E9A: Antenna Fundamentals
Before studying specific antenna designs, you need a solid grasp of the parameters used to describe and compare antennas. These foundational concepts — isotropic gain references, power calculations, efficiency, and the effects of ground — apply to every antenna you will encounter.
This lesson covers the isotropic radiator reference, effective radiated power (ERP) and effective isotropic radiated power (EIRP) calculations, the dBd-to-dBi conversion, antenna efficiency and radiation resistance, feed point impedance factors, ground gain, Fresnel zones, and the importance of ground radials and soil conductivity.
The Isotropic Radiator Reference
An isotropic radiator is a hypothetical, lossless point source antenna that radiates equal power in all directions simultaneously. It does not and cannot exist in the real world, but it serves as a convenient mathematical reference for comparing real antennas.
Antenna gain expressed in dBi (decibels relative to isotropic) compares the antenna's directional gain to an isotropic radiator. Gain expressed in dBd (decibels relative to dipole) compares the antenna to a half-wave dipole. A dipole itself has approximately 2.15 dBi of gain over an isotropic radiator.
ERP and EIRP Calculations
Effective Radiated Power (ERP) accounts for transmitter output power, all system losses (feed line, duplexer, circulator), and antenna gain in dBd. It represents the equivalent power a reference dipole would need to produce the same field strength in the direction of maximum radiation.
Effective Isotropic Radiated Power (EIRP) is the same calculation but uses antenna gain in dBi instead of dBd. EIRP = ERP + 2.15 dB.
150 W transmitter, 2 dB feed line loss, 2.2 dB duplexer loss, 7 dBd antenna gain.
Net: 7 − 2 − 2.2 = 2.8 dBd gain. ERP = 150 × 10^(2.8/10) = 150 × 1.905 ≈ 286 watts
ERP calculation example 2:
200 W transmitter, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, 10 dBd antenna gain.
Net: 10 − 4 − 3.2 − 0.8 = 2 dBd gain. ERP = 200 × 10^(2/10) = 200 × 1.585 ≈ 317 watts
EIRP calculation example:
200 W transmitter, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, 7 dBi antenna gain.
Net: 7 − 2 − 2.8 − 1.2 = 1 dBi gain. EIRP = 200 × 10^(1/10) = 200 × 1.259 ≈ 252 watts
dBd vs dBi Conversion
A dipole has 2.15 dBi of gain over an isotropic radiator. Therefore, to convert from dBi to dBd, subtract 2.15 dB. To convert from dBd to dBi, add 2.15 dB.
Antenna Efficiency and Radiation Resistance
Antenna efficiency is the ratio of power actually radiated to total power input. Electrically, it is expressed as:
Efficiency = Radiation Resistance / Total Resistance
Radiation resistance is a fictitious resistance that, if substituted for the antenna, would dissipate the same power that the antenna actually radiates. The remaining resistance in the total resistance is loss resistance — from conductors, loading coils, and ground losses. A higher ratio of radiation resistance to total resistance means a more efficient antenna.
Feed Point Impedance
An antenna's feed point impedance is not fixed — it varies with several factors. The most important is antenna height above ground, which changes the interaction between the antenna and its ground image. Transmission line length, antenna tuner settings, and transmitter power level do not affect the antenna's feed point impedance — they affect only what the transmitter sees.
Ground Gain
Ground gain is an increase in signal strength caused by ground reflections in the environment of the antenna. When signals reflect off the ground and combine constructively with the direct signal, the effective radiated power in the desired direction increases. This phenomenon is especially important for low-angle paths used in HF DX communication.
Fresnel Zones
The Fresnel zone is an elliptical region around the direct path between a transmitter and receiver. Objects inside the first Fresnel zone can cause diffraction losses. The size of the first Fresnel zone is inversely related to frequency — higher frequencies have smaller Fresnel zones. Therefore, the 5.8 GHz band has the smallest first Fresnel zone among common amateur microwave bands.
Ground Radials and Soil Conductivity
A ground-mounted quarter-wave vertical antenna uses the earth as part of its antenna system. The key factors are:
- Ground radial system: Installing buried radials dramatically improves efficiency by providing a low-resistance return path for antenna currents, reducing ground loss resistance.
- Soil conductivity: The ground losses for a ground-mounted vertical operating on HF are determined primarily by soil conductivity. Poor conductivity (rocky or sandy soil) causes high ground losses; good conductivity (moist loam or seawater) results in low losses and better radiation at low angles.
E9A Practice Questions
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E9B: Antenna Patterns and Designs →
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