Antennas Part 2
Extra Class Course
Spring 2013
Jack Lunsford, KD7RCJ
Antenna Systems
• All antennas are really Antenna Systems consisting minimally
of a radiating element and a transmission line, they need to
work together efficiently.
• Feed point impedance and transmission line impedance must
match for efficiency and they are almost never the same.
• Best place to apply matching technique is at the feed point
because feed line losses will be minimal and maximum
transfer of power to the radiating element occurs when
matched.
• When not matched some power is reflected back down the
feed line toward the source creating interference patterns. .
These are Standing Waves.
Standing Wave Ratio - SWR
• Standing Wave Ratio SWR is the ratio of the higher impedance to the
lower. It is always equal to or greater than 1:1.
Examples:
50ohm feed line connected to 200 ohm load
SWR = 200/50 = 4:1
50 ohm feed line connected to 10 ohm load
SWR = 50/10 = 5:1
5o ohm feed line connected to 50 ohm load
SWR = 50/50 = 1:1
SWR
• SWR can be measured anywhere along the feed line, commonly at the
transmitter where feed line is connected.
• SWR meters (SWR bridge) are used.
• Transmitters are designed to work with SWR at the input to the feed line of
2:1 or less, higher than that may cause the transmitter to fold back (reduce
power) .
• High SWR can damage transmitter equipment
• High SWR can be caused by mismatch between transmitter and feed line,
mismatch between feed line and antenna, a faulty feed line. Antennas too
long or short will have extreme feed point impedance and produce high
SWR.
• Tuning for best SWR does not necessary mean the antenna is resonant.
Impedance Matching
• Several factors must be considered
• Resonance and SWR
• Capacitive reactance, inductive
reactance, resistance of the antenna
and the feed line.
• Each of these depend on
frequency, ground condition and
surrounding items.
• We will look at several systems
but they all do the same thing
Hairpin Matching System (beta match)
• Popular for matching driven element of Yagi antenna.
• DE is a bit short so it has some XC, the Hairpin adds XL.
• A balanced antenna
So balun is needed
if fed with coax.
Delta Matching System
• Used to match high impedance transmission line to low
impedance antenna element.
• Z=E/I so there is a point in the antenna where Z is a high
impedance (more than 200 Ohms or so).
• Radiating element is not
broken at the center (no
center insulator required) and
it is a balanced system, no
balun required.
• Difficult to adjust because
changing either A or B changes
the other.
Gama Matching System
• Matching feed line and load eliminates reflected power and SWR is 1:1,
this is best accomplished at the antenna feed point. Low SWR in the feed
line means less feed line loss.
• Most yagi antennas have feed
point impedance of 20 to 25
ohms. Gama match at the feed
point is one way to provide a
1:1 match with the added
advantage that the driven
element
need not be insulated from the
boom.
• There are other techniques such
as beta (hair pin) match, omega
match, impedance transformers
and stubs.
Stub Match
• Adding reactance in parallel with the transmission line can be
used to create a match at the feed point.
• A + B = ¼ wavelength
• Stubs are either open or
closed depending on
whither Z is capacitive or
inductive.
Impedance Matching
• Matching at the antenna feed point is not always practical and so is often
done at the transmitter end of the feed line.
• The device used has many names, impedance matcher, trans match,
antenna coupler, antenna tuner. “Antenna tuner” does not tune the antenna
at all – it just changes the impedance of the antenna system at the end of the
feed line to match the transmitter.
• The SWR on the feed line does not
change unless matched at the feed
point of the antenna.
• Matching devices use capacitors
and inductors that can be adjusted
to establish a match.
• Balanced lines require that output
of matcher be balanced.
Feed Lines
Transmission Line
• In free space Radio waves travel at the
speed of light (30 x 10↑8 m/second)
• As a wave front encounters an antenna
it induces a current in the antenna
varying at the frequency of the wave.
• In a transmission line the applied
voltage travels along the wire causing
a current to flow.
