Download Antennas & Feedlines

Simple radio communications system
Electric field around elements
Magnetic field around elements
Relation between electric and magnetic fields
Horizontally polarized antenna
The most common, and perhaps the simplest,
antenna is the half-wave dipole antenna. As the
name suggests, it measures close to one half
wavelength from one end of the antenna to the
other. A simple dipole mounted so the conductor
is parallel to the Earth's surface is a
horizontally polarized antenna. (T9A03)
The direction that radiation is strongest from a
half-wave dipole antenna in free space is
broadside to the antenna. (T9A10)
Vertical antenna
Another kind of amateur radio antenna is the
quarter-wave vertical antenna. For vertical
antennas, the electric field is perpendicular to
the Earth. (T9A02) This makes them verticallypolarized antennas.
The approximate length of a quarter-wavelength
vertical antenna for 146 MHz is 19 inches.
(T9A08)
Antenna gain?
Antenna gain?
Beam antennas
A beam antenna is an antenna that
concentrates signals in one direction.
(T9A01)
The quad, Yagi, and dish antennas are
directional antennas. (T9A06)
The gain of an antenna is the increase in signal
strength in a specified direction when
compared to a reference antenna. (T9A11)
[Show 70cm beam.]
Hexbeam made by Bob, W1SRB
Portable, collapsible, hexbeam, on Rocky Mount
Two principal types of transmission line
Uses of coaxial cable?
Reflected energy - SWR
In general terms, standing wave ratio (SWR) is a
measure of how well a load is matched to a
transmission line. (T7C03) The reason it is
important to have a low SWR in an antenna
system that uses coaxial cable feedline is to
allow the efficient transfer of power and
reduce losses. (T9B01)
[This answer is incomplete. It ignores the possible
re-reflection by a tuner, as well as the built-in
power reducing circuits in modern radios where
no tuner is at work.]
Power lost in a feedline is converted into heat.
(T7C07)
SWR- Good, Bad, Dangerous!
1 to 1 is the reading on an SWR meter indicates a
perfect impedance match between the antenna
and the feedline. (T7C04)
2 to 1 is the approximate SWR value above which
the protection circuits in most solid-state
transmitters begin to reduce transmitter power.
(T7C05) [Bad]
An SWR reading of 4:1 means that there is an
impedance mismatch. (T7C06) [Dangerous]
[Again, the answer is misleading. Any SWR not
1:1 shows some mismatch.]
Antenna Tuner Action
Note that “SWR Meter”, when used with Antenna Tuner,
Doesn’t actually measure SWR on feedline
Measures Impedance Ratio
Between Zload and design Z = 50 Ohms
“SWR =1:1” means Zload = 50 Ohms
Transmitter
is Happy, because
it can deliver power
to antenna system!
Low SWR
Here
“Impedance
Transformer”
High SWR
on feedline is still
present and is
usually OK
6m dipole length?
Cables entering building
Professional climber changing coax