Antenna Polarization

What Is Polarization?

• The electric field pushes charges; its arrow shows direction and strength
• Polarization describes the arrow’s motion at one fixed point (end-on view, perpendicular to wave travel)
  • Linear: back and forth along a fixed line
  • Circular: rotates with constant length
  • Elliptical: rotates with changing length

Polarization Loss for Linear Antennas

• A linear receiver responds to the field component along its polarization axis
• = rotation about the line of sight between the incident field and the receiver axis (pointing fixed)

Misalignment	Coupled power	Loss
0°	100%	0 dB
45°	50%	About 3 dB
90°	0% in the ideal case	Complete rejection

Polarization Mismatch (linear–linear)

• Real antennas have finite isolation (no perfect rejection at 90°)

Circular Polarization and Real Antennas

Wave / receive polarization	Ideal coupled power	Mismatch loss
Same circular sense	100%	0 dB
Opposite circular senses	0%	Complete rejection
Circular / linear	50%	About 3 dB

• Matched circular pairs tolerate rotation about the line of sight
  • So circular polarization helps if the spacecraft rolls — but the sense (handedness) must still match at the receiver (inferred from the table) (not on slide)
• RHCP / LHCP = right- / left-hand circular polarization
• Real polarization is often elliptical; the axial ratio describes its shape
  • A circle has (0 dB)
• Use a consistent viewing convention for handedness; the actual mismatch depends on both polarizations
• References: antenna-theory.com, Polarization; Rumpf, Antenna Parameters and Figures of Merit, Continued

Polarization in the Link Budget

• is subtracted along with pointing losses: margin against a threshold

Homework 3: Polarization and Margin

Homework 3, Problem 2(b, c)

• Aligned, polarization-matched ; required received power ; all other losses included
• Total pointing loss 1.50 dB (part a); both polarizations linear

Axes differ by	(dB)	(dBW)	Margin (dB)
45°	3.01	−122.51	+0.99: meets threshold
60°	6.02	−125.52	−2.02: fails threshold

• (c) Realigning the polarization axes reduces , which removes polarization loss and restores the link without changing peak antenna gain (not on slide)