Lesson 27 Flashcards#
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1. In the L25 rule \(\sigma = A_\text{geo}\times\Gamma\times D\), which knob does RAM attack?
Reflectivity \(\Gamma\) — RAM converts incident wave energy to heat instead of echo. Shaping owns directivity \(D\).
2. Where on an airframe does RAM go, and why there?
Where shaping ran out of leverage: edges, inlet lips, seams, and cavity throats. Shaping buys the orders of magnitude; RAM buys the stubborn last dBs.
3. What is a Salisbury screen?
A resistive sheet of about \(377\ \Omega/\text{sq}\) spaced a quarter-wavelength above a metal ground plane, backed by a shorted air spacer. The sheet is matched to the impedance of free space, \(Z_0 = 377\ \Omega\).
4. Why does a quarter-wave absorber null the echo at its design frequency?
The wave reflecting off the back skin travels an extra half-wavelength round trip through the \(d = \lambda_0/4\) spacer, so it returns \(180^\circ\) out of phase with the front reflection. The two cancel at \(f_0\).
5. State the quarter-wave sizing formula and the depths it gives at 10 GHz and 1 GHz.
\(d = c/(4 f_0)\): \(7.5\) mm at 10 GHz, \(75\) mm at 1 GHz. Low-frequency RAM gets thick, hence heavy.
6. At what frequencies do a quarter-wave screen's nulls recur, and what is its \(-10\) dB bandwidth?
Nulls recur at the odd harmonics \(f_0, 3f_0, 5f_0\) (where \(\tan(\beta d)\to\infty\)). The \(-10\) dB (90% absorbed) bandwidth is about \(\pm37\%\) of \(f_0\).
7. Why do practical broadband LO treatments avoid a single resonant layer?
One notch defends one band while an IADS spans VHF to \(K_u\). Bandwidth has to be bought with thickness, added loss mechanisms, and weight — no single resonant layer covers the whole threat spectrum.
8. Name three broadband RAM families and their trade-offs.
Jaumann (stacked graded resistive sheets — wider notch but thicker); magnetic / iron-loaded (thin and broadband but heavy, the classic LO skin); structural RAM/RAS (absorber built into load-bearing skins and edge wedges). Bandwidth, thickness, weight — pick two.
9. How does polarization betray an LO aircraft?
Long conductors and edges scatter strongest in the polarization parallel to them, and trihedral corners flip circular-polarization sense — tells that a cross-polarized (polarization-diverse) radar can deliberately flag.
10. What is the design response to polarization tells?
Avoid long straight conductors, and orient or serrate the discontinuities you cannot remove so no single polarization finds a clean target — planform alignment's quiet cousin.
11. State the intercept asymmetry with its range dependence.
A passive RWR hears the emitter one-way, \(P_\text{RWR}\propto 1/R^2\); the radar needs the two-way echo, \(P_\text{echo}\propto \sigma/R^4\) with small \(\sigma\). The listener detects the transmission far beyond the radar's own detection range.
12. Why is EMCON a survivability discipline for an LO aircraft?
No coating absorbs a wave you chose to transmit. The one-way asymmetry means the enemy hears your radar before you see their echo, so you must radiate seldom and smart — whoever transmits first, telegraphs first.
13. What LPI techniques fight the intercept asymmetry?
Power management, frequency agility, and wide instantaneous bandwidth at low peak power — or refuse the duel and stay passive using an infrared search-and-track (L23) and offboard cues.
14. In the demo, what happens when the spacer depth doubles from 7.5 mm to 15 mm?
The \(X\)-band null moves; nulls appear at 5 and 15 GHz (odd harmonics), handing \(X\)-band back where fire-control threats live. Coverage stays about 47% of 2–18 GHz — broader is not free.
15. What is the one-line takeaway about resonant RAM?
A resonant absorber is a promise kept at exactly one frequency; broadband absorption costs thickness and weight.