This group covers how antennas work in general: what directional and gain mean, how polarization is defined, how length sets resonant frequency, and why compact antennas trade away performance.
Directional antennas and gain
A beam antenna (such as a Yagi) concentrates its signal in one direction instead of spreading it evenly. That focusing is called gain: the increase in signal strength in a given direction compared with a reference antenna, such as a dipole or an isotropic radiator. Gain is not extra power added by the antenna. It is power redirected, and it is gained at the expense of other directions.
An isotropic antenna is a theoretical point source that radiates equally in all directions, so it is the reference and has no gain. Of the common antennas, the Yagi offers the most gain. A 5/8-wave vertical and a J-pole have modest gain, and the dipole’s pattern is a simple figure eight.
Dipole radiation pattern
A half-wave dipole radiates most strongly broadside, meaning at right angles to the wire. Little energy leaves off the ends. It does not radiate equally in all directions, and the feed line direction does not matter.
Polarization
Polarization is the orientation of the electric field in the radiated wave. A vertical antenna produces vertically polarized signals, and a horizontal dipole produces horizontal ones. Stations on line-of-sight paths generally need to use the same polarization for best signals.
Length, resonance and loading
An antenna is resonant when its electrical length is a particular fraction of the wavelength, such as a half wave for a dipole or a quarter wave for a simple vertical. Longer antennas resonate at lower frequencies, and shorter ones at higher frequencies. So shortening a dipole raises its resonant frequency, and lengthening it lowers the frequency.
Loading makes an antenna act electrically longer than it is physically. The usual method is inserting an inductor (a loading coil) in the radiating element. Capacitive loading, such as end hats, also lowers the resonant frequency. Both are ways to fit a lower-frequency antenna into a smaller space.
The quarter-wave length in inches is about 2808 divided by the frequency in MHz. At 146 MHz that gives roughly 19 inches, which is why a 19-inch whip is a resonant quarter-wave on 2 meters.
Mobile and handheld antennas
A 5/8-wave whip has more gain than a 1/4-wave whip, which makes it popular for VHF and UHF mobile use. It concentrates its energy lower, toward the horizon.
A handheld’s short flexible antenna is much shorter than a quarter wavelength, so it is inefficient. Using a handheld inside a car makes this worse, because the metal body shields the signal and reduces strength. An outside antenna avoids this.
Key facts to memorize
- Beam antenna: concentrates signal in one direction.
- Gain: signal increase in a specified direction versus a reference antenna.
- Polarization: orientation of the electric field.
- Dipole: strongest broadside to the wire.
- Shorter antenna: higher resonant frequency; longer: lower.
- Loading coils lengthen an antenna electrically.
- Quarter-wave in inches is about 2808 / f(MHz); 19 inches is a quarter wave on 2 meters.
- A Yagi has more gain than a 5/8-wave vertical, J-pole or isotropic.
- A 5/8-wave whip has more gain than a 1/4-wave whip.
- Handheld rubber-duck antennas are low in efficiency.
Memory hooks
- Shorter wire, higher frequency: short wavelengths go with high frequencies.
- A dipole radiates like a doughnut around the wire, so broadside is strongest.