{
 "schema": "learn-deck/1",
 "module": "rf-2g4",
 "deck": "rf-2g4",
 "title": "RF at 2.4 GHz",
 "lang": "en",
 "cards": [
  {
   "id": "rf:z0-def",
   "type": "flash",
   "q": "What sets the characteristic impedance Z0 of a lossless line?",
   "a": "√(L/C): inductance and capacitance per unit length. Geometry and dielectric, not length.",
   "ex": "For microstrip: trace width, substrate height and εr.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:gamma-def",
   "type": "mcq",
   "q": "The reflection coefficient Γ at a load ZL on a Z0 line equals:",
   "choices": [
    "(ZL − Z0)/(ZL + Z0)",
    "(ZL + Z0)/(ZL − Z0)",
    "ZL/Z0",
    "Z0/ZL"
   ],
   "a": 0,
   "ex": "Γ = 0 when ZL = Z0: no reflection. |Γ| = 1 for an open, a short or a pure reactance.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:vswr-from-gamma",
   "type": "mcq",
   "q": "|Γ| = 0.33. What is the VSWR?",
   "choices": [
    "1.5:1",
    "2:1",
    "3:1",
    "1.33:1"
   ],
   "a": 1,
   "ex": "VSWR = (1 + |Γ|)/(1 − |Γ|) = 1.33/0.67 ≈ 2.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:rl-10db",
   "type": "tf",
   "q": "A return loss of 10 dB means 10% of the incident power is reflected.",
   "a": true,
   "ex": "RL = −20·log10|Γ|; 10 dB → |Γ| = 0.316, and the reflected power fraction is |Γ|² = 0.1.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:q-stub",
   "type": "tf",
   "q": "A shorted λ/4 stub looks like an open circuit at its input.",
   "a": true,
   "ex": "Zin = j·Z0·tan(βl); at l = λ/4 the tangent goes to infinity.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:qw-transformer",
   "type": "flash",
   "q": "A λ/4 line of impedance Z1 between Z0 and a load RL gives a match when:",
   "a": "Z1 = √(Z0 · RL)",
   "ex": "Zin = Z1²/RL. Exact only at the design frequency, so narrowband.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:half-wave-repeat",
   "type": "tf",
   "q": "A λ/2 length of lossless line, of any impedance, reproduces the load impedance at its input.",
   "a": true,
   "ex": "Zin repeats every λ/2. That is why cable length matters unless it is a multiple of λ/2.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:wavelength-fr4",
   "type": "mcq",
   "q": "Free-space λ at 2.45 GHz is ~12.2 cm. On FR4 microstrip (εeff ≈ 3.3) the guided λ is about:",
   "choices": [
    "6.7 cm",
    "12.2 cm",
    "3.4 cm",
    "22 cm"
   ],
   "a": 0,
   "ex": "λg = λ0/√εeff = 12.2/1.82 ≈ 6.7 cm, so a λ/4 section is roughly 1.7 cm.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:why-50-ohm",
   "type": "mcq",
   "q": "Why is 50 Ω the standard coax impedance?",
   "choices": [
    "Midway: min-loss 77 Ω, max-power 30 Ω",
    "It is the impedance of free space",
    "It matches a dipole antenna",
    "It gives the lowest possible loss"
   ],
   "a": 0,
   "ex": "Air-dielectric coax: minimum attenuation near 77 Ω, maximum power handling near 30 Ω. 50 Ω splits the difference.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:dbm-0",
   "type": "mcq",
   "q": "0 dBm corresponds to:",
   "choices": [
    "1 mW",
    "1 W",
    "1 µW",
    "0 W"
   ],
   "a": 0,
   "ex": "dBm is dB relative to 1 mW. +30 dBm = 1 W, −30 dBm = 1 µW.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:db-doubling",
   "type": "flash",
   "q": "Doubling power changes the level by how many dB? Doubling voltage?",
   "a": "+3 dB for power, +6 dB for voltage (same impedance).",
   "ex": "10·log10(2) ≈ 3. Voltage squared gives power, so 20·log10(2) ≈ 6.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:electrical-length",
   "type": "flash",
   "q": "What is the electrical length of a line?",
   "a": "Its length in wavelengths or degrees at a given frequency: βl = 2π·l/λ.",
   "ex": "The same copper is electrically longer at a higher frequency.",
   "tags": [
    "lines"
   ]
  },
  {
   "id": "rf:smith-centre",
   "type": "mcq",
   "q": "On a 50 Ω Smith chart, the centre point is:",
   "choices": [
    "50 Ω, a perfect match",
    "0 Ω, a short",
    "Infinite, an open",
    "50 + j50 Ω"
   ],
   "a": 0,
   "ex": "Normalised z = 1. The left edge is the short, the right edge the open.",
   "tags": [
    "smith"
   ]
  },
  {
   "id": "rf:smith-upper-half",
   "type": "tf",
   "q": "Points in the upper half of the Smith chart are inductive (positive reactance).",
   "a": true,
   "ex": "Upper half: +jX. Lower half: −jX, capacitive.",
   "tags": [
    "smith"
   ]
  },
  {
   "id": "rf:smith-series-l",
   "type": "mcq",
   "q": "Adding a series inductor moves an impedance point:",
   "choices": [
    "Clockwise on a constant-R circle",
    "Counter-clockwise on a constant-R circle",
