Excitation Current vs Side Lobe Level

I'm taking an intro to radar class, and we are developing models and simulations for an antenna array system in matlab. My understanding of this subject is a bit limited, but we are using the Dolph-Chebyshev Polynomial array method. We are only using odd number of array elements, and we I'm plotting the I0 current vs varying levels of Side Lobes. I was hoping you could look at my link below of the graph plotted from my matlab function, and tell me if it makes sense. To me, it seems to be wrong, because I would expect an increase in SLL as the excitation current becomes greater. Follow the link below to see the graph that I made.

imgur: the simple image sharer

Thanks for your help.

Edit: I probably should have stated that for the purpose of this project, we are only concerning ourselves with the first side lobe.
Edit2: We are also only concerned with magnitudes not phase.

Plot both your side lobe level and excitation current in log-scale (i.e. dB's vs. dB's). Since you have a simple, linear system (current produces an E-field), I'd expect a linear relationship between the input current and the output power. I think you're close.

Remember, the power from the antenna is in watts, so that's 10*log10(power in watts), but current is amps, so you'll need to take 20*log10(current in amps) to be consistent.

Also, most stuff in the radio/RF world is done in dBm (decibels referenced to 1 milliwatt), so use SLL (dBm) = 10*log10(Pout)+30.

Hi ECE12,

please clarify what is I0 current.
Regards

Z

Okay I'll give that a shot. Thanks for the heads up. But why would the sidelobe be smaller as the excitation current gets bigger? I don't really understand that concept. I thought a larger excitation current would cause a larger E&M field, and thus cause a larger side lobe. Maybe I'm not thinking about it correctly. My program is generating the correct output for the test case my professor provided, but I am wanting to make sure it is accurate for various SLL and # of elements because the program is having to do some pretty serious (what I consider serious) number crunching. Thanks for the response...






I0 would be the central array element. So for a 7 element array system, I would have I3-I2-I1-I0-I1-I2-I3.
I believe I0 will always be the largest current, because I'm using the cheybshev polynomial the excitations of the other array elements are symmetric to I0. When you find I0 - I3 you setup a ratio: I0:I1:I2:I3, and this ratio can then be used to determine the excitation of the antenna array for a given I0 excitation. That is how I understand it anyway. I emailed my professor a couple of questions about it, because I don't know if I0 will always be the largest excitation, but I haven't heard back from him...

---------- Post added at 03:34 ---------- Previous post was at 03:22 ----------

Okay. Does this look correct/more correct?

imgur: the simple image sharer Probably need to disregard the labels on the axis as far as units, but SLL and I0 are on Y and X respectively. Thanks

I took my vector of SLL = -15:-2:-45 and used the 10*log10(SLL) to convert it
I took my currents(i) vector and did 20*log10(current(i)) and plotted them against each other.
So am I correct with my logic that the SLL increases as the excitation current increases?

That new image looks much better. You are correct. If you put more current (more energy) into the antenna structure, then your field strength will increase (H = Amps/meter... more amps in same physical structure yields higher field strength), and the higher the field strength, the higher the output power.

You might do a few more points around -4 and -3 on I0, to see what's really happening at the knee of the curve.

Right. The central element should have the max I.

So, the current ratios are fixed and when you change I0 all the others (I1,... etc.) change in the same proportion?
If this is the case, then SLL should be independent of I0. As usual, I call SLL the ratio between the main lobe and the highest secondary lobe. This is a parameter of the Dolph-Chebyshev synthesis.

Rregards

Z