Feeders & Antennas

4A1: Coaxial Cable

Coaxial cable is the most common type of feeder used in amateur radio. It consists of:

• An inner conductor (centre wire or stranded wire)
• A dielectric insulator (plastic or foam)
• An outer screen/shield (braid or foil)
• A protective outer jacket

4A2: Coaxial Cable Types

Different coaxial cables are suited to different applications:

4B1: Loss Increases with Frequency

Coaxial cable loss increases with frequency. This means you lose more signal on higher frequencies than on lower ones.

• Higher frequencies = more loss per metre of cable
• Thicker cable (e.g., RG213) has less loss than thin cable (e.g., RG58)
• Keep cable runs as short as possible
• Use high-quality cable and connectors

4C1: Polarisation

Polarisation describes the orientation of the electric field of a radio wave.

4C2: Radiation Pattern

A radiation pattern is a graphical representation of how an antenna radiates energy in different directions.

• Some antennas radiate equally in all directions (omnidirectional)
• Others focus energy in specific directions (directional)
• Directional antennas have lobes (strong radiation) and nulls (weak radiation)

4C3: Gain (dBi and dBd)

Gain measures how much an antenna concentrates signal in a particular direction.

• dBi — decibels relative to an isotropic radiator (theoretical point source)
• dBd — decibels relative to a dipole antenna
• A dipole has 2.15 dBi gain, so: dBi = dBd + 2.15

4C4: Beamwidth

Beamwidth is the angular width of the main lobe of a directional antenna, measured between the points where the power drops to half (3 dB below maximum).

• Narrow beamwidth = more focused signal = higher gain
• Typical values: Yagi 30-60°, dish less than 1°

4C5: Antenna Length

The physical length of an antenna is related to the wavelength of the frequency it's designed for.

• A half-wave dipole is the most common antenna
• Length (metres) = 143 / Frequency (MHz)
• Example: For 145 MHz, a half-wave dipole is about 0.99m long

4C6: Directivity

Directivity is the ability of an antenna to concentrate radiation in one direction. It's related to gain but doesn't account for losses.

Antenna efficiency (4D1) = Gain / Directivity. Losses reduce the actual gain below the directivity.

4E1: Standing Wave Ratio (SWR)

SWR (Standing Wave Ratio) measures the impedance match between your transmitter, feeder, and antenna.

• Perfect match: SWR = 1:1 (all power goes to the antenna)
• SWR = 2:1 — acceptable, most power reaches the antenna
• SWR > 3:1 — problematic, significant power reflected back
• High SWR can damage your transmitter

4E2: SWR Measurement

SWR is measured using an SWR meter placed in the feeder line between the transmitter and antenna. Some transceivers have a built-in SWR meter.

• Measure SWR on each band you operate
• SWR varies across a band — it will be lowest at the resonant frequency
• Take readings at several points across the band

4F1: Antenna Tuner / Matching Unit

An antenna tuner (also called a matching unit) is used to match the impedance of the antenna system to the 50Ω expected by the transmitter.

• Helps achieve a low SWR when the antenna isn't perfectly resonant
• Doesn't actually change the antenna — matches the impedance
• Useful for multi-band operation with a single antenna
• Cannot fix a broken or poorly designed antenna

4G1: Popular Antenna Types

4H1: Common Connector Types