ECE4122 Chap.5 Antenna Parameters and Radiation
Antenna Parameters and Radiation
Antenna parameters separate spatial concentration from loss. Radiation intensity and pattern describe angular distribution. Directivity compares a chosen direction with the average radiation, while efficiency compares radiated with accepted power. Gain combines directivity and radiation efficiency.
Pattern reading requires labelled angular references, main lobes, nulls, sidelobes and beamwidth.
Polarisation and impedance mismatch introduce additional reception losses that should not be hidden inside an undefined efficiency.
What this chapter covers
- 01
Radiation intensity and normalised patterns
- 02
Directivity and solid angle
- 03
Radiation efficiency and gain
- 04
Linear ratios and decibels
- 05
Beamwidth, sidelobes and front-to-back ratio
- 06
Impedance and polarisation mismatch
Worked example · free
Find directivity and gain
- +2Calculate D = 4π(0.80)/2.40 = 4.1888.
- +1Convert directivity to 6.22 dBi.
- +2Calculate G = 0.75 × 4.1888 = 3.1416.
- +1Convert gain to 4.97 dBi and confirm it is below directivity.
Key terms
- Radiation Intensity
- Radiated power per unit solid angle in a specified direction.
- Directivity
- Radiation intensity relative to the average intensity over all directions.
- Radiation Efficiency
- Radiated power divided by accepted antenna power.
- Antenna Gain
- Directivity multiplied by radiation efficiency, excluding or including mismatch only when the definition states it.
- Half-Power Beamwidth
- Angular separation between directions where radiation intensity is half its maximum.
Antenna Parameters and Radiation FAQ
What is the difference between directivity and gain?
Directivity describes how strongly radiation is concentrated by pattern shape. Gain includes radiation efficiency, so conductor and dielectric loss reduce gain below directivity for a passive antenna.
When should I use ten log rather than twenty log?
Use 10log10 for a power ratio such as gain, directivity or efficiency. Use 20log10 for a field or voltage ratio under the same impedance conditions.
Can two antennas have the same pattern but different gain?
Yes. A normalised pattern shows angular shape but hides absolute power. Two designs can share directivity while having different losses and therefore different gain. The lower-efficiency design radiates less accepted power even though its angular distribution has the same shape.
How does polarisation mismatch affect received power?
The received field is projected onto the receiving polarisation. For linear fields separated by angle ψ, the power factor is cos²ψ, independent of the antenna's impedance match.
Exam move
Keep a four-column sheet for pattern, directivity, efficiency and gain. For each numerical problem, identify the reference power before writing a formula. Convert the linear result to decibels only after checking physical order, such as gain not exceeding directivity for efficiency below one. Sketch patterns from calculated maxima and nulls rather than artistic intuition.
Build a parameter ledger from a single power-flow diagram. Begin at generator available power, pass through mismatch to accepted power, through material loss to radiated power, and then through angular concentration to radiation intensity. Beside every given value, write its reference: accepted, radiated, directional, isotropic or realised.
That discipline prevents efficiency from being applied twice and reveals whether the requested result is directivity, gain or realised gain. For reception, extend the ledger through effective aperture and polarisation coupling. Keep wavelength, distance and impedance assumptions visible rather than treating the link equation as a black box. Use patterns quantitatively.
Mark the angular origin, propagation convention and whether the plotted ordinate is field magnitude, power or a decibel normalisation. Locate principal maxima, first nulls, half-power points and important sidelobes from equations or data before drawing a smooth curve. If only a two-dimensional cut is supplied, do not infer full directivity unless an approximation is explicitly justified.
Practise the solid-angle element and explain why the sine factor is needed when integrating over a sphere. Check that a pattern normalised to one contains no information about absolute efficiency. For numerical revision, alternate forward and inverse questions. From radiation intensity and radiated power, calculate directivity. From gain and directivity, recover efficiency and test that it lies between zero and one.
From gain and wavelength, calculate effective aperture. From a polarisation angle, calculate the field projection and then square it for power. Perform each calculation in linear units first and convert only the final power ratio with ten times the logarithm. When a question supplies a reflection coefficient, decide whether its mismatch factor belongs in the named gain definition before multiplying.
Finish every antenna answer with two physical checks. A passive radiation efficiency cannot exceed one, so ordinary gain cannot exceed directivity. Narrowing a main beam without moving substantial power into sidelobes should increase directivity, but conductor loss can still reduce gain.
State what a proposed measurement would establish: a normalised chamber pattern tests shape, a calibrated gain comparison tests absolute performance, and a port measurement tests mismatch. Keeping those observations distinct turns an antenna calculation into an auditable engineering argument.
Working through Antenna Parameters and Radiation in ECE4122? Sia is AskSia’s AI Electrical Engineering tutor — ask any ECE4122 Antenna Parameters and Radiation question and get a clear, step-by-step explanation grounded in how ECE4122 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
