Reading the Spec Sheet
Every antenna ever sold is described by the same seven numbers. Learn them and you can compare a $3 whip to a $30,000 dish without seeing either one.
Directivity vs Gain
Directivity is geometry. Gain is geometry after losses. At 60% efficiency you lose 10·log(0.6) = 2.2 dB; at 90%, only 0.46 dB. Datasheets quote gain; simulators print directivity.
dBi and dBd
A half-wave dipole has directivity 1.641 = 2.15 dBi (M2-L4). So a Yagi sold as 10 dBd is 12.15 dBi. Marketing quotes dBi; cautious engineers quote dBd.
Lobes, Nulls, Cuts
- Main lobe — the beam holding the peak
- Side lobes — first one 13.3 dB down for a uniform aperture
- Back lobe — front-to-back typically 20–35 dB
- Nulls — a resource: aim one at an interferer
- E-plane / H-plane — two orthogonal cuts, often nothing alike
Half-Power Beamwidth
A 1 m dish at 10 GHz (λ = 3 cm): 70 × 0.03 / 1 = 2.1°. Then G ≈ 30000/(2.1 × 2.1) = 6803 = 38.3 dBi — and the aperture formula independently says 38.2.
Pattern Explorer
Grow the aperture and watch the main lobe narrow, the side lobes multiply, and directivity climb. The second slider spends efficiency to turn D into G.
Polarization
- Linear — vertical, horizontal, or slant ±45°
- Circular — RHCP and LHCP, field rotating once per cycle
- Cross-polarized — infinite isolation in theory, 20–30 dB in practice
- Linear to circular — a flat, predictable 3 dB
- GPS uses RHCP — a known 3 dB beats an unknown fade
Impedance and Matching
VSWR 2:1 gives Γ = 1/3, so |Γ|² = 11.1% reflected — a mere 0.51 dB lost, and 9.5 dB return loss. That is why 2:1 is the usual pass mark.
Efficiency
Ohmic loss in the conductors, dielectric loss in the substrate. And the hard floor from M7-L1: shrink an antenna below resonant size and its radiation resistance collapses while its ohmic resistance does not — so small means inefficient.
What you learned
- G = e·D, and dBi = dBd + 2.15 — always check the reference
- The pattern is the real spec: main lobe, side lobes, back lobe, nulls
- HPBW ≈ 70λ/D → 2.1° for a 1 m dish at 10 GHz → 38.3 dBi
- Polarization must match; VSWR 2:1 costs only 0.5 dB; small means lossy