Lesson 6 Flashcards#
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1. Name the three features of an antenna pattern.
Main lobe (peak gain, where the radar looks), side lobes (unintended sensitivity off boresight), and the back lobe (small lobe pointing roughly opposite the main lobe).
2. Why are side lobes called "the back door for EW"?
Energy arriving through a side lobe still reaches the receiver. A jammer nowhere near the main beam can inject energy through a side lobe and degrade the radar.
3. What does HPBW measure, and where is it taken?
Half-power beamwidth: the angular width between the two points where gain falls 3 dB below the peak. It quantifies how tightly the beam is focused.
4. What is the first side-lobe level of a uniformly illuminated aperture?
About \(-13.2\) dB relative to the main lobe — the textbook constant for uniform illumination.
5. What does tapering the illumination buy, and what does it cost?
It lowers the side lobes (good for EP) at the cost of a wider main beam (larger HPBW). Lower SLL trades against beamwidth — no free lunch.
6. State the gain and beamwidth rules of thumb for an aperture of size \(D\).
\(\theta_{\text{HPBW}} \approx 70\,\lambda/D\) degrees, and \(G \approx 30{,}000/(\theta_{\text{az}}\cdot\theta_{\text{el}})\). Bigger aperture in wavelengths → narrower beam and higher gain.
7. How does an array steer its beam to angle \(\theta_s\)?
By applying a progressive phase step \(\Delta\phi = (2\pi/\lambda)\,d\,\sin\theta_s\) across the elements. The contributions add in phase at \(\theta_s\), so the main lobe points there — no moving parts.
8. What are grating lobes, and how are they avoided?
Full-strength copies of the main lobe in unwanted directions, appearing when element spacing \(d > \lambda/2\) (the angular-domain Nyquist limit). Keep \(d \le \lambda/2\) to suppress them at broadside.
9. Difference between an ESA and an AESA?
An ESA steers the beam with phase shifters. An AESA (active ESA) gives every element its own transmit/receive module.
10. List three operational advantages of an AESA.
Microsecond beam pointing (interleave search and track), graceful degradation (dead modules cost a little gain, not the radar), and multiple simultaneous beams. (Also: LPI waveforms from precise phase control.)
11. What happens to an array steered far off boresight?
Effective aperture shrinks as \(\cos\theta_s\) and HPBW broadens as \(1/\cos\theta_s\). Steering to \(60^\circ\) doubles the HPBW and costs about 3 dB of gain.
12. Name two EP techniques that close the side-lobe back door.
Side-lobe cancellation (an auxiliary antenna subtracts the side-lobe signal) and side-lobe blanking (an omni reference rejects strong pulses outside the main beam). Low-SLL (tapered) antenna design also helps.