    "Along a constant-G circle",
    "Toward the centre, always"
   ],
   "a": 0,
   "ex": "Series L adds +jX on the same resistance circle. A series C moves counter-clockwise.",
   "tags": [
    "smith"
   ]
  },
  {
   "id": "rf:smith-shunt-c",
   "type": "mcq",
   "q": "Adding a shunt capacitor moves the point:",
   "choices": [
    "Clockwise on a constant-G circle",
    "Counter-clockwise on a constant-G circle",
    "Clockwise on a constant-R circle",
    "Radially inward"
   ],
   "a": 0,
   "ex": "Shunt elements act on admittance, so use the conductance circles. Shunt C adds +jB, clockwise.",
   "tags": [
    "smith"
   ]
  },
  {
   "id": "rf:smith-admittance",
   "type": "tf",
   "q": "Series elements are easiest on the impedance chart, shunt elements on the admittance chart.",
   "a": true,
   "ex": "Series adds impedance, shunt adds admittance. An immittance chart overlays both grids.",
   "tags": [
    "smith"
   ]
  },
  {
   "id": "rf:smith-line-rotation",
   "type": "flash",
   "q": "Moving along a lossless line toward the generator does what on the Smith chart?",
   "a": "Rotates the point clockwise about the centre at constant |Γ|; one full turn is λ/2.",
   "ex": "Toward the load is counter-clockwise.",
   "tags": [
    "smith"
   ]
  },
  {
   "id": "rf:smith-q-arcs",
   "type": "tf",
   "q": "Constant-Q arcs on the Smith chart help keep a matching network broadband.",
   "a": true,
   "ex": "Staying inside a low-Q arc while matching keeps X/R small, so the match holds over a wider band.",
   "tags": [
    "smith"
   ]
  },
  {
   "id": "rf:s11-def",
   "type": "flash",
   "q": "What does S11 describe?",
   "a": "Input reflection: wave reflected at port 1 per wave into port 1, with port 2 matched.",
   "ex": "|S11| in dB is the return loss with the sign flipped.",
   "tags": [
    "s-params"
   ]
  },
  {
   "id": "rf:s21-def",
   "type": "flash",
   "q": "What does S21 describe?",
   "a": "Forward transmission: wave out of port 2 per wave into port 1, other ports matched.",
   "ex": "Gain or insertion loss in dB = 20·log10|S21|.",
   "tags": [
    "s-params"
   ]
  },
  {
   "id": "rf:s11-target",
   "type": "mcq",
   "q": "A commonly accepted 'good enough' antenna match for a 2.4 GHz product is:",
   "choices": [
    "S11 ≤ −10 dB across the band",
    "S11 ≤ −3 dB",
    "S11 = 0 dB",
    "S11 ≤ −40 dB"
   ],
   "a": 0,
   "ex": "−10 dB means 90% of the power reaches the antenna. −40 dB across a band is unrealistic.",
   "tags": [
    "s-params"
   ]
  },
  {
   "id": "rf:s-params-why",
   "type": "tf",
   "q": "S-parameters are used at RF because the opens and shorts Z or Y parameters need are hard to realise there.",
   "a": true,
   "ex": "Matched terminations are easy and stable at RF; S-parameters are defined with them.",
   "tags": [
    "s-params"
   ]
  },
  {
   "id": "rf:mismatch-loss",
   "type": "flash",
   "q": "Mismatch loss when S11 = −6 dB?",
   "a": "About 1.25 dB: |Γ|² = 0.25 is reflected, so 10·log10(1 − 0.25) = −1.25 dB.",
   "ex": "Return loss and mismatch loss are different numbers.",
   "tags": [
    "s-params"
   ]
  },
  {
   "id": "rf:match-l",
   "type": "mcq",
   "q": "An L-network matches two resistances. How many reactive parts does it use?",
   "choices": [
    "1",
    "2",
    "3",
    "4"
   ],
   "a": 1,
   "ex": "One series and one shunt element; Q = √(Rhigh/Rlow − 1).",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:match-l-q",
   "type": "flash",
   "q": "Q of an L-network matching 10 Ω to 50 Ω?",
   "a": "Q = √(50/10 − 1) = 2.",
   "ex": "Fixed by the impedance ratio: an L-network has no bandwidth knob.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:match-l-shunt-side",
   "type": "mcq",
   "q": "In an L-network, the shunt element goes on which side?",
   "choices": [
    "The higher-resistance side",
    "The lower-resistance side",
    "Either, it makes no difference",
    "The source, always"
   ],
   "a": 0,
   "ex": "A shunt element lowers the resistance seen, a series element raises it. Shunt on the high-R side.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:match-pi-why",
   "type": "mcq",
   "q": "Why do BLE module reference designs usually include a Pi matching footprint?",
   "choices": [
    "Matches R and X and lets you set Q",
    "It is the cheapest topology",
    "It needs no inductors",
    "It is broadband by nature"
   ],
   "a": 0,
   "ex": "Three elements give one more degree of freedom than an L. You populate what measurement calls for.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:match-pi-t-q",
   "type": "tf",
