diff --git a/docs/adaptive-link.md b/docs/adaptive-link.md
index 45b7c0a..c3daa3e 100644
--- a/docs/adaptive-link.md
+++ b/docs/adaptive-link.md
@@ -67,12 +67,26 @@ bit, while a faster modulation cuts airtime — and therefore PA-on time — dir
 | Modulation / FEC (time-on-air) | strong | **strong** — less airtime, fewer Joules/bit |
 | Channel / bandwidth | strong | moderate |
 | Transmit power | **strongest** | **weak** — baseline draw dominates |
+| Active receive chains | conditional | **conditional** — see below |
 
 So the energy-minimizing reflex is to ride the highest modulation the link will
 bear (short airtime) and spend only the *minimum* power that clears it. When the
 link is strong, it backs power and FEC off and lets the amplifier idle — exactly
 where a quality-maximizer would instead spend the headroom on bitrate.
 
+**The number of active receive chains is a fade-state lever.** A multi-chain
+receiver can light up one antenna or several. Each extra chain adds always-on
+baseline draw, but combining several chains improves delivery — which, for a
+fixed video rate, means less airtime spent covering losses and the baseline
+amortised over more delivered bits. Whether that trade pays depends entirely on
+how *decorrelated* the antennas are: at a static position, closely-spaced
+antennas see nearly the same channel, so combining barely improves delivery and
+the extra chains are pure baseline cost; under motion the antennas decorrelate,
+so combining fills the fades and the added chains cheapen delivery by more than
+they cost. The energy-minimizing chain count is therefore not fixed — it collapses
+toward a single chain on a still, strong link and opens up as the link fades or
+the platform moves. It adapts to the fade state, not merely to range.
+
 **The quality floor is per-layer, not a single number.** Scalable video splits
 into a base layer (decodable alone, low frame rate) plus enhancement layers that
 refine it. The base and key frames are small but indispensable; the enhancement
diff --git a/tools/precoder/chain_count_energy_sim.py b/tools/precoder/chain_count_energy_sim.py
new file mode 100644
index 0000000..20e90bd
--- /dev/null
+++ b/tools/precoder/chain_count_energy_sim.py
@@ -0,0 +1,165 @@
+"""Active-chain count as an energy lever (spatial-diversity #135).
+
+A multi-chain receiver can light up 1..N RF chains. Each active chain adds
+always-on baseline draw (LNA + mixer + ADC), but more chains combine to improve
+delivery — which, for a fixed video bitrate, means less airtime/FEC spent
+covering losses and the baseline amortised over more delivered bits. So there is
+an optimal chain count that minimises energy per *delivered* bit, and — this is
+the point — it depends on the fade state:
+
+  * static / correlated antennas (high ρ): combining barely helps delivery, so
+    extra chains are pure baseline cost → run FEWER chains;
+  * moving / decorrelated antennas (low ρ): combining fills the fades, so extra
+    chains cheapen delivery more than their baseline costs → run MORE chains.
+
+This is the energy corollary of the measured result (RX-MRC gives ~nothing at a
+static position and eliminates dropouts under motion; see
+docs/measuring-spatial-diversity.md): a ground station should adapt its chain
+count to the fade/motion state, not just to range.
+
+The model reuses the calibrated power/airtime model (energy_model.py) and adds
+two terms: a per-chain baseline, and a diversity-improved delivery parameterised
+by the antenna correlation ρ (via the same N_eff used elsewhere).
+
+CLI:
+    uv run python chain_count_energy_sim.py                 # ρ sweep table
+    uv run python chain_count_energy_sim.py --self-test
+"""
+from __future__ import annotations
+
+import argparse
+
+from energy_model import (Calib, TxPoint, airtime_fraction, load_calibration)
+
+# Per-active-RX-chain baseline draw (W) on top of the core baseline (LO + USB +
+# baseband + one chain). LNA + mixer + ADC of an extra chain — a fraction of the
+# core floor.
+P_CHAIN_W = 0.15
+
+
+def n_eff(n_ant: int, rho: float) -> float:
+    """Effective diversity order of n_ant equicorrelated branches (participation
+    ratio N/(1+(N-1)ρ²)); ρ=0 → n_ant, ρ=1 → 1. Same metric as the measurement
+    doc and space_freq_diversity_sim."""
+    if n_ant <= 1:
+        return 1.0
+    return n_ant / (1.0 + (n_ant - 1) * rho * rho)
+
+
+def deliver_multichain(d1: float, n_ant: int, rho: float) -> float:
+    """N-chain delivery from single-chain delivery d1. Combining drops the
+    per-chain outage (1-d1) with the effective diversity order: outage^N_eff."""
