Lesson 2 Flashcards#
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1. Relate wavelength and frequency.
\(\lambda f = c\), with \(c \approx 3\times10^{8}\) m/s. Radar shortcut: \(\lambda \approx 30/f\) cm for \(f\) in GHz.
2. What is the wavelength at X-band (10 GHz)?
About 3 cm (\(30/10\)). X-band is the dominant fire-control band.
3. List the IEEE radar bands from L to Ka with rough frequencies.
L (1–2 GHz), S (2–4), C (4–8), X (8–12), Ku (12–18), K (18–27), Ka (27–40 GHz).
4. What is the tradeoff between low and high radar frequency?
Lower frequency: longer range, less weather loss, but large antennas and coarse resolution. Higher frequency: fine resolution and small apertures, but more atmospheric attenuation.
5. Why do fire-control radars cluster around X-band?
X-band balances good resolution against manageable antenna size and acceptable atmospheric loss — the sweet spot for tracking and engaging aircraft.
6. Write the free-space path loss formula (km, GHz).
\(L_\text{fs,dB} = 20\log_{10}(R_\text{km}) + 20\log_{10}(f_\text{GHz}) + 92.45\).
7. State the two 6 dB rules.
Doubling the range adds 6 dB of loss; doubling the frequency adds 6 dB of loss. Both come from \(20\log_{10}(2)\approx 6\) dB.
8. How much loss does a target add by flying from 50 km to 200 km?
50→200 km is two doublings (×4), so +12 dB of one-way path loss.
9. What does polarization mismatch cost?
A full cross-polarization mismatch (e.g., horizontal into a vertical antenna) can cost 20–30 dB. Polarization is both a target-RCS factor and an EP/EA lever.
10. Relate antenna gain to aperture and wavelength.
\(G \approx 4\pi A_e / \lambda^2\). For a fixed aperture, shorter wavelength gives higher gain and a narrower beam.
11. Estimate the half-power beamwidth of an aperture.
\(\theta_\text{BW} \approx \lambda / D\), where \(D\) is the aperture dimension. Higher frequency → tighter beam from the same dish.
12. Why do missile seekers push into Ku/Ka band?
High frequency buys high gain and fine angular resolution from a small aperture — exactly what a size-limited seeker needs, accepting the higher atmospheric loss over short ranges.