Transcript Chapter 6 Notes

Chapter 6
Antennas
Antenna Basics
• Antennas contain conducting elements
(wire or metal poles).
• Radio waves are electromagnetic waves.
An electromagnetic wave contains an
electric field component and a magnetic
field component. The electric and
magnetic fields are at right angles to each
other.
• The polarization of an antenna is defined
as the orientation of the electric field
component relative to the Earth’s surface.
The electric field component is in the plane
of the elements.
• The feed point impedance is the ratio of
the RF voltage to current at the feed point.
• A resonant antenna will have only a
resistive feed point impedance (no
reactance)
• The radiation pattern of an antenna is a
plot of its signal strength in every direction
(3D)
– An azimuthal pattern is a 2D slice of the
radiation pattern in the horizontal plane.
– An elevation pattern is a 2D slice of the
radiation pattern in the vertical plane.
– Lobes are areas where the antenna is
radiating and nulls are the points between the
lobes where no, or little, occurs.
• An isotropic antenna radiates equally well in all
directions. This is a spherical pattern with one
lobe (the sphere) and no nulls. Isotropic
antennas are used as a reference.
• Omnidirectional antennas radiate equally in all
horizontal directions.
• Directional antennas has a main lobe(s) in one,
or more directions.
• Gain, measured in dB, is the ratio of power
transmitted in a given direction over the same
power from a dipole (dBd) or isotropic radiator
(dBi).
• A directional antenna has a front-to-back
ratio (measured in dB) which is the ratio of
the gain of the antenna in the forward
direction to the gain in the opposite
direction. The front-to-side ratio
(measured in dB) is the ratio of the forward
power to the power radiated at right
angles.
Dipoles
• Dipoles are simple, but effective HF
antenna. They are a half wavelength long
with the feed point in the middle.
• A dipole in free space has an feed point
impedance of 72 Ω, but varies at lower
heights. The feed point impedance
increases to several thousand ohms as it
is moved away from the center to the
ends.
• The free space length of a dipole in feet is given
by 492/f, where f is the frequency in MHz. In
practice, the actual length is closer to 468/f, but
use 492 for the test.
– Example: Calculate the free space length of a ½ wave
dipole for 3.75 MHz.
• 492/3.75 = 131.2 ft
• Always start with a longer length and then trim to
get the desired frequency.
• Dipoles cut for one frequency can be used at
odd harmonics. For example, 7 MHz (40 M)
dipole will also work at 21 MHz (15 M).
Ground Planes (Verticals)
• Ground plane antennas are essentially
vertical dipoles with the lower half
replaced by a ground plane.
• HF ground planes are typically mounted
with the vertical element on the ground
and radials laid on the ground or just
below the surface.
• The pattern is omnidirectional (turn Fig 61(B) on its side).
• The free space length in feet of a vertical
is 246/f, where f is the frequency in MHz.
– Example: Calculate the length of a ¼ wave
vertical at 21.2 MHz.
• 246/21.2 = 11.6 ft.
• Feed point impedance is about ½ that of a
free space dipole, or 35 Ω.
– The impedance can be increased by
increasing the droop angle of the radials.
• HF mobile antennas are usually verticals
which are much shorter than a full size ¼
wave. Loading is used to increase the
elements electrical size.
– Loading decreases the operating bandwidth
without retuning. Loading techniques include;
• Loading coils at the base or in the middle.
• Capacitance hats at the top of the antenna.
• Linear loading is when the antenna is folded back
on itself.
– A corona ball at the tip of the antenna acts to
eliminate high voltage discharges from the tip
of the antenna.
• Random wire antennas are non-resonant
antennas that require a antenna tuner.
– A true random wire antenna connects directly
to the antenna tuner and therefore there can
be significant RF voltages or currents present.
Effects of Height Above Ground
• An antenna’s feed point impedance varies
for different heights. This is due to the
interaction between the antenna and its
image (a good conducting ground looks
like a mirror to RF).
• The feed point impedance steadily
decreases below a height of ¼
wavelength, and is zero when on the
ground.
• The dipole’s
radiation pattern
also changes with
height. At heights
below ½
wavelength, the
pattern is nearly
omnidirectional
and is maximum
straight up.
• RF signals that are horizontally polarized
will have lower losses when reflecting off
the ground.
– Dipoles generally have a high angle of
radiation which does not favor DX.
– Verticals have a lower angle of radiation and
can work better for DX.
Yagi Antennas
• Yagi antennas are directional antennas
with good front-to-side and front-to-back
ratios to minimize interference, and good
forward gain to hear and be heard better.
• A Yagi is a parasitic array antenna made
up of two or more elements. The main
lobe is in the forward direction of the Yagi.
– The driven element is an approximately ½
wave dipole.
– The reflector is a parasitic element behind the
driven element (opposite the direction of the
main lobe), and is a bit longer than the driven
element.
– The director is a parasitic element in front of
the driven element, and is a bit shorter than
the driven element.
– A three element Yagi has a theoretical gain of
9.7 dBi and front-to-back ratio of 30 to 35 dB.
• A Yagi can have additional directors to
increase gain, but the gain is limited.