   "q": "Pi and T networks let you choose the loaded Q, unlike an L-network.",
   "a": true,
   "ex": "The extra element is a free variable; higher Q gives a narrower band.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:match-bw-q",
   "type": "flash",
   "q": "How does matching-network Q relate to bandwidth?",
   "a": "Fractional bandwidth ≈ 1/Q: higher Q, narrower match.",
   "ex": "2.4–2.5 GHz is about 4%, so keep the loaded Q well below 25.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:conjugate-match",
   "type": "tf",
   "q": "Maximum power transfer needs the load to be the complex conjugate of the source impedance.",
   "a": true,
   "ex": "Same resistance, opposite reactance, so the reactances cancel.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:cancel-reactance",
   "type": "mcq",
   "q": "An antenna measures 40 − j20 Ω at 2.44 GHz. Which single element cancels the reactance?",
   "choices": [
    "A series inductor of +j20 Ω",
    "A series capacitor",
    "A shunt inductor",
    "A shunt capacitor"
   ],
   "a": 0,
   "ex": "Series +j20 Ω cancels −j20 Ω; at 2.44 GHz that is about 1.3 nH. Only 40 → 50 Ω is left.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:reactance-anchors",
   "type": "flash",
   "q": "Reactance of 1 nH at 2.4 GHz? Of 1 pF?",
   "a": "XL = 2π·f·L ≈ 15 Ω. XC = 1/(2π·f·C) ≈ 66 Ω.",
   "ex": "Handy anchors: matching parts at 2.4 GHz are 1–10 nH and 0.5–5 pF.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:balun",
   "type": "flash",
   "q": "What does a balun do in an RF front end?",
   "a": "Converts between balanced (differential) and unbalanced (single-ended), often with an impedance change.",
   "ex": "Needed when a differential radio output feeds a single-ended 50 Ω antenna.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:srf",
   "type": "mcq",
   "q": "The self-resonant frequency (SRF) of an inductor is where:",
   "choices": [
    "Parasitic C resonates with L",
    "Its Q peaks",
    "Its loss is zero",
    "It becomes a short"
   ],
   "a": 0,
   "ex": "Above the SRF the part looks capacitive. Pick inductors with SRF well above 2.4 GHz.",
   "tags": [
    "components"
   ]
  },
  {
   "id": "rf:0402-vs-0603",
   "type": "tf",
   "q": "At 2.4 GHz a 0402 part usually behaves better than a 0603 of the same value.",
   "a": true,
   "ex": "Smaller package: less parasitic inductance and capacitance, higher SRF.",
   "tags": [
    "components"
   ]
  },
  {
   "id": "rf:cap-above-srf",
   "type": "mcq",
   "q": "Above its SRF, a capacitor looks:",
   "choices": [
    "Inductive",
    "Capacitive",
    "Resistive",
    "Like an open"
   ],
   "a": 0,
   "ex": "ESL dominates. A 100 nF 0603 self-resonates around 20–30 MHz; useless in a 2.4 GHz path.",
   "tags": [
    "components"
   ]
  },
  {
   "id": "rf:high-q-parts",
   "type": "tf",
   "q": "High-Q RF-grade capacitors and inductors in a matching network reduce insertion loss.",
   "a": true,
   "ex": "Loss in the match is power that never radiates. Use C0G/NP0 capacitors and RF inductors.",
   "tags": [
    "components"
   ]
  },
  {
   "id": "rf:sim-vs-real",
   "type": "flash",
   "q": "Why do final matching values differ from the simulation?",
   "a": "Pad and via parasitics, part tolerances, ESL and ESR, and the real ground plane all shift the impedance.",
   "ex": "Simulation gives the starting point; expect to tune by measurement.",
   "tags": [
    "components"
   ]
  },
  {
   "id": "rf:ifa",
   "type": "mcq",
   "q": "An inverted-F antenna (IFA) is essentially:",
   "choices": [
    "A shorted monopole with a feed tap",
    "A folded dipole",
    "A patch antenna",
    "A loop antenna"
   ],
   "a": 0,
   "ex": "The short to ground plus the feed position set the input impedance without extra parts.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:counterpoise",
   "type": "tf",
   "q": "For a PCB monopole or IFA, the ground plane is half of the antenna.",
   "a": true,
   "ex": "It is the counterpoise. A small plane detunes and lowers efficiency; copying the trace alone is not enough.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:mifa",
   "type": "flash",
   "q": "What is a MIFA?",
   "a": "Meandered inverted-F antenna: an IFA folded into a serpentine to shrink its footprint.",
   "ex": "Roughly λ/4 of total trace; about 20 × 4 mm on FR4 at 2.4 GHz.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:chip-antenna",
   "type": "mcq",
   "q": "Compared with a well-designed PCB trace antenna, a chip antenna typically is:",
   "choices": [
    "Smaller but less efficient",
    "More efficient",
