March 20, 2012

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Simple & Accurate Power Measurement

We've previously covered the very popular topic of Power Measurement Techniques and the wide range of equipment and projects available to us homebrewers.  But this time we're going to focus on a very practical example and chronicle a solution path that could well pan out for you.

Many of us have a simple, homebrew power meter circuit that often displays power readings to a tenth of a watt , such as 0.1 watt, or 5.2 watts. Oftentimes this is sufficient for rough estimation of one's output power level ... but at other times you need greater accuracy in order to adequately determine and set gain stages in a radio amplifier chain, determine power transfer efficiencies, or perform comparisons against known standards.  In these cases, short of having an expensive Bird or HP power meter on the bench, what can you do?

We recently needed to track down some gain uncertainties in the SDR Cube Transceiver and with some expert help of we were able to homebrew some simple building blocks that yielded very interesting results.  We will overview and describe this approach and maybe you too can get some ideas for your own bench!

~George N2APB & Joe N2CX

Audio Recording ... (Listen to MP3 recording)

Text Log ...

<20:24:23>"George - N9DXP": How is the 50 ohm impedance maintained in the attenuator?
<20:27:56>"Joe N2CX": WA2DJN
<20:28:39>"Joe N2CX": Heatsink snob he means...
<20:35:50>"Paul - wa0rse": That's the new layout you mentioned, George. Nice!
<20:36:43>"Lee KM4YY": Where you get 50 ohm resistor in TO-220 package??
<20:39:43>"Bruce - N1RX": Caddock is one mfg of the resistors. Check ur fav vendors
<20:45:03>"Ethan - KB8OJH": what is the lucite (or whatever?) between the BNC and the topside of the heat sink?
<20:45:09>"Terry WB4JFI": I love the mWattmeter from AQRP group... use it a lot
<20:45:11>"Ethan - KB8OJH": looks like the sink is on the shield?
<20:45:13>"Paul - wa0rse": Are you guys aware of the DEMI ABPM designed by W1GHZ?
<20:45:37>"Joe N2CX": It is a HDPE cutting board from a department store.
<20:46:31>"Ethan - KB8OJH": no, not that ... on the two photos of the dummy load side-by-side, the left photo seems to show a clear piece of plastic between the BNC and the aluminum of the sink
<20:46:48>"Ethan - KB8OJH": between the power tap block diagram and the power tap schematic
<20:46:56>"Terry WB4JFI": The second chart is VERY good cuz you can use it with a scope, P-P readings can be rather accurate
<20:48:11>"Terry WB4JFI": Digikey TCH35P51R0JE-ND, 51ohm, 35w for 6.98
<20:50:15>"Lee KM4YY": Is the mWattmeter from AQRP still available anywhere?
<20:50:34>"Bruce - N1RX": on the heat sink pic- I think that is a piece of sheet aluminum, not plastic. Just to provide mounting for the Bnc.
<20:50:50>"Joe N2CX": It is sheet aluminum
<20:51:04>"Ethan - KB8OJH": ok, thanks -- must just be a trick of the light
<20:52:27>"Bruce - N1RX": The AQRP mWattmeter is still avail on the page given in the notes.
<20:53:59>"George - N2APB": The 35W, 51-ohm resistor in a TO-220 package costs about $6 from Mouser, p/n 588-TCH35P-51E
<20:56:19>"Ethan - KB8OJH": no questions here due to the clarity of presentation!
<21:01:19>"Carlos - PY1EGG": May I suggest looking at OZ2CPU´s power meter at:
<21:01:32>"Joe N2CX": Caddock is a good source for that resistor
<21:02:25>"George - N2APB": The OZ2CPU power meter looks superb Carlos!  Thank you for the recommendation.
<21:12:17>"Joe N2CX": WM1 covers 150 mW to 140W
<21:17:32>"Charles WC5MC":
<21:20:39>"Carlos - PY1EGG":
<21:34:26>"George - N2APB": Here's another one for the HIQ-SDR

Checkins ...




Use of a 40 dB In-Line Power Tap ...

First comes the Dummy Load .. a TO220-packaged, 35w, 50-ohm precision resistor mounted on a good heatsink ...

Next comes the Power Tap, directly from the W7ZOI article ...

And now the combination of the Dummy Load and Power Tap, conveniently coupled with 90-degree BNC and double-female adapters ...

So now, with the DL + Tap conveniently located next to the precision KA7EXM Power Meter, the two BNCs may be connected with a short length of coax to form a single measurement pair  ...


And actually measuring the SDR Cube's output power in Tune mode ... -9.9 dBm on the meter = 1.023W (see chart below)

Here's an excellent way to go for building an accurate, high dynamic range power meter, from the AQRP Club guys: K5BCQ, W8NUE, and K5JHF ...


Readings from a Power Meter with a 40dB tap may be correlated to actual power using the Excel spreadsheet table below, from N2CX ...

Download this Excel Spreadsheet Calculator
(right-click and Save As ...)

Source:  The QRP Quarterly, Spring 2003



1. Simple RF-Power Measurement", by Wes Hayward, W7ZOI and Bob Larkin W7PUA, June 2001 QST, ARRL ...

2. "A PIC-based HF/VHF Power Meter", QEX May/June 2005. Previously available from Kanga US. PC boards and programmed PIC from KA7EXM. 

3. "A PIC-based HF/VHF Power Meter", Digital power meter by KA7EXM (son of W7ZOI), .  Similar to W7ZOI analog meter but has digital display

4. "Applications Using the PIC-Power Meter", by KA7EXM ...

5. Norcal Power Meter ... Digital and analog 10W max resolution 0.1 w ... currently out of stock 

6. RF Power Measurement for QRPers ... CWTD Nov 2011

7. AQRP mWattmeter Kit ...


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