• The magnitude of the current at any
point along the wire varies thru the full
range of values at the frequency of the
applied voltage.
• Even at the speed of light it takes time
to travel the length of the wire
Velocity of Propagation and
Electrical Length
• When the insulation between conductors is air the energy
travels along the line very near the speed of light.
• When the dielectric material is other than air the energy travels
along the line more slowly.
• VF (Velocity Factor is the ratio of actual velocity to speed of
light in a vacuum.
Electrical Length
• Electrical length of transmission line (or an antenna) is not the
same as physical length if insulation is present.
• WHY ?
Feed Line Loss
• Feed line loss increases as the frequency increases.
• Loss is greater if impedances are not matched. (high SWR)
Reflection Coefficient and SWR
• Reflection Coefficient is ratio of reflected voltage to incident
(forward) voltage. Also reflected current to forward current.
• Determined by ratio of feed line impedance to load impedance.
• The only case when reflection coefficient is 0 is when ZL = ZO.
• If reflections exist there will be a standing wave pattern along
the feed line (SWR)
Power Measurement
• Use directional RF wattmeter or power meter to rear forward
power and reflected power directly. Compute reflection
coefficient from the readings.
• Power to the load is the difference between the two readings.
PL = PF – PR
• PF is 100 watts, PR is 25 watts how much power is absorbed in
the load? 100 – 25 = 75 watts.
Smith Chart
• Used to calculate impedance
along a transmission line, SWR
values and to solve impedance
matching problems.
• Remember graphing impedance
on a rectangular coordinate
system?
• Vertical lines are constant
resistance values.
• Horizontal lines are constant
reactance values.
Smith Chart Construction
Smith Chart Anatomy
• Straight line is real
resistance
• Circles are constant
resistance circles
• Arcs are complex value
of reactance, either
capacitive or inductive.
(-jX or +jX). Constant
reactance arcs.
The Unit Circle,
Normalization and SWR
The Bold circle which crosses
the resistance axis at the center
of the chart is the Unit Circle.
If we work with a particular
characteristic impedance for
example 50Ὡ then all
reference points on the chart
are multiplied by the value of
that impedance. This is
normalization.
The prime center is the center
of a family of circles
representing constant SWR. 0
at the center increasing to
infinity at the edge of the chart.
Stubs
Sim Smith
Ward Harriman AE6TY’s
Free interactive Smith
Chart software. See QST
May 2013 Hands on Radio,
Experiment 124, page 60
.
Google search of Interactive
Smith Chart brings up several
others
Synchronous Transformers
• A series of reflections of the
right magnitude and phase so
that two different impedances
can be connected together
without creating any SWR in
the transmission lines.
• The transformer must be ¼
wavelength to function.
• To match 50 ohm feed line to
100 ohm antenna.
Antenna Analyzers
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Introduced for Amateur Radio in the 1990’s
Tunable RF source
Frequency counter
Impedance bridge
Display
Microprocessor to run it all. Battery powered
Measures Impedance, SWR, Reactance and Frequency.
Connected directly to the impedance to be tested, a circuit, a
component, a transmission line, an antenna. Has its own low
power signal source, no need to turn on the transmitter.
• Can also measure feed line length and characteristic impedance
and find shorts or opens.
Antenna Design and Antenna Modeling
• A number of programs for PC,s are available to model and
study performance of antenna designs.
• The characteristics of a design can be adjusted until the design
meets expectations.
• Programs based on Numerical Electromagnetics Code (NEC)
uses technique method of moments.
• Antenna elements modeled as “wires”, can be wires of specific
size or tubes or rods, size specified.
• The wires are modeled in segments, more segments better
accuracy.
Modeling programs for Hams
• EZNEC by Roy Lewallen W7EL
• 4NEC2 by Arie Voors, free download, tutorial
in French and German
• NEC Win Plus by Nittany Scientific
• Basic Antenna Modeling-A Hands On Tutorial
by L.B.Cebik W4RNL (SK)
• QST 4 part series also by L.B.Cebik
• ARRL Antenna Book
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