    "Free of any ground clearance",
    "Free of any matching network"
   ],
   "a": 0,
   "ex": "Its datasheet land pattern, keep-out and matching are mandatory, and it still radiates less.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:enclosure",
   "type": "flash",
   "q": "What does a plastic enclosure do to a tuned PCB antenna?",
   "a": "Its dielectric loads the antenna and shifts resonance lower; final tuning must be done inside the enclosure.",
   "ex": "Metal nearby is far worse: it can kill radiation altogether.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:keepout",
   "type": "tf",
   "q": "Ground copper must be kept out of the antenna area on every layer.",
   "a": true,
   "ex": "Copper under or beside the trace changes the antenna and steals radiation. Respect the reference keep-out.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:resonance-low",
   "type": "mcq",
   "q": "Your antenna resonates at 2.3 GHz instead of 2.44 GHz. To raise it you would:",
   "choices": [
    "Shorten it or add series C",
    "Lengthen the trace",
    "Add shunt capacitance",
    "Widen the ground plane"
   ],
   "a": 0,
   "ex": "Lower than target means electrically too long: remove length or cancel some inductance.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:dipole-length",
   "type": "flash",
   "q": "Physical length of a half-wave dipole at 2.45 GHz in air?",
   "a": "About 5.8 cm: λ/2 ≈ 6.1 cm, shortened ~5% for end effects.",
   "ex": "Each arm is about 2.9 cm.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:efficiency",
   "type": "flash",
   "q": "What is antenna efficiency?",
   "a": "Radiated power over power delivered to the antenna; the rest is conductor, dielectric and mismatch loss.",
   "ex": "A perfect S11 with 20% efficiency is still a bad antenna.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:gain-directivity",
   "type": "tf",
   "q": "Gain equals directivity times efficiency.",
   "a": true,
   "ex": "Directivity is the pattern alone; gain also counts losses. dBi is relative to an isotropic radiator.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:microstrip-z0",
   "type": "mcq",
   "q": "What sets a microstrip trace's impedance?",
   "choices": [
    "Width, substrate height and εr",
    "Length",
    "Copper thickness only",
    "Frequency only"
   ],
   "a": 0,
   "ex": "A wider trace or thinner dielectric lowers Z0. Length changes phase, not Z0.",
   "tags": [
    "layout"
   ]
  },
  {
   "id": "rf:cpwg-vias",
   "type": "tf",
   "q": "Coplanar waveguide with ground (CPWG) needs ground vias stitched along both sides of the trace.",
   "a": true,
   "ex": "The via fence keeps the side grounds at one potential and stops parallel-plate modes.",
   "tags": [
    "layout"
   ]
  },
  {
   "id": "rf:feed-rules",
   "type": "flash",
   "q": "Three layout rules for the 50 Ω feed line.",
   "a": "No stubs or test pads on it; shunt pads get a via to ground right at the pad; nothing routed under it.",
   "ex": "Mitre or curve bends and keep the reference ground continuous beneath.",
   "tags": [
    "layout"
   ]
  },
  {
   "id": "rf:pi-placement",
   "type": "mcq",
   "q": "In a Pi network on a 2.4 GHz feed, the series element sits:",
   "choices": [
    "In series between the two shunt pads",
    "At the antenna feed only",
    "Anywhere on the board",
    "Across the antenna to ground"
   ],
   "a": 0,
   "ex": "Shunt–series–shunt along the feed. Unused shunt pads stay empty; the series pad gets 0 Ω if unused.",
   "tags": [
    "layout"
   ]
  },
  {
   "id": "rf:ufl-debug",
   "type": "tf",
   "q": "A U.FL or SMA at the antenna feed is worth the space on every RF prototype.",
   "a": true,
   "ex": "Without a connector you cannot calibrate at the feed or measure S11 properly.",
   "tags": [
    "layout"
   ]
  },
  {
   "id": "rf:sol-cal",
   "type": "mcq",
   "q": "SOL(T) calibration of a VNA moves the measurement reference plane to:",
   "choices": [
    "Where the standards were connected",
    "The VNA front panel",
    "The antenna centre",
    "Nowhere; it only removes noise"
   ],
   "a": 0,
   "ex": "Calibrate at the cable end that touches the board, or every cm of cable rotates your Smith chart point.",
   "tags": [
    "vna"
   ]
  },
  {
   "id": "rf:cal-check",
   "type": "flash",
   "q": "Where do open, short and load land on the Smith chart after a good calibration?",
   "a": "Open at the right edge, short at the left edge, load at the centre.",
   "ex": "Quick sanity check before trusting any antenna measurement.",
   "tags": [
    "vna"
   ]
  },
  {
   "id": "rf:nanovna-sweep",
