Antennas

Overview

Resonance

Current / Voltage

When a radio transmits, it generates an AC current which is applied to an antenna. The oscillating electrons in the antenna generate oscillating electric and magnetic fields that radiate away from the antenna as radio waves.

The same happens during recieve, but the opposite way around.

Radio waves couple to electrons in the antenna and create an AC current that the reciever than detects.

There is a place on any antenna where the AC current will be naturally highest, and the typical referance example for this is the centre of a resonant (correct length) dipole.

Centre of dipole = best signal at radio.

The trouble with centre fed dipoles is that they are ugly to implement in a mobile installation. Not to worry! We can cheat.

If you place a mirror on the floor, and place the upper part of a dipole (1/4 wave whip) vertically on that mirror, you will see the real whip and an upside down reflection of that whip. A groundplane does something similar for RF.

When you place a 1/4 wave whip onto a sufficiently large groundplane, the RF "sees" the whip and acts as though the other half of a full 1/2 wave dipole is below the groundplane.

The highest current point of an antenna like this is at the bottom of the whip (where you want it so that it can carried by the coax to the radio).

It also has the added benefit of giving the feedpoint of the whip an impedence of around 50 ohms (not exactly but close enough) just like the coax and radio.

With no groundplane, the other half of the antenna is no longer present. The system no longer behaves like a centre fed dipole with the highest current at the feedpoint, and the impedence will be in the THOUSANDS of ohms.

For the avoidence of doubt, this is bad.

A groundplane is absolutely VITAL to a well fuctioning antenna of this design.