What Does Antenna Gain Mean?
- Question: how can an antenna increase the signal without increasing transmitter power?
- Gain describes radiation in a particular direction, relative to a lossless isotropic antenna with the same input power
- It redistributes power in direction; it doesn’t create power (cf. EIRP)
- The engineering question: where does that stronger signal need to go? (see Antenna Coverage)
Isotropic and Omnidirectional
- Isotropic: equal radiation in every direction
- Omnidirectional: uniform in one plane
- Dipole example (3-D pattern plus two cuts):
- Azimuth-plane cut: a circle — omnidirectional
- Elevation-plane cut: a figure-eight with nulls along the dipole axis
WARNING
- Omnidirectional ≠ isotropic: a dipole still has nulls along its axis
The Receive Antenna and Effective Area
- Receive antenna captures
- : incident power flux density (W/m²)
- : effective collecting area (m²)
- : available received power (W)
- Physical area of a circular aperture, and effective area:
- = aperture efficiency; use for our examples
- Illumination, spillover, blockage, and surface accuracy affect how well the physical aperture contributes
- Aperture efficiency’s meaning differs from radiation efficiency alone
Frequency and Antenna Size
| Frequency | Wavelength | A 1 m aperture spans |
|---|---|---|
| 2.1 GHz | 0.143 m | 7 wavelengths |
| 8.4 GHz | 0.0357 m | 28 wavelengths |
- The antenna’s size in wavelengths determines much of its behavior
Gain from Diameter and Frequency
- Gain = effective area ÷ isotropic effective area
Reflector Gain
- in meters, in GHz, in dBi; circular aperture, all logs base 10
- 17.81 includes efficiency 0.55 and all constants
- Doubling diameter or frequency increases gain by
A 1 m Antenna at 8.4 GHz
- Given , (assumes )
- Predict, then check:
- Double , fixed frequency:
- Double frequency, fixed :
- Double both: gain increases 12.04 dB
- Higher gain comes with a narrower beam: beamwidth
- For links, frequency changes gain and path loss: Frequency Selection
Far-Field Model (Optional)
- The gain/pattern model assumes the far field; for an electrically large aperture:
- and in meters, in GHz; also require and
- (This is with )
- At 8.4 GHz:
- 3 m aperture: about 504 m
- 70 m aperture: about 274,000 m
Gain in Linear Form
- Class 4 (lunar link tools) writes the same model linearly:
- For :
- Lunar link at 8.4 GHz: 0.3 m transmit dish → 25.8 dBi; 34 m receive dish → 66.9 dBi (signal budget)
Homework 3: Gain and Beamwidth
Homework 3, Problem 1
- Circular reflector, , , aperture efficiency 0.55
- (a) Peak gain in dBi and full HPBW in degrees
- (b) Double at fixed : gain change, new gain, new HPBW
- (c) Restore and double to 15.00 GHz: gain change and HPBW; why is this insufficient to determine the received-power change?
- Using and degrees:
Case Gain (dBi) Change (dB) HPBW (°) m, GHz 33.37 Reference 3.50 Double to 1.60 m 39.39 +6.02 1.75 Double to 15.00 GHz 39.39 +6.02 1.75
- Part (c): see why gain alone is insufficient