have a few new receivers though that have seem to have more antennas than radio modules though so they might be doing somethign new there...multiple antennas for the same radio module though I would think that might mess with phase. Or my impression of the number of radio modules is wrong.
 
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Mikebits

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For RX the antenna length should not be that big of a deal. I am sure the frequency is close enough that the loss would be insignificant. Maybe you can make antenna length the average of the two. After all, FM receivers use one antenna and covers 88 to 108 MHz.
 

dknguyen

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Well, I'm not trying to make anything. I'm just trying to dig past their marketing specs.
 

Mikebits

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Well, I'm not trying to make anything. I'm just trying to dig past their marketing specs.
In other words, you question their claims? I would think it is possible and valid.
 

dknguyen

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No, I don't question their claims, but their product specs aren't listed very consistently so it's hard to tell what actually has what.

Some things have certain features listed while others things obviously have those features but they aren't listed. For example, the redundant transmission feature- not all the receivers that do it have it listed as a feature. THey have two things- different polarity antennas (they call it MultLink) and dual-channel redundant transmission (DualLink). It is almost implied by the advertising that each channel has more than one receiving antenna arranged in different orientations to cover multiple polarities. Though this seems to be true for the most part, some receivers seem to only have enough antennas to receive one frequency at one polarity- so that if the first channel is being jammed, but the second channel is out of view because it is at the wrong orientation, you just lost connection with your plane.

That means in certain orientations during flight, you are effectively flying without a redundant backup channel.
 
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Tesla23

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Most radio manufacturers would know how to drive multiple receivers from a single antenna (it's basically what Mike says). Are you sure it isn't a diversity receiver.
 

Mikebits

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Most radio manufacturers would know how to drive multiple receivers from a single antenna (it's basically what Mike says). Are you sure it isn't a diversity receiver.
A diversity system requires two antenna's. Dk is talking about just one antenna and two receivers.
 

dknguyen

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Most radio manufacturers would know how to drive multiple receivers from a single antenna (it's basically what Mike says). Are you sure it isn't a diversity receiver.
Please see my previous post which should answer this. It is a diversity system, but each "module" often has two parallel antennas right beside each other on the receiver. Between the two antennas on each module, they are providing neither diversity from orientation or blocking. The diversity is provided by using multiple modules (though one module actually has two antennas that are mounted 90 degrees to each other and this was the one that made me start to wonder whether it was receiving two signals at two different polarities each, or two signals at only one polarity each which is dependent on whether the antenna is able to simultaneously receive multiple signals).
 
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Mikebits

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Please see my previous post which should answer this. It is a diversity system, but each "module" often has two parallel antennas right beside each other on the receiver. They are providing neither diversity from orientation or blocking between themselves. Multiple modules are used to provide diversity.
Gadzooks, make up yer mind.

Okay, so I know that a single antenna can be designed to transmit/receive multiple frequencies.
So is it a single antenna or multiple?
 

Tesla23

Member
Please see my previous post which should answer this. It is a diversity system, but each "module" often has two parallel antennas right beside each other on the receiver. They are providing neither diversity from orientation or blocking between themselves. Multiple modules are used to provide diversity.
Well for a diversity system you do need parallel receive channels up to the point where the signals are combined. It is possible to get some benefit simply using a switch at the antennas, but you generally gain more using parallel receive channels.

So the answer is no, you can't implement a spatial diversity system with one antenna. You can often get a similar performance using polarization diversity which may have a dual polarized antenna (may be more compact than separate antennas), but apart from that the system is similar.

Depending on the propagation conditions, diversity can give you many dBs of gain.
 

dknguyen

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I think they didn't bother to add extra circuitry to use the same antenna to simultaenously receive two signals (now that I know it is possible and from looking at some of the receiver PCBs). They needed to have 2 radio modules anyways for dual frequency redundancy, and they either had the choice of extra circuitry (so they could use just one antenna), or extra wire (an extra antenna). I guess they went with the piece of wire.

Oh yeah, I've been talking about wire antennas the whole time. None of those special dual polarity ones.

What initially sparked this question (in case it's gotten lost), was the receiver had two antennas- each oriented 90 degrees to each other. I knew it transmitted on two frequencies, but I was not sure whether it was receiving one frequency per antenna (Channel A at Polarity X and CHannel B at Polarity Y) or if it was receiving two frequencies per antenna (Channel A at Polarity X&Y and Channel B at Polarity X&Y).
 
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