USAFA DFEC

ECE 495 — Electromagnetic Warfare and Survivability

Block 3: IR EW and Signature Management
Lesson 27: RAM, Polarization, and EMCON
Juan Jurado, Col, USAF, Ph.D.
Permanent Professor and Head
Electrical and Computer Engineering
Fall 2026
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Block 3 · Lesson 27 · Where We Left Off

Where We Left Off

L26 handed out the verdicts: shape what you can, mask the cavities, manage the seams — and left the edges and antennas to be treated.

Today we pay those debts. How do materials absorb what shaping could not redirect — and what is left to hide after that?

Today
Absorbers on the edges shaping could not fix, the tells polarization leaves behind, and the one emission no coating can touch.
The loudest thing on a stealth aircraft is its own transmitter.

Block 3 · Lesson 27 · Objectives

Lesson Objectives

By the end of this lesson, you will be able to:

  • Explain how RAM attacks the reflectivity knob $\Gamma$ and where it goes on an airframe
  • Use quarter-wave cancellation to size a resonant absorber and its notch
  • Describe how polarization betrays edges and wires, and the design response
  • Argue EMCON from the intercept asymmetry: one-way $1/R^2$ beats two-way $\sigma/R^4$

Block 3 · Lesson 27 · Part One

Absorbing What Shaping Could Not Redirect

Shaping bought the orders of magnitude. Materials buy the stubborn last decibels — and they charge for every one of them.

Block 3 · Lesson 27 · RAM

RAM: Buying Down $\Gamma$

Shaping chose where energy goes; RAM decides how much survives the trip.

  • In L25's rule, RAM attacks $\Gamma$ — convert the wave to heat instead of echo
  • It goes where shaping ran out: edges, inlet lips, seams, and cavity throats
  • Absorbers trade bandwidth, thickness, and weight — you may pick two
  • RAM finishes the budget: shaping bought the orders of magnitude, RAM buys the stubborn last dBs
RAM is a finishing tool, never a substitute for good planform.

Block 3 · Lesson 27 · Light Rule

The Resonant Trick: Quarter-Wave Cancellation

Put a resistive sheet a quarter wavelength above the metal skin:

Quarter-wave cancellation
$$d \;=\; \frac{\lambda_0}{4} \;=\; \frac{c}{4 f_0}$$
  • Front and rear reflections return $180^\circ$ apart — and cancel
  • Off frequency, the phase geometry breaks and the echo comes back
  • The catch is in the formula: $d$ is fixed hardware, so the null lives at one $\lambda_0$
  • At 10 GHz, $d$ = 7.5 mm; at 1 GHz, 75 mm — low-band RAM gets thick fast
A resonant absorber is a promise kept at exactly one frequency.

Block 3 · Lesson 27 · The Notch

One Screen, One Promise

A 7.5 mm spacer nulls at 10 GHz and holds $-10$ dB from 6.3 to 13.7 GHz — a notch sitting squarely on $X$-band.

One layer, 47% of 2–18 GHz protected. Double the spacer and what happens?

Block 3 · Lesson 27 · The Notch

Double the Depth, Move the Promise

15 mm halves $f_0$ to 5 GHz, and the third harmonic at 15 GHz slides in from above. Two windows, each half as wide — and $X$-band handed straight back.

Two windows now, where there was one. Read the two sub-bands off the plot — is that more protection than the 7.5 mm screen bought, or the same amount moved somewhere else?

Twice the thickness. And X-band is bare.

Block 3 · Lesson 27 · Bandwidth

The Broadband Problem

The threat does not transmit at your design frequency as a courtesy:

  • An IADS spans VHF to $K_u$ — a single notch defends one band and betrays the rest
  • Layered (Jaumann) absorbers stack graded sheets: wider notch, more thickness
  • Magnetic RAM (iron-loaded coatings) absorbs broadband and thin — but heavy
  • Structural RAM builds the absorber into the skins and edge wedges themselves
  • Every choice re-runs L26's budget: dBs per pound, per inch, per maintenance hour
At VHF the quarter-wave depth is meters. That is not a materials problem any longer — it is a regime problem, and materials cannot fix it.

Block 3 · Lesson 27 · Polarization

Polarization: the Tell in the Echo

The echo carries orientation information — and edges talk too much.

Long conductors Scatter strongest in the polarization parallel to them
Corners Flip circular polarization — radars use that to reject rain
Cross-pol channel Hunts the LO features the co-pol channel misses
Design response: no long straight conductors; orient and serrate the discontinuities — planform alignment's quiet cousin.

Block 3 · Lesson 27 · Part Two

The Signature You Choose to Radiate

Every trick so far shrinks an echo somebody else caused. The last one is entirely yours.

Block 3 · Lesson 27 · EMCON

The Signature You Choose to Radiate

After shaping, masking, and RAM, one megawatt-class contributor remains.