   "type": "tf",
   "q": "A NanoVNA S11 sweep from 2.0 to 3.0 GHz is enough to find a 2.4 GHz antenna's resonance and bandwidth.",
   "a": true,
   "ex": "Look for the S11 minimum and its −10 dB width, then tune toward 2.44 GHz.",
   "tags": [
    "vna"
   ]
  },
  {
   "id": "rf:tune-rule",
   "type": "flash",
   "q": "The rule for tuning a matching network by measurement.",
   "a": "Change one component at a time, re-measure, and watch the Smith chart trajectory.",
   "ex": "A series L walks the point clockwise on its R circle; a shunt C clockwise on its G circle.",
   "tags": [
    "vna"
   ]
  },
  {
   "id": "rf:tinysa",
   "type": "mcq",
   "q": "What can a tinySA add to antenna work that a VNA cannot?",
   "choices": [
    "See the radiated spectrum and spurs",
    "Measure S11",
    "Calibrate the cable",
    "Draw the Smith chart"
   ],
   "a": 0,
   "ex": "A spectrum analyser sees what the transmitter actually puts out; the VNA only sees the match.",
   "tags": [
    "vna"
   ]
  },
  {
   "id": "rf:range-test",
   "type": "flash",
   "q": "Simplest end-to-end check that a match improved an antenna?",
   "a": "A range or RSSI test at a fixed distance, before and after tuning.",
   "ex": "Relative rather than calibrated, but it catches matches that look fine on the VNA and radiate badly.",
   "tags": [
    "vna"
   ]
  },
  {
   "id": "rf:return-path",
   "type": "tf",
   "q": "A signal's return current follows the path of least impedance, which at RF is directly under the trace.",
   "a": true,
   "ex": "A slot in the ground under a trace forces a detour: extra inductance, radiation and crosstalk.",
   "tags": [
    "sipi"
   ]
  },
  {
   "id": "rf:decoupling-loop",
   "type": "flash",
   "q": "What makes a decoupling capacitor effective at high frequency?",
   "a": "Low loop inductance: short traces and vias straight to the planes, close to the pin.",
   "ex": "The capacitor's value matters less than the loop it sits in.",
   "tags": [
    "sipi"
   ]
  },
  {
   "id": "rf:rise-time-bw",
   "type": "flash",
   "q": "Bandwidth of a digital edge with rise time tr?",
   "a": "≈ 0.35/tr. A 1 ns edge carries content to about 350 MHz.",
   "ex": "This decides whether a 'digital' trace needs RF care.",
   "tags": [
    "sipi"
   ]
  },
  {
   "id": "rf:loaded-q-def",
   "type": "flash",
   "q": "Loaded Q of a resonant circuit?",
   "a": "Centre frequency over 3 dB bandwidth, Q = fc/(f2 − f1), measured with the real source and load attached.",
   "ex": "Bowick ch. 2: set by source R, load R and component Q.",
   "tags": [
    "resonators"
   ]
  },
  {
   "id": "rf:loaded-q-rp",
   "type": "flash",
   "q": "Parallel resonator between Rs and RL: loaded Q formula?",
   "a": "Q = Rp/Xp with Rp = Rs || RL and Xp the reactance of L (or C) at resonance.",
   "ex": "Bowick eq. 2-6: low terminations de-Q the tank; small L and large C raise Q for a given Rp.",
   "tags": [
    "resonators"
   ]
  },
  {
   "id": "rf:q-raise-source",
   "type": "mcq",
   "q": "A 50 Ω source gives a tank Q of 1.1. Replacing it with a 1000 Ω source gives about...",
   "choices": [
    "Q ≈ 22",
    "Q ≈ 1.1",
    "Q ≈ 0.05",
    "Q ≈ 1000"
   ],
   "a": 0,
   "ex": "Bowick fig. 2-10: Q scales with the parallel resistance seen by the tank.",
   "tags": [
    "resonators"
   ]
  },
  {
   "id": "rf:series-parallel-q",
   "type": "flash",
   "q": "Convert a series R–X to its parallel equivalent (Q < 10)?",
   "a": "Rp = (Q² + 1)·Rs and Xp = Rp/Q, with Q = Xs/Rs.",
   "ex": "Bowick eq. 2-7/2-8. For Q > 10: Rp ≈ Q²·Rs and Xp ≈ Xs.",
   "tags": [
    "resonators"
   ]
  },
  {
   "id": "rf:insertion-loss-q",
   "type": "flash",
   "q": "Why does finite inductor Q cause insertion loss in an LC tank?",
   "a": "The coil loss appears as a shunt Rp = Q·Xp across the tank, which divides the signal with the load.",
   "ex": "Bowick: a Q = 10 coil made 4.5 kΩ across 1 kΩ terminations, costing 0.9 dB; it adds up over cascaded stages.",
   "tags": [
    "resonators"
   ]
  },
  {
   "id": "rf:shape-factor",
   "type": "flash",
   "q": "Filter shape factor?",
   "a": "60 dB bandwidth divided by 3 dB bandwidth; 1 is the ideal brick wall.",
   "ex": "Bowick ch. 2. Ripple = passband flatness; insertion loss = what the filter costs at fc.",
   "tags": [
    "resonators"
   ]
  },
  {
   "id": "rf:xfmr-tap",
   "type": "flash",
   "q": "How do you get high Q between two low impedances?",
   "a": "Transform them up: tapped-C or tapped-L divider, or an impedance transformer, so the tank sees a large Rp.",
   "ex": "Bowick ch. 2 'impedance transformation'.",
   "tags": [
    "resonators",
    "matching"
