How polarization, length, loading and gain decide where an antenna sends your signal.
11 pool questions1 on the exam
Syllabus: Antennas: vertical and horizontal polarization, concept of antenna gain, definition and types of beam antennas, antenna loading, common portable and mobile antennas, relationships between resonant length and frequency, dipole pattern
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Compendium articles on the ideas in this lesson, beyond what the exam asks.
Polarization is the direction of the electric field
An antenna's polarization is the orientation of the electric field it radiates. A vertical wire gives vertical polarization. A horizontal wire gives horizontal.
The wire's direction sets the electric-field direction (red). A receiving antenna works best when it points the same way.
Transmit antenna
Receive antenna
RememberPolarization = electric field direction. Match the antennas.
Shorter antenna, higher resonant frequency
An antenna is resonant when its length is a simple fraction of the wavelength. Higher frequency means a shorter wavelength, so a shorter antenna resonates at a higher frequency.
Reference: a dipole cut for 146 MHz. Shorten it and its resonant frequency goes up.
Resonant frequency146MHz
On 2 meters (λ about 2 m) a quarter-wave is a short 19-inch whip, so that's the classic 2 m vertical.
Same band, three lengths drawn to scale. The 19-inch whip is one quarter of the wave.
RememberLonger wire, lower frequency. Shorter wire, higher frequency. 19 inches on 2 m is a quarter-wave.
A dipole radiates broadside, not off its ends
The wire is the hole of the donut. Signal is strongest straight out from its side, weakest off its ends.
Think of a donut with the wire through the hole. Signal is strongest broadside (straight out from the side) and weakest off the ends.
RememberBroadside to the wire, never the ends.
Gain is signal pushed into one direction
Gain is the increase in signal strength in a specified direction compared with a reference antenna. No power is added: it is moved from other directions.
A beam antenna concentrates signals in one direction. The Yagi is the classic beam: a reflector, a driven element and directors.
Dashed circle = isotropic reference. Same power in each case: gain is signal moved into one direction, not power added.
A 5/8-wave vertical also has gain over a ¼-wave: it flattens the pattern toward the horizon, not up.
Side view, equal power. The 5/8 wave sends more signal toward the horizon, where the other station is.
Antenna
Gain
Isotropic
reference, none
5/8 wave, J pole
all-around, modest
Yagi
highest of these
RememberGain = more signal one way, less elsewhere. A Yagi has the most.
Short antennas trade efficiency for size
Loading makes a short antenna act longer by adding an inductor (a coil) in the radiating element. It resonates, but is less efficient than a full-size antenna, like a handheld's rubber duck.
Short antennas trade efficiency for size. A car body shields a handheld.
Inside a vehicle, the metal body shields a handheld's signal. An outside antenna avoids that.
RememberLoading coil = electrically longer. Short antenna = low efficiency.