TECHNICAL NOTE / ANTENNAS
An antenna system includes more than its radiating element. It includes the feed, balun or choke, transmission line, mount, nearby objects and ground environment. Antenna gain quoted in free space does not automatically describe the installed radiation pattern.
1. Frequency and electrical size
Wavelength in free space is approximately 300 / frequency in MHz, in meters. A nominal quarter-wave radiator is around one-fourth of that wavelength before end effects and conductor diameter are considered. These are starting estimates, not finished dimensions.
2. Pattern and polarization
Antenna gain is directional concentration, not additional transmitter power. For VHF terrestrial work, horizontal polarization is common for SSB and weak-signal operating; FM activity often uses vertical polarization. Cross-polarization can cause serious loss even when two stations have ample transmit power.
3. The installed environment
- Keep antennas clear of obstructions and nearby metal when practical; a nearby support structure can detune the feed and distort pattern.
- Model or measure the installed height and terrain horizon when long-range terrestrial contacts are the goal.
- Plan a safe rotor loop with suitable bend radius and strain relief.
- Assess conducted and radiated common-mode current; an excellent match at the coax connector is not proof that the system is balanced.
4. Measure, do not assume
Start with a calibrated VNA measurement at the antenna or de-embed the feed line. Record frequency, VSWR, impedance and installation state. VSWR alone says little about efficiency or directivity: a dummy load can present a beautiful match.
For a narrowband directional antenna, compare beam shape and contact performance with consistent power, direction and conditions. Do not publish claimed modeled gain as measured gain without a valid measurement method.