   ]
  },
  {
   "id": "rf:l-match-q",
   "type": "flash",
   "q": "L-network between Rs (series side) and Rp (shunt side): Q?",
   "a": "Qs = Qp = √(Rp/Rs − 1); Xs = Qs·Rs, Xp = Rp/Qp; Xs and Xp of opposite type.",
   "ex": "Bowick eq. 4-1 to 4-3. Example: 100 to 1000 Ω gives Q = 3, Xs = 300 Ω, Xp = 333 Ω.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:l-match-how",
   "type": "flash",
   "q": "What does each element of an L-network do?",
   "a": "The shunt element transforms the larger resistance down to the smaller real part; the series element cancels the leftover reactance.",
   "ex": "Bowick ch. 4: 1000 Ω with −j333 shunt looks like 100 − j300; add +j300 in series.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:absorption",
   "type": "flash",
   "q": "'Absorption' and 'resonance' when matching a complex load?",
   "a": "Absorb strays into same-type elements and subtract them; if strays are too big, resonate them out first, then match the resistance.",
   "ex": "Bowick: series L absorbs stray series L, shunt C absorbs stray shunt C.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:pi-virtual-r",
   "type": "mcq",
   "q": "In a Pi network the virtual resistance R must be...",
   "choices": [
    "Larger than both",
    "Smaller than both terminations",
    "Equal to the source",
    "Equal to 50 Ω"
   ],
   "a": 1,
   "ex": "Two back-to-back L-networks whose series legs meet at R; Q = √(RH/R − 1) with RH the larger termination.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:t-network-q",
   "type": "flash",
   "q": "T network: where is the virtual resistance and what sets Q?",
   "a": "R is larger than both terminations (shunt legs joined); Q = √(R/Rsmall − 1). Used to match two low impedances at high Q.",
   "ex": "Bowick eq. 4-5. The L-network Q is the minimum a Pi or T can have.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:topology-choice",
   "type": "flash",
   "q": "Bowick's three reasons to pick one matching topology over another?",
   "a": "Eliminating stray reactances, harmonic filtering (low-pass vs high-pass form), and passing or blocking DC.",
   "ex": "Low-pass L (series L, shunt C) also attenuates harmonics.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:mismatch-100-1000",
   "type": "mcq",
   "q": "100 Ω source driving 1000 Ω directly loses about...",
   "choices": [
    "0.5 dB",
    "10 dB",
    "20 dB",
    "4.8 dB"
   ],
   "a": 3,
   "ex": "Bowick fig. 4-5: a third of the available power; the L-network recovers it.",
   "tags": [
    "matching"
   ]
  },
  {
   "id": "rf:series-only-limit",
   "type": "flash",
   "q": "Load 25 − j75 Ω: can a single series element match it to 50 Ω?",
   "a": "No. A series part only moves along the 25 Ω constant-R circle; you need a shunt element to change the real part.",
   "ex": "Lau ch. 7: series L to the 20 mS admittance circle, then shunt L or C to the centre.",
   "tags": [
    "matching",
    "smith"
   ]
  },
  {
   "id": "rf:loop-vs-short-dipole",
   "type": "flash",
   "q": "Which L-network suits a loop antenna vs a short monopole?",
   "a": "Loop is inductive: use a series C variant. Short monopole is capacitive with high parallel R: shunt L to cancel and step down.",
   "ex": "Lau ch. 7.4.3.",
   "tags": [
    "matching",
    "antenna"
   ]
  },
  {
   "id": "rf:rfc-dcblock",
   "type": "flash",
   "q": "Why do RFIC PA outputs use a shunt L and series C L-network?",
   "a": "The inductor is the RF choke feeding DC to the drain (swing up to 2×Vdd); the capacitor blocks DC and matches.",
   "ex": "Lau fig. 7.10. Also protects 50 Ω test gear from the bias.",
   "tags": [
    "matching",
    "components"
   ]
  },
  {
   "id": "rf:inductor-q",
   "type": "flash",
   "q": "Inductor Q and what happens at the SRF?",
   "a": "Q = 2πfL/Rloss. Q collapses at the SRF where L resonates with its winding capacitance; above it the part is capacitive.",
   "ex": "Lau ch. 7.5: choose SRF well above 2.4 GHz.",
   "tags": [
    "components"
   ]
  },
  {
   "id": "rf:lumped-vs-distributed",
   "type": "mcq",
   "q": "Above roughly what frequency does Lau prefer distributed over SMD matching?",
   "choices": [
    "300 MHz",
    "~3 GHz",
    "30 GHz",
    "1 MHz"
   ],
   "a": 1,
   "ex": "SMD caps lose in the dielectric, inductors in dielectric or ferrite; lines are lower loss but larger.",
   "tags": [
    "components",
    "matching"
   ]
  },
  {
   "id": "rf:dipole-r",
   "type": "flash",
   "q": "Radiation resistance of a half-wave dipole, and its 'resistive' length?",
   "a": "73 Ω at λ/2, slightly inductive; trimming to 0.475 λ gives a purely resistive 67 Ω.",
