Electromagnetics Comparison Tables

Smith chart connecting normalized impedance and reflection coefficient. — A Smith chart is itself a compact comparison map, relating many impedance states to their corresponding reflection coefficients. Image source: Wikimedia Commons.

Use these tables when the question is not “what is the equation?” but “which model, instrument, architecture, or technology fits this problem?”

TEM vs TE vs TM

Mode	Longitudinal $E_z$	Longitudinal $H_z$	Typical structure	Key point
TEM	0	0	coax, two-wire, ideal parallel plate	no cutoff in ideal homogeneous two-conductor line
TE	0	nonzero	metallic waveguide	cutoff exists
TM	nonzero	0	metallic/dielectric guides	cutoff exists

Reactive near field vs radiating near field vs far field

Region	Dominant idea	$E/H$ relation	Use when
Reactive near field	stored electric/magnetic energy	not generally $\eta_0$	close to small antennas, inductive/capacitive coupling
Radiating near field (Fresnel)	radiation present but angular pattern distance-dependent	locally complex	large apertures, finite range
Far field (Fraunhofer)	locally plane-wave-like	$E/H\approx\eta$ in simple media	antenna gain/pattern/link calculations

Common antenna families

Antenna	Strength	Limitation	Use when
Dipole	simple, fundamental, predictable	moderate gain	reference antenna, basic RF links
Monopole	compact over ground plane	depends strongly on ground	mobile/vehicle/platform mounting
Loop	magnetic-field coupling, compact variants	efficiency can be low when electrically small	near-field magnetic sensing / compact receive
Patch	planar, PCB-compatible	bandwidth and efficiency can be limited	wireless devices, arrays
Horn	broadband, well-behaved aperture	physically large at low frequency	microwave measurement, feeds, calibration
Reflector	very high gain	pointing/mechanical size	satellite, radio astronomy, radar
Phased array	electronic steering, multifunction	cost, calibration, thermal/coupling complexity	AESA, satcom, mmWave

FEM vs FDTD vs MoM

Method	Natural domain	Excellent for	Main cost/limitation	Use when
FEM	frequency or time, volume mesh	complex materials/geometry, waveguides, cavities	meshes entire volume	finite complex domains, resonators, components
FDTD	time domain, volume grid	broadband transients, pulse response	small cells/time steps; staircasing unless advanced mesh	broadband field evolution
MoM	usually frequency domain, surfaces/wires	conducting antennas/scattering	dense matrices for classical formulations	open-region conductors, wire/surface antennas
Asymptotic PO/GO/SBR	high frequency	electrically huge structures	less accurate near resonances/edges unless corrected	large reflectors, spacecraft, vehicles

VNA vs spectrum analyzer vs VSA vs oscilloscope

Instrument	Measures directly	Best for	Does not replace
VNA	complex S-parameters	impedance, return loss, insertion loss, group delay	spectrum analyzer for unknown emissions
Spectrum analyzer	power vs frequency	spurs, harmonics, noise, emissions	VNA phase-coherent network characterization
VSA	complex modulated waveform/IQ	EVM, constellation, demodulation, modulation quality	broadband time-domain scope for arbitrary transients
Oscilloscope	voltage vs time	transients, timing, pulse shape, baseband/IF	calibrated RF network measurement

Homodyne vs heterodyne

Architecture	LO relation	Advantage	Challenge	Use when
Homodyne / zero-IF	LO at carrier	simple direct baseband, low IF bandwidth	DC offsets, $1/f$ noise, IQ imbalance	compact coherent receivers
Heterodyne	nonzero IF	moves signal away from DC, filtering/selectivity	image/spur/mixer planning	robust RF receivers and precision beat detection

Pulsed vs FMCW vs CW Doppler radar

Radar	Measures naturally	Strength	Limitation
Pulsed	range + Doppler with waveform processing	high peak power, long range	timing, pulse compression, blind ranges depending design
FMCW	range from beat frequency; velocity with chirp processing	compact, continuous low peak power	chirp linearity, leakage, range-Doppler coupling
CW Doppler	velocity	simple and sensitive	no direct absolute range without modulation

Analog vs digital vs hybrid beamforming

Beamforming	RF chains	Strength	Limitation
Analog	few	low power/cost	limited simultaneous beams / flexibility
Digital	one per element/channel	maximum flexibility, MIMO, multiple beams	converters/data/power/thermal load
Hybrid	intermediate	compromise	architecture/algorithm complexity

Helmholtz coils vs Halbach arrays

Source	Tunability	Power	Field reversal	Homogeneity	Use when
Helmholtz coils	excellent	continuous electrical power	easy	excellent near center when designed well	calibration, sweeps, precision control
Halbach permanent array	limited	essentially zero steady-state coil power	mechanical/reconfiguration needed	geometry dependent; can be excellent	compact strong permanent bias field
Halbach + trim coils	fine electronic trim	low	trim only unless designed otherwise	can be optimized	compact atomic/portable systems

Atomic magnetometry approaches

Approach	Core observable	Typical regime	Strength	Limitation
Mx/Mz OPM	driven Larmor resonance	finite bias field	direct frequency-field relation	RF drive/systematic shifts
Bell–Bloom	optically driven spin precession	broad range	no separate RF drive required	modulation/light-shift control
SERF	near-zero-field spin polarization	near zero field, high density	extraordinary sensitivity	shielding, heating, limited large-field operation
CPT/EIT	ground-state coherence	optical two-photon resonance	narrow optical feature	laser coherence/systematics
Rydberg EIT	excited-state electric-dipole response	RF electric fields	SI-linked RF field sensing, broadband tunability	optical complexity, state/environment sensitivity

Ground-state vs Rydberg sensing

Feature	Ground-state atom	Rydberg atom
Dominant sensing interaction	magnetic dipole / hyperfine/Zeeman	very large electric dipole; also Zeeman
Coherence	potentially very long	shorter, environment-sensitive
Common field	DC/AC magnetic	RF electric
Typical readout	optical rotation/absorption	EIT/AT/optical transmission
Main advantage	magnetic sensitivity	strong RF electric response and tunability

S-parameter vs field measurement

Question	Better starting measurement
Is this filter matched?	VNA $S_{11}$
What is insertion loss?	VNA $S_{21}$
What spurious emissions exist?	spectrum analyzer
What is the radiated field at a point?	calibrated field probe/antenna
What is antenna gain/pattern?	antenna range / chamber
What is modulation quality?	VSA
What is transient voltage/current?	oscilloscope + suitable probe

Ideal model vs engineering model

Layer	Includes	Use
Ideal analytic	dominant physics, symmetry, lossless approximations	intuition and first estimate
Corrected analytic/circuit	losses, parasitics, finite $Q$, mismatch	design-space exploration
Numerical full-wave	real geometry/materials	fields, coupling, resonance, radiation
Experimental calibration	fixtures, cables, probes, drift, uncertainty	defensible measurement

Decision ruleChoose the simplest model that retains the mechanism you are trying to predict. Add complexity only when the omitted effect is comparable to the quantity of interest.