An Accurate Power Meter For QRP Ops
Created by on 2000-07-19An Accurate Power Meter For QRP Ops
By AI2Q, Alex Mendelsohn
Have you ever wondered what the real power output of your QRP rig is? How about being able to measure QRPP?
Here's a circuit using two junkbox transistors and a garden variety op-amp that will let you accurately measure your rig's flea power down to milliwatts. It makes use of your ordinary shop multi-meter. If your DMM or analog voltmeter's accuracy is good, your results will be too. It works because power is proportional to the square of the voltage across a resistor, assuming the resistance stays the same.
Although it can't measure a rig's output power while operating into an antenna, it will tell you how much power your QRP rig puts out into 50-ohm resistive loads like a well-matched coaxial feedline. The best thing is that it's accurate!
Simply Use A Bulb
The basis of the circuit is an ordinary filament type bulb. If you build a small attenuator as well, you can measure up to 5 W. This circuit was originated by G4COL. My version uses parts available here in the US. The semiconductors can be obtained from Radio Shack. You can build it on a piece of perf-board, or Manhattan-style (more about that in a later article), and then subsequently mount it in a small cabinet.
Click on this image to view theschematic full size
As you may know, the resistance of a bulb changes depending on how much current goes through it (how hot it gets). This simple circuit allows you to adjust the resistance by passing a DC current through the bulb. The bulb's brightness is monitored by an infrared phototransistor.
The phototransistor's output feeds an op-amp that keeps the current constant. The pot R8 sets the brightness. The brightness of the lamp is always held constant because of the closed feedback loop; transistor Q1 is fed by the op-amp. How much it conducts sets the current through the bulb.
The current flows from ground (I prefer the old fashioned electron flow convention), up through the RF choke, then through R2/R3 combo, through Q1 from collector to emitter and finally back to the positive side of the 12 V power supply. To see this, trace the current flow with a colored pencil. If you study it, you'll see that the bulb brightness is always the same, providing a constant impedance match over all the bands from 160 meters up through ten meters.
Your RF is fed into capacitor C1 and is coupled to the bulb. The bulb is also fed with DC, although the RF is blocked from Q1 by the RF choke (it passes only DC). When the bulb lights, its light causes current flow in the phototransistor Q2, causing a voltage drop across R5. This voltage is "felt" by the non-inverting input of the op-amp, and is amplified and appears as output.
The output voltage of the op-amp will change (although it happens too fast for you to see on a voltmeter) until it is equal to whatever level is set by the three-resistor voltage divider across the +12V line (R7, R8 and R9 in series). The lamp is always at constant brightness and therefore at constant impedance. That's the neat part!
Ohm's Law Applied!
Power and lamp resistance is monitored by your external meter. With S1 in the lower position and no RF applied, the voltage across the bulb is measured. Since R2 and R3 in parallel equal 50 ohms, and the current through the two is the same current through the bulb (they are in series after all) the voltage across the 50-ohm resistor pair can be set to equal the voltage across the bulb. You can check this by flipping the switch to the upper position and then to the lower. Then you set the pot R8 until they're equal. Easy, eh?
DC power is found by using an ordinary calculator. Measure the voltage, square the value found, and divide that result by 50. With no RF applied, this is the maximum power that can be read.
When you apply RF, the lamp will absorb it and the amount of DC required to maintain brightness will, naturally, be less! A new lower voltmeter reading will be obtained. The difference between it and the previous (no RF applied) reading is used to quickly calculate your actual applied RF power.
| AE1X | 2004-06-10 | |
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| RE: An Accurate Power Meter For QRP Ops | ||
| Great idea. I've been looking for techniques to measure the output of my Pixie2 and my Rock-Mite. My commerical equipment doesn't measure below 1 Watt very well. What a shame someone would be little concept such as this. This is precisely what amateur radio is all about. I applaud all of those involved in this work. Thanks... Ken, AE1X Reply to a comment by : N5LB on 2003-09-20 Clever concept. It intriqued me that this approach might work. Though not a complex technical article, this work represents some of the best in ham radio's approach to technical needs, pragmatic, with a strong link to basic concepts. I don't run QRP (yet!) but I'll experiment with the idea anyway. Off to the lab bench....... Liomel N5LB | ||
| N5LB | 2003-09-20 | |
|---|---|---|
| An Accurate Power Meter For QRP Ops | ||
| Clever concept. It intriqued me that this approach might work. Though not a complex technical article, this work represents some of the best in ham radio's approach to technical needs, pragmatic, with a strong link to basic concepts. I don't run QRP (yet!) but I'll experiment with the idea anyway. Off to the lab bench....... Liomel N5LB | ||
| AC5WA | 2000-08-15 | |
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| QRP Power Meter | ||
| Isn't this similar to a bolometer? In the late seventies, early eighties, I remember hunting down an extremely tiny incandescent lamp for a ham friend who was trying to optomize a microwave oscilator for a satelite reciever. The circuit he described seems to be similar to the "QRP Power Meter". If the working principle is the same, it could, with proper layout and couppling, be accurate up to SHF and beyond. | ||
| G3YSX | 2000-08-02 | |
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| RE: QRP Watt meter | ||
