Antenna Basics for Beginners.pdf

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ANTENNA BASICS
FOR BEGINNERS
INTRODUCTION
VERTICALS
MULTIBAND VERTICALS
DIPOLES
MULTIBAND DIPOLES
RF CHOKES
1
HOW DO ANTENNAS WORK?
An Antenna Is A Basic Transducer
For transmitting,
you generate an RF signal on a conductor.
-Electric fields arise from a voltage rapidly changing
-Magnetic fields arise from a current rapidly changing
Generally people don't think of radio-frequency radiation in terms of discrete
particles (oscillating electrons and photons) -they typically use the
wave
model
instead, as it's much easier to use.
An efficient resonant antenna (1/4 wavelength or longer) produces a large-
amplitude EM wave for a given feed power, and produces little heat. An
inefficient antenna produces a small-amplitude EM wave for the same feed
power, and converts most of the power into heat.
For receiving,
the same resonance issues apply. It's just that when
receiving, the currents induced on the antenna by the passing EM field
cause a terminal voltage at the feedpoint of the antenna, which generates
a propagating signal down the coax to the receiver's input amplifier circuit.
2
VERTICAL and HORIZONTAL POLARIZATION
The
Electric field
or E-plane determines
the polarization or orientation of the radio
wave.
For a vertically-polarized antenna, the E-
plane usually coincides with the vertical/
elevation plane.
For a horizontally-polarized antenna, the
E-plane usually coincides with the
horizontal/azimuth plane.
The
Magnetizing field
or H-plane lies at a
right angle to the E-plane.
For a vertically polarized antenna, the H-
plane usually coincides with the horizontal/
azimuth plane.
For a horizontally-polarized antenna,
the H-plane usually coincides with the
vertical/elevation plane.
3
ANTENNA LENGTH
λ/2
(180°)
1 WL (meters) = 300 = Lambda
(λ)
F
MHz
λ/4
(90°)
Antenna Length
is usually
described as wavelength (WL)
in meters or degrees:
90°
180°
TIME
360°
Frequency Wavelength
(MHz) (Meters) (Feet)
1.8
160
510
3.75
80/75 252
5.36
60
175
7.15
40
131
10.125
30
92.4
14.175
20
66
18.1
17
51.2
21.225
15
44
24.9
12
37.6
28.5
10
33
52
6
18
360 deg
deg
=
Freq Length (ft)
ft
Example:
50 ft vertical used on 160 m
360 deg
deg
= 510 ft
50 ft
360 X 50 / 510 = 34.6 degrees
4
VERTICALS
Basic Vertical (Monopole) Radiation Resistance
Radiation Resistance
(R
rad
) is that
portion of the antenna input resistance
that radiates power.
Radiation Resistance
=
Power radiated / input current squared
The other portions are ground loss and
antenna structure loss that dissipate
power as heat.
Feedpoint Radiation Resistance vs Degrees
(Double for Dipole)
Example: 160 m 50 ft vertical = 34.6 deg = 6 Ohms
5

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