Homebrew HF SWR/Power Meter
Bill Leonard
NØCU
NAØTC - 285 TechConnect Radio Club
http://www.naØtc.org/
5/4/2012
Theory
•Standing Waves
SWR = 3:1
Dynamic Demo: http://www.walter-fendt.de/ph14e/stwaverefl.htm
5/4/2012
Theory
•SWR Calculation
•Not a direct measurement
1+
SWR =
1-
PR
PF
PR
PF
=
EF + ER
EF - ER
EF = Forward Voltage
ER = Reverse Voltage
PF = Forward Power
PR = 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.
5/4/2012
RF voltmeter connected at the output of the transmitter
Theory
•How should RF Power be measured?
No!
1.
RF voltmeter connected at the output of the transmitter
1.
Voltage reading is dependent upon load impedance at the point
of measurement
• Power meters on Ameritron Power Amplifiers
To determine power, we need independent measurements of
voltage and current
2.
5/4/2012
Theory
•How should RF Power be measured?
1.
2.
5/4/2012
RF voltmeter connected at the output of the transmitter
Use a TRUE POWER meter that can measure both current
and voltage of both the forward and reflected waves
RF Power Meter Components
RF Power Meter
Sensor
RF
IN
Directional
Coupler
X
RF
OUT
X
*Meter can be:
• Analog or digital
• Current or voltage
•0-100 uA typical (non-powered)
Envelope
Detectors
Forward
Reflected
5/4/2012
Switch
Meter*
SWR/Power Meter - Digital
Sensor
RF
IN
Directional
Coupler
X
RF
OUT
X
Envelope
Detectors
Forward
Reflected
5/4/2012
Digital
Processor
Digital
Display
Alpha 4520 Digital Power/SWR Meter
5/4/2012
SWR/Power Meter - Digital
Sensor
RF
IN
Directional
Coupler
X
RF
OUT
X
Envelope
Detectors
Forward
Reflected
5/4/2012
Processor
Digital
Display
SWR/Power Meter - Digital
Sensor
RF
IN
Directional
Coupler
X
RF
OUT
X
Envelope
Detectors
Forward
Reflected
5/4/2012
My Digital Power Meter
uV
Resolution:
100W/40000 uV = 0.025 W/uV
5/4/2012
Digital Power/SWR Meters
What is the main difference?
Sensor
Forward
Reflected
5/4/2012
Alpha 4520
Digital Power/SWR Meters
Cost!
Sensor
$30 up
Alpha 4520
Forward
Reflected
$800
~$0-50
5/4/2012
Digital Power/SWR Meters
Accuracy varies from ~5% to ?
•Power reading accuracy is very dependent on Sensor
calibration accuracy (both Forward & Reflected)
Sensor
Alpha 4520
Forward
Reflected
Accuracy: 5-10% achievable
5/4/2012
Accuracy Spec = <5%
Ultimate Limit on Accuracy?
•Sensor Calibration!
•Initial CAL accuracy
•Volts out vs Power In
•SWR is a calculation, not a measurement
•Volts out vs Frequency
•Traceable to NIST?
•Drift with time
•Load impedance drift with heating (1-3 KW???)
•Having a digital readout:
•Doesn’t improve accuracy
•Improves resolution
•May 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% maximum
•3% (typical)
•NIST traceable factory calibration
•What does this mean?
•eHam rating: 5.0/5 (121 reviews)
•$435
5/4/2012
Directional Coupler
Sensor
RF
IN
Directional
Coupler
X
5/4/2012
X
RF Power Meter
RF
OUT
Directional Coupler
•Only couples power flowing in one direction
•Only couples a small sample of the power flowing in the desired direction
RF
IN
X
X
RF
OUT
Forward
Coupler
Green = desired coupling
Red = undesired coupling
Forward
•Coupling factor represents the primary property of a directional coupler
•To reduce 100 w to 100 mw => Coupling factor = -30 dB
•Directivity is the measure of how well a coupler isolates two oppositetravelling (forward and reverse) signals
•Creates region of uncertainty around all measurements
5/4/2012•Bird 43: Directivity >30 dB
Dual Directional Coupler
RF
IN
X
X
Forward
Coupler
X
Reverse
Coupler
Green = desired coupling
Red = undesired coupling
Forward
5/4/2012
X
Reflected
RF
OUT
Common Directional Couplers
Coupled Transmission Line Coupler
