homebrew swr meter

Bill LeonardNCU

NATC - 285 TechConnect Radio Clubhttp://www.natc.org/

Homebrew HF SWR/Power Meter

5/4/2012

Theory

Standing Waves

Dynamic Demo: http://www.walter-fendt.de/ph14e/stwaverefl.htm

SWR = 3:1

5/4/2012

Theory

SWR CalculationNot a direct measurement

SWR = EREF

1+PRPF

1-PRPF

= +

EREF -

EF = Forward VoltageER = Reverse VoltagePF = Forward PowerPR = Reverse Voltage

SWR accuracy is only as good as the power measurement accuracy (both Forward & Reflected)

5/4/2012

Theory

How should RF Power be measured?1. RF voltmeter connected at the output of the transmitter

5/4/2012

Theory

How should RF Power be measured?1. RF voltmeter connected at the output of the transmitter

No!1. Voltage reading is dependent upon load impedance at the point

of measurement Power meters on Ameritron Power Amplifiers

2. To determine power, we need independent measurements of voltage and current

5/4/2012

Theory

How should RF Power be measured?1. RF voltmeter connected at the output of the transmitter2. Use a TRUE POWER meter that can measure both current

and voltage of both the forward and reflected waves

5/4/2012

RF Power Meter Components

Forward

Reflected

Sensor

Meter*

RFIN

RFOUT

X X

DirectionalCoupler

EnvelopeDetectors

*Meter can be: Analog or digital Current or voltage0-100 uA typical (non-powered)

RF Power Meter

Switch

5/4/2012

SWR/Power Meter - Digital

Forward

Reflected

Sensor

DigitalDisplay

RFIN

RFOUT

X X

DirectionalCoupler

EnvelopeDetectors

DigitalProcessor

5/4/2012

Alpha 4520 Digital Power/SWR Meter

5/4/2012

SWR/Power Meter - Digital

Forward

Reflected

Sensor

DigitalDisplay

RFIN

RFOUT

X X

DirectionalCoupler

EnvelopeDetectors

Processor

5/4/2012

Forward

Reflected

Sensor

RFIN

RFOUT

X X

DirectionalCoupler

EnvelopeDetectors

SWR/Power Meter - Digital

5/4/2012

My Digital Power Meter

Resolution:100W/40000 uV = 0.025 W/uV

uV

5/4/2012

Digital Power/SWR Meters

Forward

Reflected

What is the main difference?

SensorAlpha 4520

5/4/2012

Digital Power/SWR Meters

Forward

Reflected

Sensor

Cost!

$800

~$0-50

$30 upAlpha 4520

5/4/2012

Digital Power/SWR Meters

Forward

Reflected

Sensor

Accuracy varies from ~5% to ?Power reading accuracy is very dependent on Sensor

calibration accuracy (both Forward & Reflected)

Accuracy Spec =

Ultimate Limit on Accuracy?

Sensor Calibration!Initial CAL accuracy

Volts out vs Power InSWR is a calculation, not a measurement

Volts out vs FrequencyTraceable to NIST?Drift with timeLoad impedance drift with heating (1-3 KW???)

Having a digital readout:Doesnt improve accuracyImproves resolutionMay improve repeatability

Having a digital processor does allow for better calibration of sensor characteristics

5/4/2012

Telepost LP-100A Digital Vector Wattmeter

Accuracy: Same specs as Alpha 4520

5% maximum3% (typical)

NIST traceable factory calibrationWhat does this mean?

eHam rating: 5.0/5 (121 reviews)$4355/4/2012

Directional Coupler

Sensor

RFIN

RFOUT

X X

DirectionalCoupler

RF Power Meter

5/4/2012

Only couples power flowing in one directionOnly couples a small sample of the power flowing in the desired direction

Directional Coupler

RFIN

RFOUTX

ForwardCoupler

X

Green = desired couplingRed = undesired coupling

Coupling factor represents the primary property of a directional couplerTo reduce 100 w to 100 mw => Coupling factor = -30 dB

Directivity is the measure of how well a coupler isolates two opposite-travelling (forward and reverse) signals

Creates region of uncertainty around all measurementsBird 43: Directivity >30 dB

Forward

5/4/2012

Dual Directional Coupler

RFIN

RFOUTX

ForwardCoupler

X X XReverseCoupler

Green = desired couplingRed = undesired coupling

Forward Reflected

5/4/2012

Common Directional Couplers

Coupled Transmission Line Coupler

Bird 435/4/2012

Common Directional Couplers

Tandem Match Coupler

Bruene Bridge*

*Most common type found in commercial amateur transmitters

5/4/2012

Common Directional Couplers

Tandem Match Coupler

Bruene Bridge*

*Most common type found in commercial amateur transmitters

How do we get Voltage & Current?5/4/2012

Common Directional Couplers

Tandem Match Coupler

Bruene Bridge*

5/4/2012

Common Directional Couplers

Null adjustment

Tandem Match Coupler

Bruene Bridge*

5/4/2012

Tandem Match CouplerThis coupler has some nice features:

Simplicity, excellent directivityScalable to other power levels, and50- load impedances on all ports

Covering 1.8-30 MHz requires careful transformer designInput VSWR can degrade rapidly as frequency drops below 7 MHz

Bruene BridgeRequires comparatively little spaceMost commonly used design by Ham equipment manufacturersPrimary challenges with this design:

1. Parasitic lead inductance associated with C22. High values for C23. Excessive secondary wire length on T1, and4. Impedance control in the bifilar secondary winding

The lead inductance and C2 result in a series resonance that progressively deteriorates bridge balance as the frequency is raised

Common Sensors

5/4/2012

Tandem Match Coupler

SWR Sensor (from 2010 ARRL Antenna Handbook)

5/4/2012

SWR Sensor (from 2010 ARRL Antenna Handbook)

At 100 watts, IFWDOUT (into a short) > 6 mAAt 100 watts, VFWDOUT (into an open) > 2.0 vdc

Any small signal diode will work(Germaniums are best for QRP)

Shield on coax used as a Faraday shield (grounded on one end only)

Tandem Match Coupler

5/4/2012

Tandem Match CouplerCaution: Germanium diodes dont like heat

5/4/2012

Tandem Match Coupler Using Balun CoreDX Zone.com Digital QRP SWR/ Power Meter by KD1JV

Compensation Diodes(I dont recommend)

5/4/2012

Processor/DisplayDX Zone.com Digital QRP SWR/ Power Meter by KD1JV

http://kd1jv.qrpradio.com/http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=18048

Whitmans Sampler tin

5/4/2012

Sensor

RFIN

RFOUT

X X

EnvelopeDetectors

RF Power Meter

Envelope Detector

5/4/2012

Common Envelope Detector

Forward

Reflected

100K

100K

0.01 uF

0.01 uF

Diode

Diode

Diodes:Type not criticalGermanium best for QRPMatched is desirable, but not required

51

51

5/4/2012

Diode Options

Silicon:1N3600 => VD ~0.7 volt

Germanium:1N34, 1N60, 1N270 => VD ~ 0.3 volt

5/4/2012

Diode Matching -Tandem Match Coupler

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60 70 80 90 100

1N34A-1

1N34A-2

Watts

VDC

Diode type dependent

?

No matching attempted

5/4/2012

Isolated Meter Circuit

+

-1 Forward OUT

3

2

1

11

4

+

-2 Reflected OUT

5

6

7

+VIN+X VDC

1 uF

780X

1 uF

LM324 or equivalent

100K

100K

0.01 uF

0.01 uF

Forward

Reflected

51

51

Diode

Diode3

1

4

2 vdc

~ 300 ohm

Source

5/4/2012

Meter Adjustment

+

-1 Forward OUT

3

2

1

11

4

+

-2 Reflected OUT

5

6

7

+VIN+X VDC

1 uF

780X

1 uF

100K

100K

0.01 uF

0.01 uF

FWDLevel

Adjust

REFLevel

Adjust

Forward

Reflected

51

51

Diode

Diode

5/4/2012

A SIMPLE SWR METER FOR QRP (1 WATT) LEVELS

Not required

XX

X

5/4/2012

0.00

0.50

1.00

1.50

2.00

2.50

0 2 5 10 20 30 40 50 60 70 80 90 95 100

0

10

20

30

40

50

2 5 10 20 30 40 50 60 70 80 90 95

0

20

40

60

80

100

0 10 20 30 40 50 60 70 80 90 100

VDC

mVDC

Watts

Watts

Performance Power Measurement

Homebrew(Tandem Match)

Bird(Coupled Line/Digital)

Pride(Breune)

Diode type dependent

5/4/2012

Performance SWR Measurement

1.0

1.5

2.0

2.5

3.0

Tandem

Bird

Breune

25 ohm Load @ PF = 60 watts

1.8 MHz14 MHz

28 MHz

5/4/2012

SWR Protection Circuit

+

-1

3

2

1

11

4

+

-2

ReflectedReflected

Power

5

6

7 +

-4

1K

12

1314

1.1K

+13.5VDC

+8 VDC

1 uF

To+4 VDC

FoldbackTreshold

7808

390 W

SWR LEDRadio Shack

276-0017

100K