SWR
Seeing Watts Reflected
by
Rick Hiller – W5RH
SWR
Seeing What’s Reflected
by
Rick Hiller – W5RH
Agenda
•
•
•
•
•
The Antenna System
What is SWR
How it is developed
How it is measured
Why/How we use it
Handout
(See last page of PDF)
The Antenna “System”
Antenna The Load
Transmission Line - TL
Transceiver – The Source
Defacto Standards
• 50 ohm (nominal) coax is the most
commonly used in Ham Radio RF
– Transmitter outputs designed for 50
ohms
– Antennas are commonly designed for
50 ohms
The Requirement
(our desire)
• Maximum power transfer from the
transmitter to the antenna (minimize
losses)
• This is accomplished by having a
matched system – i.e. 50/50/50 ohms
The Transmission Line
• Also called “Feed Line”
• Passive Device
• Primarily, for getting RF power from an RF
power source to an antenna
• Characteristic Zo
• Different types have differing loss numbers
• Different physical configurations
•
”If I had one wish in Ham Radio it
would be that both my antenna
and transmission line would be
totally lossless…..we can get
close, but no cigar.
“Neither the line or the antenna
actually consumes power, they
are simply passing it off to
something else”
George Grammer, W1DF – QST -1957.
What happens?
Xmitter
For demonstration purposes,
TL is many times longer than a typical Ham Station feed line
A
N
T
E
N
N
A
What happens?
FORWARD POWER
Xmitter
Voltage is
generated by
the transmitter
Zero Reflected
If the system is “matched” all power
will be accepted by the antenna
A
N
T
E
N
N
A
What happens?
FORWARD POWER
Xmitter
Reflected Power
If the system is “NOT matched”
some power will be reflected.
How much is reflected is dependent
on the value of the mis-match
A
N
T
E
N
N
A
What happens?
Xmitter
Forward wave and reflected wave travel
independently, but mix/add into a
static condition called the Standing Wave
A
N
T
E
N
N
A
Creating an envelope, similar to AM modulation
wave form.
What happens?
Xmitter
Standing Wave Envelope
The waves will add at some places
and subtract at others,
producing mins and maxes
It is almost as if the Reflected wave
is modulating the forward wave
A
N
T
E
N
N
A
SWR Demo Bessernet Web
Note: URL is on the Handout
Standing Wave Envelope
Maximum
Minimum
Standing Wave Envelope
Movie -- Similarities of Wave Behavior
(Note: URL is on the Handout)
Standing Wave Ratio
Maximum Envelope Voltage
versus
Minimum EnvelopeVoltage
SWR = E max
E min
The differential of the max to the min is
dependent on the size of the
reflected voltage wave
Standing Wave Ratio
If, the differential of the max to the min is
dependent on the size of the
reflected voltage wave…….
Then -- SWR can be used to indicate the quality
of the match between the
Transmission line and the antenna.
SWR = E max = I max = Z Ant
E min I min Z TL
SWR Cardinal Rules
• Always determined by the load
– Load vs Zo of line (which is constant)
• Does not change over the length of a
lossless transmission line (theoretical)
• Will change over the length of a normal line
• SWR is greatest at the load
SWR
• Due to inherent line loss SWR will decrease
as you approach the line input point
• Absolute and always greater than 1
SWR= Zo / Zl or Zl / Zo
• Singular – never SWR’s or s’wers
How To Measure SWR?
• With an SWR Meter, of course !!!
• Power meter – Forward / Reflected
• Cross Needle Meter
• VNA Vector Network Analyzer
Measuring SWR
Measuring SWR – Bird
Crossed Needle Meter
VNA Based Meter
Antenna Analyzer
Also use an antenna analyzer -- with caution
Very low drive signal so…..
High RF fields can distort the measurements
“Blame it on IZO” BVARC Tech Articles
(Note: URL is on the Handout)
Living in the Emperical World
A few insights and caveats
SWR Excursions
SWR
1:1
2:1
3:1
4:1
6:1
10:1
Z
50
100/25
150/16.6
200/12.5
300/8.3
500/5
50ohm +/-j
0
36
58
75
102
142
Antenna System Philosophy
• The good thing is that Hams have
“bands of frequencies” to use
• The bad thing is that hams have “bands
of frequencies” to cover
Antenna load Z’s
• Unless you are rich and famous and
own a self tuning antenna system (ie
Stepper IR) then you do not have a
resonant or matched antenna across
the “Band”.
• Your antenna generates a range of
input feed impedances which can vary
extremely.
80 Meter Dipole
Feed Z Excursions
5.29 SWR 43 –j86 ohms
3.95 SWR 79 +j88 ohms
1.18 SWR 59 +j1.3
Should we worry about low SWR?
Should we worry about high SWR?
Should we worry about low SWR?
YES
Should we worry about high SWR?
Yes
SWR is best at the Xmitter
X
M
I
T
T
E
R
A
N
T
Mis-matched loss graph
A
d
d
I
T
I
O
N
A
I
L
O
S
S
SWR
At
Antenna
4
2.25
SWR By example – Reference Sheet
It’s all math
Transmission Line Apps
• TL Details Dan McGuire AC6LA
• Provides a Smith Chart on a computer
• ARRL TLW – Dean Straw
•
ARRL Antenna Book CD
SWR Vs. Reflected E & P
Loss -- dB vs Watts
100 watt reference
dB watts(loss)
0= 0
-1 = 10
-2 = 37
-3 = 50 – half power
-4 = 40
-5 = 69
-6 = 75 – 1 ‘S’ Unit
-10 = 90
Where should I put my ATU?
