# Why is the total radiated power of array calculated by sum of square of excitation coefficients different from the result by integral of power pattern

Consider an array with same isotropic antennas, $$P_{ele}^n$$ is the input power of the nth antenna. As I consider antenna radiation efficiency = 1, the $$P_{ele}^n$$ is also the radiated power of the nth antenna. If the antenna is matched, this normalized power input is proportional to the square of the input signals, or (in a normalized form) $$P_{ele}^n=|w_n|^2\tag{1}$$ where the $$w_n$$ is the complex weight of the nth antenna. Then, the radiated power for the whole array is given by the sum of the excitation coefficients at each antenna, $$P_{rad}=\sum_{n}P_{ele}^n=\sum_{n}|w_n|^2\tag{2}$$

Meanwhile, as mentioned in , , the total radiated power can be calculated by the integral of the radiated intensity and the square of beampattern is proportional to the radiated intensity. Therefore, the beampattern can be used to calculated the total radiated power, $$P_{rad}=\int |B(\theta,\phi)|^2d\Omega=\int_{0}^{2\pi}\int_{0}^{\pi}|\mathbf{w}^H\mathbf{a}(\theta,\phi)|^2\sin\theta d\theta d\phi\tag{3}\label{eq3}$$ where $$B(\theta,\phi)$$ is the beampattern, $$\mathbf{w}\in \mathbb{C}$$ is the weight, $$\mathbf{a}(\theta,\phi)$$ is the array manifold, $$\theta$$, $$\phi$$ are azimuth and elvation angles, and $$d\Omega=\sin\theta d\theta d\phi$$ is the infinitesimal solid angle.

However, the simulation results im MATLAB of this two calculations are not equal. For a half-wavelength spaced 64-antenna URA (uniform rectangular array), with uniform beamformers applied ($$|w_n|=1$$), the total radiated can be calculated as $$P_{rad}= \sum_{n}|w_n|^2=64 W\tag{4}$$ However, the result based on \eqref{eq3} is much larger than 64$$W$$. Then, I thought that the excitation coefficients in calculating beampattern $$B(\theta,\phi)$$ should be $$\mathbf{w^{\prime}}=\frac{1}{2\sqrt{\pi}}(w_1,...,w_{64})$$, as each isotropic antenna radiates equally in angular space. Therefore, each isotrropic antenna has $$\frac{1}{4\pi}$$ power intensity, and $$\frac{1}{2\sqrt{\pi}}$$ field intensity, which indicates $$\mathbf{w^{\prime}}=\frac{1}{2\sqrt{\pi}}(w_1,...,w_{64})$$. But the result is still not equal to the 64$$W$$.

I don't know why these two results are not the same. Is there a scaling factor that I miss? Or I just calclulated wrong? If the scaling factor exists, then why the second calculation does not guarantee energy conservation?

Reference

Balanis, Constantine A. Antenna theory: analysis and design. John wiley & sons, 2015.

Element and Array Radiation and Response Patterns

• What is $B(\theta,\phi)$ - the usual expression is something like ${sin({N\over2}\Psi)}/{sin({{\Psi}\over{2}})}$ where $\Psi=kdcos(\phi)$ It won't depend on $\theta$ if the elements are omnidirectional. How are you integrating it? Sep 23 at 16:04
• i know, it was a joke because of all the hieroglyphs Sep 23 at 20:13
• That's known as calculus. ;) Sep 23 at 20:16
• @tyrela OK but is $\int{\int{B}}d{\theta}d{\phi} = 1$ ? Otherwise that would bias the array pattern too. What else is there to go wrong? Sep 24 at 3:02
• @tomnexus I expected the consequence of the integration would be $\int{\int{|B|^2\sin \theta}}d{\theta}d{\phi} = 4\pi \times P_{rad}=64\times 4\pi$, which means $\frac{1}{4\pi}\int{\int{|B|^2\sin \theta}}d{\theta}d{\phi} = P_{rad}=64$, where $|w_1|=...=|w_{64}|=1$. But, the result is not what I expected. Do you mean the integration of one element power pattern should be 1? Which is $\int_{0}^{2\pi}{\int_{0}^{\pi}{\frac{1}{4\pi}\sin \theta}}d{\theta}d{\phi}=1$. $\frac{1}{4\pi}$ reprents the radiation intensity of a isotropic antenna with $1W$ radiated power over entire solid angle of $4\pi$. Sep 24 at 3:35

It's all to do with mutuals, and you can't wish them away - even for 'ideal' antennas.

Consider a simple case, two elements a reasonable distance apart. Give them excitation amplitudes +1 and -1. From your assumptions, the radiated power is 2, irrespective of their locations. The power is no longer radiated isotropically, there is some array pattern, but you assume it integrates to 2 (this is unlikely to be exactly true, but I won't argue at this point).

Now consider the case when you move them closer together, $$\lambda/2$$, $$\lambda/4$$, $$\lambda/8$$ ... As they get closer, their far fields start to cancel more and more over more and more solid angle, and so the radiated power, as measured in the far field reduces - but from your assumption the radiated power is still 2. When they are very close, you still think you are radiating 2, but the far fields are very small. Something is very wrong with your model.

