Antenna basics
The 1/4 wave, 5/8wave and the vertical dipole have a radiation pattern which
spreads out all round and are said to be omni-directional.
The yagi on the other hand is a directional aerial and has gain due to its
ability to focus the signal in a relatively narrow beam. Gain is the
apparent increase in output that the aerial gives when compared to a dipole.
The higher the gain the more directional.
The ERP (Effective Radiated Power) is the result of multiplying the power
to the aerial by the gain of the aerial. Power x gain = ERP
The aerial must be designed to work at the frequency of the transmitted signal.
If the antenna is not properly designed then it will not work effectively
and power will be returned from the antenna to the feeder
and not radiate.
When working on HF if the aerials is not correctly
tuned to the frequency of the transmission then it is possible
to match the aerial to the Transmitter by the use of an ATU (Aerial
Tuner Unit). |