This cheat sheet covers common antenna types and the radiation patterns used to describe how antennas send and receive electromagnetic waves. Students need it to connect physical antenna shape with performance terms such as gain, beamwidth, directivity, and polarization. It is useful for introductory communications, electronics, robotics, and radio frequency engineering lessons.
The goal is to make antenna selection and pattern interpretation faster and clearer.
Key Facts
- Wavelength is found from lambda = c / f, where c is about 3.00 x 10^8 m/s and f is frequency in hertz.
- A common half-wave dipole has total length L approximately lambda / 2, with each arm approximately lambda / 4.
- A quarter-wave monopole over a ground plane has length L approximately lambda / 4 and often acts like half of a dipole.
- Antenna gain in decibels is G_dB = 10 log10(P_directional / P_reference), so higher gain means more power is concentrated in certain directions.
- Effective isotropic radiated power is EIRP = P_transmit x G_antenna when using linear units for gain.
- Power density in free space decreases with distance as S = P / (4 pi r^2) for an ideal isotropic radiator.
- Beamwidth is the angular width between the half-power points, where power drops to 1/2 of the maximum or about -3 dB.
- Polarization must match between transmitting and receiving antennas because mismatched polarization reduces received signal strength.
Vocabulary
- Radiation pattern
- A radiation pattern is a graph showing how strongly an antenna transmits or receives energy in different directions.
- Dipole antenna
- A dipole antenna is a simple antenna made of two conductive elements, commonly with total length about one half wavelength.
- Monopole antenna
- A monopole antenna is a single conductive element that usually operates above a ground plane with length about one quarter wavelength.
- Gain
- Gain describes how much an antenna concentrates radiated power in a direction compared with a reference antenna.
- Beamwidth
- Beamwidth is the angle across the main lobe of a radiation pattern, usually measured between the -3 dB points.
- Polarization
- Polarization is the orientation of the electric field of the radio wave, such as vertical, horizontal, or circular.
Common Mistakes to Avoid
- Confusing gain with creating extra power is wrong because gain redirects power rather than increasing the transmitter output power.
- Using frequency in megahertz directly in lambda = c / f is wrong because the formula requires frequency in hertz unless a converted shortcut is used.
- Reading a radiation pattern as physical antenna shape is wrong because the pattern shows field strength versus direction, not the metal geometry.
- Ignoring polarization is wrong because a vertical antenna and a horizontal antenna can lose significant signal even when pointed correctly.
- Assuming every omnidirectional antenna radiates equally in all 3D directions is wrong because many are omnidirectional only around the horizontal plane and have nulls above or below.
Practice Questions
- 1 A radio system operates at 100 MHz. Find the wavelength and the approximate total length of a half-wave dipole.
- 2 A quarter-wave monopole is designed for 300 MHz. Estimate the antenna length using c = 3.00 x 10^8 m/s.
- 3 An antenna receives maximum power at 0 degrees and half-power at -25 degrees and +25 degrees. What is its half-power beamwidth?
- 4 Explain why a high-gain directional antenna may be better for a point-to-point link but worse for a moving receiver.