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”

Transceiver – The Source

Antenna -

The Load

Transmission Line - TL

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.

Xmitter

A

N

T

E

N

N

A

What happens?

For demonstration purposes,

TL is many times longer than a typical Ham Station feed line

Xmitter

A

N

T

E

N

N

A

What happens?

Voltage is

generated by

the transmitter

FORWARD POWER

If the system is “matched” all power

will be accepted by the antenna

Zero Reflected

Xmitter

A

N

T

E

N

N

A

What happens?

FORWARD 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

Reflected Power

Xmitter

A

N

T

E

N

N

A

What happens?

Forward wave and reflected wave travel

independently, but mix/add into a

static condition called the Standing Wave

Creating an envelope, similar to AM modulation

wave form.

Xmitter

A

N

T

E

N

N

A

What happens?

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

SWR Demo Bessernet WebNote: URL is on the Handout

Standing Wave Envelope

Maximum

Minimum

Standing Wave EnvelopeMovie -- Similarities of Wave Behavior

(Note: URL is on the Handout)

Standing Wave RatioMaximum 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…….

SWR = E max

E min

Then -- SWR can be used to indicate the quality

of the match between the

Transmission line and the antenna.

= I max = Z AntI 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 Z 50ohm +/-j1:1 50 0

2:1 100/25 36

3:1 150/16.6 58

4:1 200/12.5 75

6:1 300/8.3 102

10:1 500/5 142

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

A

N

T

X

M

I

T

T

E

R

Mis-matched loss graphA

d

d

I

T

I

O

N

A

I

L

O

S

S

4

2.25

SWR By example – Reference Sheet

SWR

At

Antenna

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 Watts100 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

A

N

T

E

N

N

AHigh SWR at xmitter

Reduced power output

In-line matching

Matched

Xmitter

A

N

T

E

N

N

A

ATU

SWR 4:1SWR 1:1

Additional losses in TL between ATU and Antenna

Matched TL losses between xmitter and ATU

In-line matching

Matched

Xmitter

A

N

T

E

N

N

A

ATU

SWR 4:1SWR 1:1

All power, except for losses, makes it to the antenna

Reflected power is re-reflected at the ATU

Circulating current between ATU and antenna

Re-reflected PowerVideo – “Standing Up for Standing Waves”

(Note: URL is on the Handout)

Mis-matched loss graphA

d

d

I

T

I

O

N

A

I

L

O

S

S

Loss -- dB vs Watts100 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!!!!!!!

SWR

Seeing Watts Reflected

Rick Hiller – W5RH

The End

SWR—Seeing Watts Reflected HamCom June 12, 13 2015 by W5RH – Rick Hiller rhiller@sdicgm.com

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.

=================================

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

SWR—Seeing Watts Reflected HamCom June 12, 13 2015 by W5RH – Rick Hiller rhiller@sdicgm.com

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 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

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