| I tested the G4COL design against an adret signal generator, which was in turn calibrated against an HP power meter. The design is as accurate as the DVM that you use to take the measurements an is flat up to VHF. The limitation in this type is the reliance on the the choke to seperate the bias from the bulb and the need to do the calculation. However it was a very neat design by Ian G4COL. G3GRO and I did quite a lot of work on bulb based power meters - see our article in RadCom Jan/Feb 2000. Note that the linear approximation we used to get direct rteading is comfortably within acceptable measurement error for most purposes. Stewart G3YSX Reply to a comment by : W6DX on 2000-07-24 The writer claims that the power meter is "accurate" but never states how he arrived at that conclusion. While the feedback circuit does hold the DC resistance of the bulb constant, there is no assurance that the AC impedance is anywhere near 50 Ohms. Light bulbs are notoriously bad loads, in part because the filaments are made of coiled wire and are thus inductive. This is the same reason we don't use 50 Ohm wire-wound resistors as dummy loads. The author's claim for accuracy would be better supported if he were to compare the power calculations from his instrument with the measurements from an accepted standard such as a Hewlett-Packard power meter, at several different frequencies and power levels. The device may in fact turn out to be an extremely accurate power indicator, but no evidence has been presented to justify such a conclusion. This is not intended to criticize the author's efforts but rather to point out that the accurate measurement of low power RF signals is not a trivial exercise, and any effort aimed at simplifying the process must be carefully researched and thoroughly verified. Reply to a comment by : WA4CNG on 2000-07-23 The "Mystery Writer" is what we all call an "Appliance Operator", someone who wants everything ready for take-out on the Master-Card Charge plan, he can't build it so the garbage criticizm of it, nuff said. Very good article, low power wattmeters are NOT the norm, and this article addresses many QRP issues, mainly are you really QRP and how do I know if I am. In commercial applications this would be a "Quality Measurement Tool", here it is. Reply to a comment by : on 2000-07-21 This is a "simple" watt meter ??? It would take less effort to buy an extant watt meter than to read this whole pile of mumbo jumbo. Get real! How much rec time do you get at the "bin" ? Try finger painting next time and leave the word processor to the more lucid ones on your floor. | ||
| W6DX | 2000-07-24 | |
|---|---|---|
| RE: QRP Watt meter | ||
| The writer claims that the power meter is "accurate" but never states how he arrived at that conclusion. While the feedback circuit does hold the DC resistance of the bulb constant, there is no assurance that the AC impedance is anywhere near 50 Ohms. Light bulbs are notoriously bad loads, in part because the filaments are made of coiled wire and are thus inductive. This is the same reason we don't use 50 Ohm wire-wound resistors as dummy loads. The author's claim for accuracy would be better supported if he were to compare the power calculations from his instrument with the measurements from an accepted standard such as a Hewlett-Packard power meter, at several different frequencies and power levels. The device may in fact turn out to be an extremely accurate power indicator, but no evidence has been presented to justify such a conclusion. This is not intended to criticize the author's efforts but rather to point out that the accurate measurement of low power RF signals is not a trivial exercise, and any effort aimed at simplifying the process must be carefully researched and thoroughly verified. Reply to a comment by : WA4CNG on 2000-07-23 The "Mystery Writer" is what we all call an "Appliance Operator", someone who wants everything ready for take-out on the Master-Card Charge plan, he can't build it so the garbage criticizm of it, nuff said. Very good article, low power wattmeters are NOT the norm, and this article addresses many QRP issues, mainly are you really QRP and how do I know if I am. In commercial applications this would be a "Quality Measurement Tool", here it is. Reply to a comment by : on 2000-07-21 This is a "simple" watt meter ??? It would take less effort to buy an extant watt meter than to read this whole pile of mumbo jumbo. Get real! How much rec time do you get at the "bin" ? Try finger painting next time and leave the word processor to the more lucid ones on your floor. | ||
| WA4CNG | 2000-07-23 | |
|---|---|---|
| RE: QRP Watt meter | ||
| The "Mystery Writer" is what we all call an "Appliance Operator", someone who wants everything ready for take-out on the Master-Card Charge plan, he can't build it so the garbage criticizm of it, nuff said. Very good article, low power wattmeters are NOT the norm, and this article addresses many QRP issues, mainly are you really QRP and how do I know if I am. In commercial applications this would be a "Quality Measurement Tool", here it is. Reply to a comment by : on 2000-07-21 This is a "simple" watt meter ??? It would take less effort to buy an extant watt meter than to read this whole pile of mumbo jumbo. Get real! How much rec time do you get at the "bin" ? Try finger painting next time and leave the word processor to the more lucid ones on your floor. | ||
| VK3YE | 2000-07-22 | |
|---|---|---|
| RE: QRP Watt meter | ||
| A good article and a good project. Power measurement is a real need for all amateurs, whether QRP or QRP. That coward 'Anonymous' should do us all a good turn and contact the FCC, turn his licence in and sell all his equipment. He's the type of person who rubbishes anyone who tries to make a contribution to ham radio. Amateur radio does not need people like him. Let all his CQs go unanswered, his antennas get struck by lightning and the sharp end of the Woulff Hong be shoved up an unmentionable orifice! AI2Q has built a piece of equipment and voluntarily shared the information with other hams. Surely a better person than 'Anonymous', hiding behind his keyboard, who does nothing but complain! Reply to a comment by : on 2000-07-21 This is a "simple" watt meter ??? It would take less effort to buy an extant watt meter than to read this whole pile of mumbo jumbo. Get real! How much rec time do you get at the "bin" ? Try finger painting next time and leave the word processor to the more lucid ones on your floor. | ||
| N.A. | 2000-07-21 | |
|---|---|---|
| QRP Watt meter | ||
| This is a "simple" watt meter ??? It would take less effort to buy an extant watt meter than to read this whole pile of mumbo jumbo. Get real! How much rec time do you get at the "bin" ? Try finger painting next time and leave the word processor to the more lucid ones on your floor. | ||
