Homebrew SWR Meter

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Transcript of 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