   "ex": "Wallace ch. 3. Shorter than λ/4 per arm becomes capacitive and needs a series L.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:monopole-r",
   "type": "mcq",
   "q": "Input impedance of a λ/4 monopole over a large ground plane?",
   "choices": [
    "73 Ω",
    "50 Ω",
    "300 Ω",
    "36.5 Ω"
   ],
   "a": 3,
   "ex": "Half the dipole: the image in the ground plane supplies the other arm. Bending the ground down toward the feed gives 50 Ω.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:folded-dipole",
   "type": "flash",
   "q": "Why is a folded dipole ~300 Ω?",
   "a": "Current splits between two parallel radiators, so for the same radiated I²R the impedance is 4× the 73 Ω dipole.",
   "ex": "Wallace ch. 3: matches 300 Ω ribbon feeders.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:dbi-dbd",
   "type": "flash",
   "q": "dBi vs dBd?",
   "a": "dBi is gain over an isotropic radiator; dBd over a half-wave dipole. dBi = dBd + 2.15.",
   "ex": "Wallace: dipole 2.15 dBi, 78° beamwidth; monopole over ground 5.15 dBi.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:thick-dipole",
   "type": "flash",
   "q": "Why does a thick dipole have wider bandwidth?",
   "a": "Its R and X vary less with frequency, so the resonator is lower Q.",
   "ex": "Wallace ch. 3.3.3. Same idea as wide PCB traces for broadband IFAs.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:near-far-field",
   "type": "flash",
   "q": "Near field vs far field of a dipole?",
   "a": "Near: stored reactive energy, E and H 90° apart, no net power. Far: E and H in phase, spherical spreading. Equal at ~λ/2.",
   "ex": "Wallace: reactive field negligible after a few wavelengths.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:polarisation-loss",
   "type": "flash",
   "q": "Linear antenna receiving a circularly polarised wave?",
   "a": "Loses 3 dB (one vector component). Circular vs wrong-hand circular: nothing received.",
   "ex": "Wallace ch. 3.3.6.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:end-capacitance",
   "type": "flash",
   "q": "An antenna is too short for the band. Which trick keeps the length?",
   "a": "Add end capacitance (top hat or wider tip) or series inductance to pull the resonance down.",
   "ex": "Wallace 3.3.11; Lau: inverted-L is a bent monopole, lower R than 36.5 Ω with large C to ground.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:ifa-tline-model",
   "type": "flash",
   "q": "Transmission-line view of an inverted-F antenna?",
   "a": "Shorted end = 0 Ω, open end = very high; the feed tap between them lands near 50 Ω, so the antenna matches itself.",
   "ex": "Lau ch. 4: alternatively a capacitive divider (C1/C2 set by feed position) resonated by the shorting-stub inductance.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:meander-effect",
   "type": "flash",
   "q": "Effect of meandering a dipole of fixed height?",
   "a": "Longer electrical path lowers the resonant frequency: 500 MHz straight fell to 320, 300 and 260 MHz with more segments.",
   "ex": "Lau eq. 4.1: Ld = 2N(W + L). Costs efficiency and bandwidth.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:pifa-vs-ifa",
   "type": "flash",
   "q": "PIFA vs IFA?",
   "a": "IFA is a folded monopole; PIFA is a patch relative with more volume, wider bandwidth and slightly better efficiency (air dielectric).",
   "ex": "Pandey ch. 1. Both need an electrically large ground plane.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:patch-length",
   "type": "flash",
   "q": "Rectangular patch: first-cut length and width?",
   "a": "L ≈ c/(2f√εeff) minus 2ΔL for fringing (ΔL ≈ 0.412h·f(W/h, εeff)); W = c/(2f)·√(2/(εr + 1)).",
   "ex": "Pandey ch. 2: L is 0.3–0.5 λ0, h is 0.003–0.05 λ0; low εr radiates better.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:patch-edge-z",
   "type": "flash",
   "q": "Patch edge impedance and how to feed it?",
   "a": "ZA ≈ 90·(εr²/(εr − 1))·(L/W)² Ω, typically 100–300 Ω; match with an inset feed or a λ/4 line of ZL = √(Z0·ZA).",
   "ex": "Pandey ch. 2.4. Probe feed: move the pin inward to lower R.",
   "tags": [
    "antenna",
    "lines"
   ]
  },
  {
   "id": "rf:vswr2-power",
   "type": "mcq",
   "q": "At VSWR 2:1 the fraction of power actually accepted by the antenna is...",
   "choices": [
    "50 %",
    "88.9 %",
    "96 %",
    "75 %"
   ],
   "a": 1,
   "ex": "|Γ| = 1/3, reflected 11.1 %. VSWR 1.5 (−14 dB) reflects only 4 %.",
   "tags": [
    "antenna",
    "lines"
   ]
  },
  {
   "id": "rf:trp-tis",
   "type": "flash",
   "q": "TRP and TIS?",