If you want to minimize loss, place
the ATU at the antenna feedpoint.
What if I can’t ?
In-line matching
No Matching
Xmitter
High SWR at xmitter
Reduced power output
A
N
T
E
N
N
A
In-line matching
Matched
ATU
Xmitter
SWR 1:1
SWR 4:1
A
N
T
E
N
N
A
Matched TL losses between xmitter and ATU
Additional losses in TL between ATU and Antenna
In-line matching
Matched
ATU
Xmitter
SWR 1:1
SWR 4:1
A
N
T
E
N
N
A
Reflected power is re-reflected at the ATU
Circulating current between ATU and antenna
All power, except for losses, makes it to the antenna
Re-reflected Power
Video – “Standing Up for Standing Waves”
(Note: URL is on the Handout)
Mis-matched loss graph
A
d
d
I
T
I
O
N
A
I
L
O
S
S
Loss -- dB vs Watts
100 watt reference
dB watts(loss)
0= 0
-1 = 10
-2 = 37
-3 = 50 – half power
-4 = 40
-5 = 69
-6 = 75 – 1 ‘S’ Unit
-10 = 90
Summary
• Understand what your SWR measurement
is trying to tell you.
• Use the published TL specifications to
establish a base line for losses.
• Further you knowledge of antennas and
TL’s by reading more and using the free
tools available to you. Get hold of the
ARVN Video “SWR…etc.
Summary
• Use the appropriate coax for the situation.
• Use the best that you can afford.
• In general -- The length of transmission
line that is recommenred in the length that
gets you from your transceiver to tyour
antenna – not 1/2wl, not 1/wl, etc.
• .
Summary
• Remember the antenna and TL and
matching network are a “system”.
• Better Antenna Systems mean more
contacts………..
– More contacts mean more fun
• .......and that is what this hobby
is all about!!!!!!!
The End
SWR
Seeing Watts Reflected
Rick Hiller – W5RH
SWR—Seeing Watts Reflected
HamCom
SWR—Seeing Watts Reflected
June 12, 13 2015
HamCom
by W5RH – Rick Hiller rhiller@sdicgm.com
June 12, 13 2015
by W5RH – Rick Hiller rhiller@sdicgm.com
Terms and Definitions
Terms and Definitions
Incident (Forward) Power – the power delivered to the feed line input by the
source/generator/transmitter
Reflected Power – the portion of the incident (forward) power, sent back
toward the generator from the load, as a result of the mismatch between
the line characteristic Zo and the load Z.
Standing Wave – the wave created on a transmission line, being not
terminated in its’ characteristic impedance, by the sum of the incident and
reflected voltage waves.
VSWR (SWR) -- Voltage Standing Wave Ratio – the mathematical ratio of
the standing wave’s voltage max to voltage min. It is an easily made
measurement that is used as a reference to define the quality of the line
to load match.
=================================
Incident (Forward) Power – the power delivered to the feed line input by the
source/generator/transmitter
Reflected Power – the portion of the incident (forward) power, sent back
toward the generator from the load, as a result of the mismatch between
the line characteristic Zo and the load Z.
Standing Wave – the wave created on a transmission line, being not
terminated in its’ characteristic impedance, by the sum of the incident and
reflected voltage waves.
VSWR (SWR) -- Voltage Standing Wave Ratio – the mathematical ratio of
the standing waves voltage max. to voltage min. It is an easily made
measurement that is used as a reference to define the quality of the line
to load match.
=================================
Reference Articles and Publications
*The Why’s of Transmission Lines – Series
George Grammer QST Jan, Feb. March 1965
*Understanding SWR By Example –
Darrin Walraven – K5DVW – QST November 2006
*SWR, Reflected Power—What Do They Mean –
Joel Hallas, W1ZR, QST June 2011
*Reflections (I, II and III) –
Walt Maxwell, World Radio Books Publishing
*Transmission Lines, Antennas and Wave Guides—
King, Mimno and Wing – 1945 US Army
*ARRL Antenna Handbook –
ARRL Publishing -- Any printing from 1939 to 2007
Reference Articles and Publications
*The Why’s of Transmission Lines – Series
George Grammer QST Jan, Feb. March 1965
*Understanding SWR By Example –
Darrin Walraven – K5DVW – QST November 2006
*SWR, Reflected Power—What Do They Mean –
Joel Hallas, W1ZR, QST June 2011
*Reflections (I, II and III) –
Walt Maxwell, World Radio Books Publishing
*Transmission Lines, Antennas and Wave Guides—
King, Mimno and Wing – 1945 US Army
*ARRL Antenna Handbook –
ARRL Publishing -- Any printing from 1939 to 2007
*
Web Sites
* SWR DEMO -- http://www.bessernet.com/Ereflecto/tutorialFrameset.htm
* Bell Labs Movie -- https://www.youtube.com/watch?v=DovunOxlY1k
* Standing Waves Video --http://arvideonews.com/hrn/HRN_Episode_0199.html
* TL Details Download -- http://www.ac6la.com/tldetails.html
* BVARC Articles -- http://www.bvarc.org/index.php?page=tech
Web Sites
* SWR DEMO -- http://www.bessernet.com/Ereflecto/tutorialFrameset.htm
* Bell Labs Movie -- https://www.youtube.com/watch?v=DovunOxlY1k
* Standing Waves Video --http://arvideonews.com/hrn/HRN_Episode_0199.html
* TL Details Download -- http://www.ac6la.com/tldetails.html
* BVARC Articles -- http://www.bvarc.org/index.php?page=tech