Feedlines and Antenna Systems

Introduction

Transmission Lines and Antennas — SWR, impedance matching, antenna types, polarization, and wavelength calculations

Transmission Line — carries radio frequency energy from transceiver to antenna with characteristic impedance as the voltage to current ratio

Coaxial Cable — unbalanced line with centre conductor, dielectric, braided shield, and jacket, 50 ohms characteristic impedance
Balanced Line — two conductors at equal voltage above and below ground, ladder line 300 to 600 ohms

Common Connectors — PL-259 for HF, N-type for VHF and UHF, BNC for test equipment, SMA for handhelds

Signal Loss — increases with length and frequency due to skin effect, expressed in dB per 100 feet or 100 metres

SWR — ratio of maximum to minimum voltage on the line, 1 to 1 is perfectly matched, below 1.5 to 1 acceptable
High SWR Causes — open or short in feedline, wrong impedance antenna, damaged connector, moisture in cable

Antenna Tuner — adjustable LC network that transforms impedance to 50 ohms for the transceiver

Half-Wave Dipole — two quarter-wave elements fed at centre, feed point approximately 73 ohms in free space

HF DX often uses horizontal polarization for lower man-made noise pickup and better skywave performance.

Wavelength — 300 divided by frequency in megahertz gives wavelength in metres (lambda = 300/f)

Yagi Antenna — driven element, reflector longer, one or more directors shorter, parasitic elements not directly fed
Yagi Design — traps allow multiband operation on 20, 15, and 10 metres, longer boom gives higher gain

Quarter-Wave Vertical — quarter-wave radiator plus ground plane with radials, formula 75 divided by frequency in megahertz
Loading Coil — makes a short antenna appear electrically longer, cancels capacitive reactance

Quad Antenna — two or more square loops of wire, each loop a full wavelength in perimeter
Delta Loop — triangular loop with bottom element parallel to ground, polarization depends on feed point

Transmission Line Types — coax 50 ohms unbalanced or ladder line 300 to 600 ohms balanced, impedance determined by geometry and dielectric
Balun — converts balanced to unbalanced, needed when connecting dipole to coax
Connectors — PL-259 for HF, N-type for VHF and UHF, BNC for test equipment, SMA for handhelds
Line Loss — increases with length and frequency, RG-58 has excessive loss at 70 cm, use RG-213 or LMR-400 for UHF
SWR — 1 to 1 is perfect match, below 1.5 to 1 acceptable, high SWR causes reflected power back to final amplifier
Antenna Tuner — transforms impedance to 50 ohms, does not make antenna resonant, protects solid-state finals
Dipole Formula — 150 divided by frequency in megahertz gives total length in metres, feed point approximately 73 ohms in free space
Polarization — electric field orientation, VHF and UHF FM uses vertical, HF DX uses horizontal for lower ground loss
Wavelength Formulas — 300 divided by frequency gives wavelength, quarter-wave is 75 divided by frequency, half-wave is 150 divided by frequency
Yagi — driven element, reflector longer, directors shorter, parasitic elements energized by electromagnetic field
Vertical Antenna — quarter-wave plus ground plane, requires radials, loading coil for mobile operation on lower bands
Loop Antennas — quad uses full-wave loop per element, delta loop is triangular, polarization depends on feed point