Bird 43
5/4/2012
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
Tandem Match Coupler
Bruene Bridge*
Null adjustment
5/4/2012
Common Sensors
•Tandem Match Coupler
•This coupler has some nice features:
•Simplicity, excellent directivity
•Scalable to other power levels, and
•50-Ω load impedances on all ports
•Covering 1.8-30 MHz requires careful transformer design
•Input VSWR can degrade rapidly as frequency drops below 7 MHz
•Bruene Bridge
•Requires comparatively little space
•Most commonly used design by Ham equipment manufacturers
•Primary challenges with this design:
1. Parasitic lead inductance associated with C2
2. High values for C2
3. Excessive secondary wire length on T1, and
4. 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
5/4/2012
Tandem Match Coupler
SWR Sensor (from 2010 ARRL Antenna Handbook)
5/4/2012
Tandem Match Coupler
Shield on coax used as a Faraday shield (grounded on one end only)
SWR Sensor (from 2010 ARRL Antenna Handbook)
Any small signal diode will work
(Germaniums are best for QRP)
At 100 watts, IFWDOUT (into a short) > 6 mA
At 100 watts, VFWDOUT (into an open) > 2.0 vdc
5/4/2012
Tandem Match Coupler
Caution: Germanium diodes don’t like heat
5/4/2012
Tandem Match Coupler Using Balun Core
DX Zone.com ”Digital QRP SWR/ Power Meter” by KD1JV
Compensation Diodes
(I don’t recommend)
5/4/2012
Processor/Display
DX Zone.com ”Digital QRP SWR/ Power Meter” by KD1JV
“Whitman’s Sampler” tin
•http://kd1jv.qrpradio.com/
•http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=18048
5/4/2012
Envelope Detector
Sensor
RF
IN
RF
OUT
X
X
Envelope
Detectors
5/4/2012
RF Power Meter
Common Envelope Detector
Forward
Diode
0.01 uF
51
Reflected
100K
Diode
0.01 uF
51
5/4/2012
100K
Diodes:
•Type not critical
•Germanium best for QRP
•Matched is desirable, but not required
Diode Options
Silicon:
•1N3600 => VD ~0.7 volt
Germanium:
•1N34, 1N60, 1N270 => VD ~ 0.3 volt
5/4/2012
Diode Matching -Tandem Match Coupler
No matching attempted
2.5
2
Diode type dependent
1.5
VDC
1N34A-1
1N34A-2
1
0.5
?
0
0
10
20
30
40
50
Watts
5/4/2012
60
70
80
90
100
Isolated Meter Circuit
+X VDC
780X
1 uF
Forward
+VIN
1 uF
Diode
3
0.01 uF
51
100K
+
-
2
4
1
1
Forward OUT
1
11
LM324 or equivalent
Reflected
Diode
5
0.01 uF
51
Source
~ 300 ohm
2 vdc
5/4/2012
100K
6
+
-
2
7
Reflected OUT
3
4
Meter Adjustment
+X VDC
780X
1 uF
Forward
1 uF
Diode
3
0.01 uF
51
Reflected
100K
2
5
0.01 uF
5/4/2012
+
-
4
1
1
11
Diode
51
100K
+VIN
6
+
-
2
7
FWD
Level
Adjust
REF
Level
Adjust
Forward OUT
Reflected OUT
A SIMPLE SWR METER FOR QRP (1 WATT) LEVELS
X
X
X
Not required
5/4/2012
Performance – Power Measurement
2.50
Homebrew
(Tandem Match)
2.00
VDC
1.50
Diode type dependent
1.00
0.50
0.00
0
2
5 10 20 30 40 50 60 70 80 90 95 100
2
5
10
0
10
50
Bird
(Coupled Line/Digital)
40
30
mVDC
20
10
0
20 30 40
50 60
70 80
90 95
100
Pride
(Breune)
80
60
Watts
40
20
0
5/4/2012
20
30
40
50
Watts
60
70
80
90 100
Performance – SWR Measurement
25 ohm Load @ PF = 60 watts
3.0
2.5
Tandem
2.0
Bird
1.8 MHz
1.5
1.0
5/4/2012
Breune
14 MHz
28 MHz
SWR Protection Circuit
+8 VDC
+13.5
VDC
7808
1 uF
1 uF
15K
100
3
+
-
2
4
1
LM324
0.1 uF
1
0.1 uF
1.21M
1N3600
Latch
Treshold
Reflected
5
100K
6
+
-
2
1.0 uF
7
FWD
17 mA
REF
12
10
9
+
-
8
3
10K
2N3501
To
+8 VDC
Latch Relay1
Reflected
Power
13
0.1 uF
+
-
4
9
Reset
4
6
13
5.1K
8
14
2N3501
To
+4 VDC
15K
1N3600
To SWR
Shutdown
5.1K
2N3501
390
½W
+
-
100K
+1.6 VDC
5/4/2012
-
16
11
1K
Forward
1.0 uF
+
1N3600
~1 sec Delay
to Latch
200K
1
+12 VDC
11
150K
Latch
Relay1
+4 VDC
15K
Foldback
Treshold
10K
1.1K
2N3501
SWR LED
Radio Shack
276-0017
To
+13.5 VDC