• Larger diameter elements improves the
bandwidth of at Yagi. This is also true for
other antennas.
• Element spacing, boom length, and the
number of elements all effect the SWR
and performance of a Yagi.
• Yagi antennas have a feed point
impedance of around 20 to 25 ohms.
• A gamma match is used to match the
Yagi’s feed point impedance to 50 Ω. The
driven element can be electrically
connected to the boom which makes
construction easier.
Loop Antennas
• Loops can be 4 sided (quad loop) or three
sided (delta loop). Radiation is maximum
broadside to the plane of the loop.
• A directional antenna can be made from
much like a Yagi with a driven loop and
parasitic loops.
– A quad antenna uses a full wave driven
element ¼ wave on a side.
– A delta antenna uses a full wave driven
element 1/3 wavelength on a side.
– Director elements for a quad or delta are
about 5% shorter than the driven element.
Reflectors for a quad or delta are about 5%
longer than the driven element.
– The gain of a two element quad or delta is
about the same as a 3 element Yagi, but the
front-to-back ratio is better for the 3 element
Yagi.
– The polarization of the quad antenna can be
changed by moving the feed point.
Specialized Antennas
• Near vertical incidence sky-wave (NVIS) is
an antenna that is designed to radiate
mostly straight up (high vertical angle).
– NVIS is useful for short skip to cover a
regional area.
– Useful for emergency communications
– A dipole mounted λ/10 to λ/4 above the
ground is an NVIS antenna.
– A horizontal loop is an NVIS antenna.
• Stacked Antennas
– Vertically stacking
antennas narrows the
elevation beamwidth of
the antenna and
increases the gain.
Spacing of about λ/2
provides about 3 dB
gain. For another 3 dB
gain, the array must be
doubled to four.
• Log Periodic
– Log periodic antennas are a driven array.
They can be designed to be very broad band
(10:1), but have less gain and lower front-toback ratio than a Yagi.
– TV antennas are usually log-periodic.
– Length and spacing of the elements varies
logarithmically.
• Travelling Wave Antenna
– The beverage antenna is an example of a
travelling wave antenna.
– The beverage antenna is used for directional
receiving, but is not a very efficient radiator.
• Multiband Antennas
– A trap dipole uses LC
traps so that the dipole
looks resonant for more
than one band.
• At their resonant
frequency the trap looks
like an open circuit and
cuts off the rest of the
antenna.
• At lower frequencies the
trap adds inductance
making the antenna look
electrically longer
• At higher frequencies the
trap adds capacitance
making the antenna look
electrically shorter.
Feed Lines
• All feed lines have two conductors
• All feed lines have a characteristic
impedance (Z0).
• The characteristic impedance of parallel
feed lines (balanced feed lines) is
determined by the radius of the conductors
and the distance between them.
– TV type twin lead has an impedance of 300 Ω.
– Open wire or ladder line has impedances from
300 to 600 Ω.
• The characteristic impedance of coaxial
transmission lines is determined by the
diameter of the inner and outer
conductors, and their spacing.
– The insulating material between the inner and
outer conductors affects the feed line loss and
velocity factor (how fast a wave travels down
the cable).
– 50 Ω and 75 Ω cables are the most common.
Forward and Reflected Power
• A feed line delivers all the power to the
antenna when the antenna’s feed point
impedance and the feed line’s
characteristic impedance are the same.
• A reflection occurs when the impedances
don’t match. Forward power is power
traveling toward the antenna and reflected
power is power reflected back due to the
impedance mismatch.
• The forward and reflected power create
standing waves on the transmission line.
• A standing wave ratio (SWR) of 1:1
represents a perfect match (no reflected
power). A higher SWR means more
reflected power.
• SWR is the ratio of the antenna
impedance to the feed line characteristic
impedance, and is always greater than 1.
– Example: What is the SWR in a 50 Ω
transmission line when connected to an
antenna with a feed point impedance of 25 Ω?
• SWR = 50/25 = 2:1
– Example: What is the SWR in a 50 Ω
transmission line when connected to an
antenna with a feed point impedance of 250
Ω?
• SWR = 250/50 = 5:1
• A high SWR can damage a transmitter
because of the reflected power returning
the transistors (or tubes) in the final power
amplifier.
• Matching the antenna to the feed line will
maximize the power delivered from the
transmitter to the antenna.
• A device to match the feed line to the
antenna is called an impedance matcher,
transmatch, antenna coupler, or antenna
tuner.
• A section of transmission line connected in
parallel, called a stub, can be used to
match impedances.
• Impedance matching does not change the
SWR in the feed line from the matching
device to the antenna. Only the SWR
between the transmitter and the
impedance matching device will be low.
Feed Line Loss
• All feed lines will dissipate some energy as
heat. This loss effects both receive and
transmit.
• Air insulated transmission lines tend to
have the lowest loss.
• Loss is measured in dB/100 feet.
• Loss increases with frequency for all
transmission lines.
– Example: RG-8 has a loss of 1.08 dB/100 ft at
30 MHz and 2.53 dB/100 ft at 150 MHz.