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37. ANTENNAS

The purpose of an antenna is to transmit and receive electromagne�c radia�on. When the
antenna is not connected directly to the transceiver, we need a transmission line (feed line)
to transfer the received or transmi�ed signal to and from the antenna.
I will mostly be talking about transmission. Whatever is said about transmission applies
equally to recep�on. We know this as reciprocity. An antenna's characteris�cs are the same
for transmission and recep�on.
In a perfect transmi�ng system, the transmission line would transfer all the power from the
transmi�er to the antenna with no losses. The antenna should then radiate all the power it
receives as electromagne�c radia�on.
We have discussed the mechanism by which an electromagne�c wave is radiated by an
antenna. If you have forgo�en this, refer back to the chapter on electromagne�c radia�on.

THE HALF-WAVE DIPOLE

If you take a basic balanced transmission line such as 300Ω TV ribbon, split it and pull it
apart, you will form a dipole antenna (refer to Figure 37-1). Each side of the halfwave dipole
will be ¼ wavelength. We learned earlier that the free-space wavelength of an

electromagne�c wave is found from:

Equation 37-1. The simplified wavelength equation
This equation is derived from:

Equation 37-2. The frequency wavelength equation

Where c ≈ 300,000,000 metres/second. The constant ‘c’ is the velocity of any
electromagnetic wave in free space.
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