Antenna Basics — How a Wire Radiates - Antennas & Feedlines

Antenna Basics — How a Wire Radiates

What an Antenna Really Does

An antenna converts electrical energy (RF current from your transmitter) into electromagnetic waves that travel through space. On receive, it does the reverse.

The simplest effective antenna is a half-wave dipole — just a piece of wire, half a wavelength long, split in the middle and fed with coax.

The Half-Wave Dipole

How long should it be? The formula accounts for the "end effect" (the electrical length is slightly shorter than the physical length):

\( L_{metres} = \frac{143}{f_{MHz}} \)

Examples:

40m band (7.1 MHz): 143 / 7.1 = 20.1 metres total
20m band (14.2 MHz): 143 / 14.2 = 10.1 metres total
2m band (146 MHz): 143 / 146 = 0.98 metres total

Gain — What It Really Means

Antenna gain doesn't create extra power — it focuses the existing power in a preferred direction, like squeezing a balloon. More gain in one direction = less in others.

Gain is measured in two ways:

dBi: Compared to an imaginary perfect point source (isotropic radiator)
dBd: Compared to a half-wave dipole

\( \text{dBi} = \text{dBd} + 2.15 \)

A dipole has 2.15 dBi (or 0 dBd — it IS the reference).

Watch out: Antenna advertisements sometimes use dBi to make gain look higher. A "7 dBi" antenna is only about 5 dBd. Always check which reference is being used.

Radiation Resistance

An antenna has a property called radiation resistance — it's the "resistance" that accounts for the power actually being radiated. For a half-wave dipole in free space:

\( R_{rad} \approx 73 \, \Omega \)

This is close to 50 Ω but not exact — which is why a dipole has a slight SWR when fed with 50 Ω coax. Still perfectly usable!

Polarisation

Horizontal: Dipole mounted horizontally — standard for HF (ionosphere scrambles polarisation anyway)
Vertical: Common for VHF/UHF mobile and base antennas
Circular: Used for satellites — handles random Faraday rotation in the ionosphere

Cross-polarisation loss (horizontal antenna receiving a vertical signal) is about 20 dB — significant! This is why matching polarisation matters on VHF/UHF.

Effective Radiated Power (ERP)

The power that appears to be transmitted, taking antenna gain and feedline losses into account:

\( ERP_{dBW} = P_{tx(dBW)} + G_{dBd} - L_{feedline(dB)} \)

With 400W and a 3-element Yagi (7 dBd) but 1 dB feedline loss: ERP = 26 + 7 − 1 = 32 dBW = 1585 W ERP. Your 400W radio looks like nearly 1600W in the antenna's main beam direction!