  • The fire-control radar, the datalinks, the altimeter — every emission is a beacon with your name on it
  • B2's ES lesson returns: their receivers are patient, passive, and everywhere
  • No coating absorbs a wave you chose to transmit
  • So the last LO discipline is emissions control: radiate seldom, radiate smart
Everything before this slide was about an echo. This one is about a transmission.

Block 3 · Lesson 27 · Light Rule

The Asymmetry: Why the Listener Wins

The physics is rigged in the receiver's favor:

Intercept asymmetry
$$\underbrace{P_\text{RWR} \propto \frac{1}{R^{2}}}_{\text{they hear you}} \qquad\qquad \underbrace{P_\text{echo} \propto \frac{\sigma}{R^{4}}}_{\text{you see them}}$$
  • Your signal reaches the enemy RWR one way; your echo pays the round trip and their small $\sigma$
  • Result: they detect your radar far beyond where your radar detects them
  • LPI fights back — manage power, hop frequencies, spread the waveform thin
  • Or refuse the duel: stay passive, let the IRST (L23) and offboard cues do the finding
Whoever transmits first, telegraphs first.

Block 3 · Lesson 27 · Looking Ahead

Where This Is Headed: Every Other Signature

The RF story is told. But the aircraft is still seen, heard, and felt.

  • Visual: contrails, glint, and paint schemes — the oldest sensor is the human eye
  • Acoustic: engines and airflow announce low-altitude penetrators
  • EO/IR: L22's five components never left — they fuse with everything above
  • L28 closes the loop: multi-spectral fusion, and the rule that you are only as quiet as your loudest band

Block 3 · Lesson 27 · Quick Exercise

Quick Exercise / Type-along: Move the Notch

Type this with me — the 15 mm screen, twice the $X$-band depth.

c = 3e8; Z0 = 377; Rs = 377;         % free space, matched sheet
d = 15e-3;                           % spacer depth [m]
f = linspace(2e9, 18e9, 3201);       % the band the threat picks
f0 = c/(4*d);                        % quarter-wave null
nulls = f0*(1:2:7); nulls = nulls(nulls <= 18e9);
Zs  = 1j*Z0*tan(2*pi*f/c*d);         % shorted air spacer
Zin = (Rs*Zs)./(Rs + Zs);            % resistive sheet in parallel
G   = 20*log10(abs((Zin - Z0)./(Zin + Z0)));
b   = G <= -10;                      % the 90%-absorbed window
lo  = f(diff([0 b]) ==  1)/1e9;
hi  = f(diff([b 0]) == -1)/1e9;
fprintf('d=%g mm: nulls at %sGHz\n', d*1e3, sprintf('%g ',nulls/1e9));
fprintf('protected: %sGHz\n', sprintf('%.1f-%.1f  ', [lo; hi]));
fprintf('at 8 GHz: d = c/(4f) = %.3f mm\n', c/(4*8e9)*1e3);

Twice the depth. Why did the protected fraction not go up?

HW3 sizes one for 6 GHz. Thicker or thinner than the 8 GHz answer — and why?

Block 3 · Lesson 27 · Quick Problem

Quick Problem: Size the Screen

A Salisbury screen tuned to a 4 GHz acquisition radar, checked against the threat band and against a VHF early-warning radar.

  • Matched sheet, $R_s = 377\ \Omega/\text{sq}$, over an air spacer on the metal skin
  • Design frequency $f_0 = 4$ GHz
  • Threat band: 2–18 GHz
  • VHF early-warning radar: $150$ MHz
  • $c = 3\times10^8$ m/s
  1. (a) The spacer depth $d$, in mm.
  2. (b) The screen’s nulls inside the 2–18 GHz threat band.
  3. (c) The depth a 150 MHz screen would need, and how many times deeper than (a) that is.
(a) $d = 18.75$ mm  ·  (b) 4 and 12 GHz  ·  (c) 500 mm, 26.7 times deeper

Today's Interactive Demo: the Quarter-Wave Notch

Companion script: L27_QuarterWaveRAM.m
Live demo

Quarter-Wave RAM

usafa-ece.github.io/ece-495-ew

Block 3 · Lesson 27 · Wrap-Up

Wrap-Up

Key ideas
  • RAM attacks $\Gamma$: quarter-wave notches, layered and magnetic for bandwidth, structural for edges
  • $d = \lambda_0/4$ is a one-frequency promise — broadband absorption costs thickness and weight
  • Polarization betrays straight edges and corners; discipline the geometry
  • EMCON: one-way $1/R^2$ beats two-way $\sigma/R^4$ — radiate seldom and smart, or go passive
Next lesson
  • L28: Multi-Spectral Signatures — the loudest band sets the detection range
  • Work the L28 pre-flight before class — you could be the one up front
Materials and discipline buy the last dBs that shaping left on the table.