   "a": "Total radiated power and total isotropic sensitivity: the whole product (antenna + radio) integrated over the sphere in a chamber.",
   "ex": "Lau ch. 6: certification metrics; efficiency = gain/directivity when matched.",
   "tags": [
    "antenna",
    "vna"
   ]
  },
  {
   "id": "rf:percent-bw",
   "type": "flash",
   "q": "Why quote antenna bandwidth in percent?",
   "a": "Percent bandwidth is constant relative to frequency; a typical printed antenna manages ~10 % at −10 dB return loss.",
   "ex": "Pandey ch. 1: 2.40–2.50 GHz is 4 %, comfortable for an IFA.",
   "tags": [
    "antenna"
   ]
  },
  {
   "id": "rf:speed-fr4",
   "type": "flash",
   "q": "Signal speed and wiring delay on FR4?",
   "a": "~6 in/ns (15 cm/ns), i.e. ~170 ps per inch, from 12 in/ns divided by √Dk with Dk ≈ 4.",
   "ex": "Bogatin ch. 7 rules of thumb.",
   "tags": [
    "lines",
    "sipi"
   ]
  },
  {
   "id": "rf:50ohm-aspect",
   "type": "mcq",
   "q": "Rule of thumb aspect ratio (width/height) of a 50 Ω FR4 microstrip?",
   "choices": [
    "2 : 1",
    "1 : 2",
    "1 : 1",
    "4 : 1"
   ],
   "a": 0,
   "ex": "Stripline: 1 : 2 between planes. Wider trace = more C per length = lower Z0.",
   "tags": [
    "lines",
    "sipi"
   ]
  },
  {
   "id": "rf:50ohm-lc",
   "type": "flash",
   "q": "C and L per length of any 50 Ω line in FR4?",
   "a": "About 3.3 pF/in and 8.3 nH/in; C halves and L doubles if Z0 doubles.",
   "ex": "Bogatin ch. 7: Z0 = √(L/C), delay = √(LC).",
   "tags": [
    "lines",
    "sipi"
   ]
  },
  {
   "id": "rf:reflection-sign",
   "type": "flash",
   "q": "Sign of a reflection at an impedance step?",
   "a": "Γ = (Z2 − Z1)/(Z2 + Z1): positive when the signal enters a higher impedance, negative into a lower one.",
   "ex": "Bogatin ch. 8: a 75 Ω line and a 75 Ω resistor reflect identically.",
   "tags": [
    "lines",
    "sipi"
   ]
  },
  {
   "id": "rf:discontinuity-rt",
   "type": "flash",
   "q": "Rise-time hit from a small shunt capacitance on a 50 Ω line?",
   "a": "About Z0·C: 1 pF adds ~50 ps. The reflection from a short discontinuity lasts its round-trip delay.",
   "ex": "Bogatin ch. 8-9: fast edges reflect more, so bandwidth drops.",
   "tags": [
    "sipi"
   ]
  },
  {
   "id": "rf:xtalk-budget",
   "type": "mcq",
   "q": "Typical acceptable crosstalk on a victim line?",
   "choices": [
    "~5 % of the swing",
    "~25 %",
    "~0.1 %",
    "~50 %"
   ],
   "a": 0,
   "ex": "Bogatin ch. 10: 60 mV on a 1.2 V swing. RF receivers may need ~100 dB isolation from digital noise in-band.",
   "tags": [
    "sipi"
   ]
  },
  {
   "id": "rf:xtalk-knobs",
   "type": "flash",
   "q": "Two geometry knobs that cut crosstalk?",
   "a": "Bring the return plane closer to the signals and spread the signal lines apart; a wide solid plane always confines fringe fields best.",
   "ex": "Bogatin ch. 10: mutual C and mutual L model the coupling.",
   "tags": [
    "sipi",
    "layout"
   ]
  },
  {
   "id": "rf:pdn-low-z",
   "type": "flash",
   "q": "Six features of a low-impedance PDN?",
   "a": "Power close to ground, short paths, wide planes, adjacent power/ground layers, thin dielectric, near the top layer.",
   "ex": "Bogatin ch. 1: it is all about loop inductance.",
   "tags": [
    "sipi",
    "layout"
   ]
  },
  {
   "id": "rf:skin-depth-cause",
   "type": "flash",
   "q": "What causes the skin effect?",
   "a": "Above ~1 MHz loop inductance dominates; current redistributes to minimise it: toward the return path and away from itself.",
   "ex": "Bogatin ch. 9. Dielectric loss instead comes from bound dipoles rotating; it grows linearly with frequency.",
   "tags": [
    "sipi",
    "lines"
   ]
  },
  {
   "id": "rf:dispersion",
   "type": "flash",
   "q": "Dispersion?",
   "a": "Frequency-dependent signal speed (Dk falls with frequency), so edges smear; Z0 also drifts under ~10 % across the band.",
   "ex": "Bogatin ch. 7.",
   "tags": [
    "lines",
    "sipi"
   ]
  },
  {
   "id": "rf:rule9",
   "type": "flash",
   "q": "Bogatin's rule #9?",
   "a": "Anticipate the result before any measurement or simulation; if reality differs, do not use it until you know why.",
   "ex": "Rules of thumb start the analysis; sign-off needs a verified numerical simulation.",
   "tags": [
    "vna"
   ]
  },
  {
   "id": "rf:series-r-receiver",
   "type": "tf",
   "q": "A 30 Ω series resistor at a high-impedance receiver input changes the reflection there.",
   "a": false,
   "ex": "Bogatin 8.7: the signal still sees the open; series R only matters at the source (series termination).",
   "tags": [
    "sipi"
   ]
  }
 ],
 "version": "1.1.0",
 "updated": "2026-09-14T14:57:54Z"
}