So this clearly demonstrates that with two elements with amplitudes +1 and -1, you have $$|a_n|=1$$, but the radiated power is not constant at 2, but depends on the spacing between the elements.

Now let's consider a more realistic situation, where the antennas have terminals, and the radiation amplitude of each element is proportional to the current flowing into the terminal (e.g. a dipole), but the power radiated by an element is not simply proportional to the square of the current (independent of other elements), but equal to the product of the voltage and current. When I have two elements, then:

$$v_1 = z_{11} i_1 + z_{12} i_2$$

$$v_2 = z_{21} i_1 + z_{22} i_2$$

What you will find is that when the elements get very close that $$z_{12} \rightarrow z_{11}$$ (look at the mutual impedances of dipoles for example). So then what you find is that if $$i_1 = 1$$ and $$i_2 = -1$$ then

$$v_1 = z_{11} - z_{12} \rightarrow 0$$ as the spacing tends to 0

When the fields tend to cancel in the far field, this is reflected in the input impedance that no power can enter the array.

So for the initial comment where the two elements were 'reasonably spaced', unless $$z_{12}=0$$, then the presence of the second driven element will affect the radiated power from the first and so the radiated power is unlikely to be 2. The same effects occur for N elements, just the maths is messier, so mutuals means that you can't assume that what you think is unity element excitation will achieve unity radiated power.

So to bring it back to your question, you are assuming that by maintaining the element excitation amplitudes at unity $$|a_n|=1$$, that the power radiated by each element is also 1. This is not true for two elements as my example above shows, and it appears in the power equations as a mutual interaction. It is not true in general unless you assume mutual interactions are zero, which is not physically possible. If you are dealing in impedances, mutual interactions cause currents in one element alter the voltage across others, if you are dealing in scattering parameters, mutuals cause some of the power you feed to one element to come out the feedpoint of another.

• If I understand you correctly, you're saying that the problem is that the equations in the question are integrating scalar quantities, but the integrations should be of vectors, which don't sum like scalars. Is my understanding correct? Sep 30 at 23:39
• No, the problem is not with your maths, it's with your physics. Antennas that radiate to the same far field locations, but that don't interact locally, don't exist. You can't assume that the far field amplitude contributed by an element is related to the power radiated by the element, independent of of other elements in the array. Work through my simple example. Sep 30 at 23:59
• Really thanks! I get it now why even without mutual interaction, the integration of radiated intensity won't be equal to the sum of weights. I didn't consider the signal cancellation in far field before. Thanks again! Oct 1 at 13:29

Consider a simpler problem: Stack two dipoles at 0.01 wavelength separation. Connect the endpoints and feed power to one of the dipoles and short the feed-point of the other. What you then have is a folded dipole. Impedance 300 ohms. Then remove the short and feed both feed-points. The impedance of each one will be 150 ohms. They are series connected on a line at a separation of 1 wavelength. Current is the same at both feed points but voltage is half. In the first case you would have to supply 300 V to get 1A and 300 W power. In the second case 150V at two places to get 1A at each place
for a total power of 300W. The radiation pattern will be exactly the same and the current in both elements will be 1 A in both cases. If you disconnect the endpoints there will be no difference, just a small length adjustment might be needed. You might also put both elements in parallel all the way along and then there will be just a single feed point with 75 ohms which you would need to feed with 150V to get 2A and 300W power and still the same pattern.

Mutual coupling causes impedance changes. That has to be taken into account.

• This is true, but I think the question is about an ideal uniform array, with current as specified by Wn, so no need to worry about coupling etc. Sep 29 at 4:19
• yes, as @tomnexus mentioned, this is a question about ideal uniform array. So I think all the elements have unit impedance and the $U^2$ or $I^2$ represents the power. $\mathbf{a}(\theta,\phi)$ is the phase difference of each element towards $(\theta,\phi)$ in far-field. $\mathbf{w}$ is the weight used to compensated the phase difference and steer the array beam towards direction $(\theta,\phi)$ in far-field. And the magnitude of the weight on each element is the current, which is also the power, as unit impedance is applied. Sep 29 at 12:29
• Basically, my question is, in ideal uniform array, Is there difference between sum of modulus square of weight on each element and the integration of radiated intensity (array factor is used to represent the radiated intensity) over the angular space? As mentioned in (3), they use a scaling factor $\mathbf{C}$ to make sure two calculations are equal. Sep 29 at 12:39
• When you stack 8x8 isotropic antennas at 0.5 wl and force the currents of all elements to become equal, the mutual coupling between the elements will make the voltages different at the different feedpoints. You will need to compute the impedances to get the voltages in order to compute power. I suggest you verify theory by computing for two isotropic radiators stacked at 0,1 wavelengths. Then try 20 wavelengths. Try to vary the phase between the currents and see how the discrepancy you have noted will vary very much as the impedance at the feedpoints vary due to the mutual interaction. Sep 29 at 13:51
• I wonder if it would be good to rephrase your answer with fewer questions in it. If you have questions for the OP, it would be better to include those as comments to the original question rather than using an answer to ask them. :) :) Oct 1 at 15:41