+    return 1.0 - (1.0 - d1) ** n_eff(n_ant, rho)
+
+
+def energy_per_bit_nchain(p: TxPoint, src_bps: float, overhead: float,
+                          payload: int, d1: float, n_ant: int, rho: float,
+                          calib: Calib, p_chain_w: float = P_CHAIN_W) -> float:
+    """J per delivered source bit with `n_ant` active RX chains at correlation ρ:
+
+        (P_core + n_ant*P_chain + airtime*P_pa) / (src_bitrate * deliver_N)
+
+    n_ant chains add baseline but raise deliver_N; +inf if infeasible / dead."""
+    dN = deliver_multichain(d1, n_ant, rho)
+    if dN <= 0.0:
+        return float("inf")
+    af = airtime_fraction(p, src_bps, overhead, payload, calib)
+    if af > 1.0:
+        return float("inf")
+    p_avg = calib.p_baseline_w + n_ant * p_chain_w + min(1.0, af) * calib.pa_w(p.txagc)
+    return p_avg / (src_bps * dN)
+
+
+def optimal_chains(p: TxPoint, src_bps: float, overhead: float, payload: int,
+                   d1: float, rho: float, calib: Calib, max_chains: int = 4,
+                   p_chain_w: float = P_CHAIN_W) -> tuple[int, list[float]]:
+    """Return (best_n, [E/bit for n=1..max_chains]) at fade correlation ρ."""
+    eb = [energy_per_bit_nchain(p, src_bps, overhead, payload, d1, n, rho,
+                                calib, p_chain_w) for n in range(1, max_chains + 1)]
+    best = min(range(len(eb)), key=lambda i: eb[i]) + 1
+    return best, eb
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description=__doc__,
+                                 formatter_class=argparse.RawDescriptionHelpFormatter)
+    ap.add_argument("--d1", type=float, default=0.6,
+                    help="single-chain delivery at the operating point (marginal link)")
+    ap.add_argument("--src-mbps", type=float, default=4.0)
+    ap.add_argument("--overhead", type=float, default=0.25)
+    ap.add_argument("--payload", type=int, default=1024)
+    ap.add_argument("--mcs", type=int, default=3)
+    ap.add_argument("--txagc", type=int, default=32)
+    ap.add_argument("--max-chains", type=int, default=4)
+    ap.add_argument("--p-chain-w", type=float, default=P_CHAIN_W)
+    ap.add_argument("--self-test", action="store_true")
+    args = ap.parse_args()
+    if args.self_test:
+        return self_test()
+
+    cal = load_calibration()
+    p = TxPoint(mcs=args.mcs, txagc=args.txagc)
+    src = args.src_mbps * 1e6
+    print(f"energy/delivered-bit vs active chains, by fade correlation ρ\n"
+          f"(single-chain delivery d1={args.d1}, {args.src_mbps:g} Mbps video, "
+          f"MCS{args.mcs}, P_chain={args.p_chain_w} W, P_core={cal.p_baseline_w} W)")
+    hdr = "  " + " ".join(f"N={n}" for n in range(1, args.max_chains + 1))
+    print(f"{'ρ':>5} {'N_eff':>6} {'deliverN(best)':>15}   E/bit nJ [{hdr.strip()}]   best")
+    for rho in (0.9, 0.7, 0.5, 0.3, 0.1, 0.0):
+        best, eb = optimal_chains(p, src, args.overhead, args.payload, args.d1,
+                                  rho, cal, args.max_chains, args.p_chain_w)
+        dN = deliver_multichain(args.d1, best, rho)
+        cells = " ".join(f"{e*1e9:5.1f}" for e in eb)
+        print(f"{rho:>5.1f} {n_eff(args.max_chains, rho):>6.2f} {dN:>15.3f}   "
+              f"[{cells}]   N={best}")
+    print("\nHigh ρ (static): diversity ~nil, so extra chains are pure baseline "
+          "cost → best N=1.\nLow ρ (mobile): diversity fills the fades, so extra "
+          "chains cheapen delivery > their baseline → best N grows.")
