effect of scan angle in phased array?

I asked this question before and really didn't get a decent answer. My question is straightforward:

1. Why would I pick a phase of 45 degrees vs 90 degrees, for example?

2. Does a 45 degree have a narrower beam than a 90 degrees phase? If so, when would I need a narrower beam than a wider beam?

3. If I have a 5-element array, can I have non-symmetrical phases for each of the element, i.e.

element 1 = 25 degrees
element 2 = 50 degrees
element 3 = 60 degrees
element 4 = 80 degrees
element 5 = 90 degrees


Please some straight to the point answers.

Thanks all.

kae_jolie, an answer is pretty hard to come up with when we don't have a complete picture of what your environment or array situation is ...


1. What are you applying the different phase signals too? 1/4 wave vertical antennas spaced what, maybe 1/4 wave apart?

2. Question does not quite make sense. Must know the array spacing/elements to make a call.

3. How is the array spaced or what is the array: Vertical elements all in a line? Star-pattern? Four-square? Waveguide slots?

With more info, more people might be inclined to offer advice or an opinion.

Regards,
RF_Jim

Thanks RF_Jim for your reply. To answer your question:

Please see attached image of three 1/2 wave dipole antenna elements. They are placed horizontally in a hemispherical shape. The distance between them is about 3cm apart. The wavelength is ~33 cm for 915MHz center frequency.

I assume this is some sort of homework question rather than an engineering task you are trying to do. You have two options here:

1. Simulate it using some software - probably the quickest from an engineering point of view.
2. Calculate it analytically - more useful, and probably what is needed for your homework.

At first, I thought the second would be difficult, but for reasons I don't know, I went to look up in a book how one might do it. Whilst not trivial, it is not as hard as I first thought it would be. The calculation can be achieved by pattern multiplication. See "Antenna for All Applications", (Kraus and Merhefka, 3rd edition, 2003), where it says:

In general, the field pattern E(θ,Φ) of a volume array as a function of θ and Φ is:

E(θ,Φ) = Es(θ,Φ) Ex(θ,Φ) Ey(θ,Φ) Ez(θ,Φ)

where:

Es(θ,Φ) is the pattern of a single element.
Ex(θ,Φ) is the pattern of a linear array of point sources in the x direction
Ey(θ,Φ) is the pattern of a linear array of point sources in the y direction
Ez(θ,Φ) is the pattern of a linear array of point sources in the z direction

The pattern of linear array of N point sources in one dimension is easy to find. The total field E from a linear array of point sources at a large distance in the direction Φ is given by

E = sin(N Ψ/2)/(N sin(Ψ/2))

where Ψ is the total phase difference of the fields from adjacent sources - i.e.

Ψ = 2 π d cos(Φ)/λ + δ

where δ is the phase difference between adjacent sources.

The field from a dipole is easy to find, and is independent of Φ,

Es(θ,Φ) = cos((π/2)cos(θ))/sin(θ)

So it looks to me that with some care with the maths, it would be possible to predict this all analytically.

For non-symmetical situations, it might become more difficult. Also, the spacing between your antennas (around λ/10 I think), would change the characteristics of the antennas individually, so perhaps computer simulation is going to be your only possible method to analyse this.

Dave

Thanks for detailed answer. Actually I am using simulation for this, but was just wondering if you could answer this question only:

Does a 45 degree have a narrower beam than a 90 degrees phase? If so, when would I need a narrower beam than a wider beam?

To my knowledge, for a structure like you show, there is no easy way to know whether 45°, 90° or some other angle will have a wider or narrower beam. You will have to simulate it and see. I'm not sure how useful this is going to be to you though. Long term, the knowledge gained is not much use, as it will vary from setup to setup, and what you have is not simple.

You want a narrower beam when you require higher gain in one direction, and are happy to sacrifice gain in others. There is no free lunch. The antenna will only radiate the power it is fed (ignoring losses), so if you make the beam narrower, you must reduce the radiation in other directions.

What about in a linear standard array structure? In theory, the smaller the scan angle, the narrower the beam?


related question:

Definition: A phased array forms multiple beams in different directions at the same time.

Question1: I think the phrase "at the same time" is misleading. Phase array should be scanning within split second, but it's not exactly same time, correct?

Question2: For a non-phased array, would the beam be fixed in one direction only? Just need confirmation.

I don't understand your definition of a phased array - it is not one I'd use.

To answer your second question, no a non-phased array would not have the beam fixed in one direction. A dipole is a pretty good example of that. Make it a half-wave and it has two clear peaks in beam direction. Make the frequency much higher and there will be many peaks. You will change the direction of the peak too. This is just a simple dipole - not a phase array.

If non-phased array would not have the beam fixed in one direction, then what's the difference between phased and non-phased arrays?

If I were you, I'd look at the IEEE definitions. But I guess if there are two or more antennas fed from a single source, it must be a phased array.

I'm not sure if one would consider splitting power between a Yagi and a long-wire to be a phased array or not. But one is not likely to want to do that. But if you did it, you could control the overall radiation pattern by changing the phase between the two signals.

I suggest you take a look at the IEEE standard definitions of antenna terms. Once you have done that, then I'd look at a section on phased arrays in any professional antenna book - Kraus, Balanis or similar.