+    return 0
+
+
+def self_test() -> int:
+    print("=== chain_count_energy_sim self-test ===")
+    ok = True
+    cal = load_calibration()
+    p = TxPoint(mcs=3, txagc=32)
+    src = 4e6
+
+    # N_eff limits
+    c = abs(n_eff(4, 0.0) - 4) < 1e-9 and abs(n_eff(4, 1.0) - 1) < 1e-9
+    ok &= c
+    print(f"[{'ok' if c else 'FAIL'}] N_eff(4): ρ0={n_eff(4,0):.2f} ρ1={n_eff(4,1):.2f}")
+
+    # decorrelated (low ρ) improves delivery; correlated (high ρ) barely does
+    d_lo = deliver_multichain(0.6, 4, 0.1)
+    d_hi = deliver_multichain(0.6, 4, 0.9)
+    c = d_lo > 0.95 and d_hi < d_lo
+    ok &= c
+    print(f"[{'ok' if c else 'FAIL'}] deliver(4ch): low-ρ={d_lo:.3f} > high-ρ={d_hi:.3f}")
+
+    # the headline: static prefers fewer chains, mobile prefers more
+    best_static, _ = optimal_chains(p, src, 0.25, 1024, 0.6, 0.9, cal)
+    best_mobile, _ = optimal_chains(p, src, 0.25, 1024, 0.6, 0.1, cal)
+    c = best_static < best_mobile
+    ok &= c
+    print(f"[{'ok' if c else 'FAIL'}] optimal chains: static N={best_static} < "
+          f"mobile N={best_mobile}")
+
+    # static optimum is 1 chain (extra chains pure cost when diversity is nil)
+    c = best_static == 1
+    ok &= c
+    print(f"[{'ok' if c else 'FAIL'}] static optimum is a single chain (N={best_static})")
+
+    # a more marginal link (lower d1) makes MORE chains worth their baseline
+    best_marg, _ = optimal_chains(p, src, 0.25, 1024, 0.35, 0.1, cal)
+    c = best_marg >= best_mobile
+    ok &= c
+    print(f"[{'ok' if c else 'FAIL'}] more-marginal link opens up more chains: "
+          f"d1=0.35 mobile N={best_marg} >= d1=0.6 mobile N={best_mobile}")
+
+    print("=== PASS ===" if ok else "=== FAIL ===")
+    return 0 if ok else 1
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/tools/precoder/test_chain_count_energy_sim.py b/tools/precoder/test_chain_count_energy_sim.py
new file mode 100644
index 0000000..70b928b
--- /dev/null
+++ b/tools/precoder/test_chain_count_energy_sim.py
@@ -0,0 +1,51 @@
+"""Tests for the active-chain-count energy sim (chain_count_energy_sim.py)."""
+from energy_model import TxPoint, load_calibration
+import chain_count_energy_sim as cc
+
+
+def _setup():
+    return load_calibration(), TxPoint(mcs=3, txagc=32), 4e6
+
+
+def test_n_eff_limits():
+    assert abs(cc.n_eff(4, 0.0) - 4.0) < 1e-9
+    assert abs(cc.n_eff(4, 1.0) - 1.0) < 1e-9
+    assert 1.0 < cc.n_eff(4, 0.5) < 4.0
+
+
+def test_delivery_improves_only_when_decorrelated():
+    # correlated antennas barely lift delivery; decorrelated lift it a lot
+    assert abs(cc.deliver_multichain(0.6, 4, 1.0) - 0.6) < 1e-9   # rho=1: no help
+    assert cc.deliver_multichain(0.6, 4, 0.1) > 0.95
+    assert cc.deliver_multichain(0.6, 4, 0.1) > cc.deliver_multichain(0.6, 4, 0.9)
+
+
+def test_static_prefers_single_chain():
+    cal, p, src = _setup()
+    best, eb = cc.optimal_chains(p, src, 0.25, 1024, 0.6, 0.9, cal)
+    assert best == 1
+    # and energy strictly increases with each added chain when diversity is nil
+    assert eb[0] < eb[1] < eb[2] < eb[3]
+
+
+def test_mobile_prefers_more_chains():
+    cal, p, src = _setup()
+    best_static, _ = cc.optimal_chains(p, src, 0.25, 1024, 0.6, 0.9, cal)
+    best_mobile, _ = cc.optimal_chains(p, src, 0.25, 1024, 0.6, 0.1, cal)
+    assert best_mobile > best_static
+
+
+def test_more_marginal_link_opens_more_chains():
+    # at a deeper marginal link, more chains earn their baseline
+    cal, p, src = _setup()
+    best_mid, _ = cc.optimal_chains(p, src, 0.25, 1024, 0.6, 0.1, cal)
+    best_deep, _ = cc.optimal_chains(p, src, 0.25, 1024, 0.35, 0.1, cal)
+    assert best_deep >= best_mid
+    assert best_deep == 4    # a deep marginal + decorrelated link wants all four
+
+
+def test_cheaper_chains_shift_optimum_up():
+    cal, p, src = _setup()
+    n_expensive, _ = cc.optimal_chains(p, src, 0.25, 1024, 0.6, 0.1, cal, p_chain_w=0.15)
+    n_cheap, _ = cc.optimal_chains(p, src, 0.25, 1024, 0.6, 0.1, cal, p_chain_w=0.05)
+    assert n_cheap >= n_expensive